With the Compliments of the Anth er. M 'ihelvtMis CONTRIBUTIONS from the CHEMICAL LABORATORY OF HARVARD COLLEGE. By HENRY B. HILL. [Reprinted from the Proceedings of the American Academy' of Arts and Sciences.] \ m OF ARTS AND SCIENCES. 125 X. CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF HARVARD COLLEGE. By Henry B. Hill. Presented February 1, 1882. I. On Dibromacrylic Acid. In a communication upon furfurol and certain of its derivatives which I laid before the Academy a year ago, I described a dibro- macrylic acid * which O. R. Jackson and I had some time before ob- tained from mucobromic acid by the action of alkalies. Although we had not been able to prepare the acid in a state of perfect purity, still our results seemed to us sufficient for its identification, and since it then appeared that a more extended study of it would interfere with other investigators in the same field, further work upon it had been for the time given up. Not long afterward it became evident that our hesitation upon this account had been quite unnecessary ; but it was not until recently that I was able to take up again the study of this acid. I have now obtained results which correct our previous observations in several important particulars. For the preparation of the acid O. R. Jackson and I used chiefly the barium salt, which crystallized well from water or dilute alcohol, and which gave us constant analytical results. The air-dried salt lost nothing in vacuo over sulphuric acid, or when heated to 80°, and the percentage of barium which it contained agreed closely with that required by the formula Ba(C 3 HBr 2 0 2 ) 2 . We therefore with little hesitation considered the salt anhydrous, and were inclined to ascribe the slight loss of weight which we noticed at 100° to a slow decomposition. The acid made from carefully-prepared barium salt crystallized well, melted quite sharply at 83-84°, but on analysis * These Proceedings, Vol. XVI. (n. s. viii.) p. 192. 126 PROCEEDINGS OF THE AMERICAN ACADEMY proved to contain too high a percentage of bromine. Since the acid made from the lead salt had given us precisely the same unsatisfactory results, we thought it probable that the impurities which were found in the acid thus made were introduced by a decomposition of the acid, itself in its liberation from its salts. A more careful study of this reaction subsequently convinced me, however, that such was not the case. Certainly no carbonic dioxide, bromacetylen, or hydrobromic acid could be detected as resulting from such decomposition when dilute sulphuric acid was added in slight excess to a boiling aqueous solution of the barium salt. I therefore turned my attention to a further purification of the salts. Since repeated recrystallization of the barium salt failed to give me any much better product, I thought it worth while to determine the variation of composition introduced by one set of crystallizations from water. I therefore dissolved 30 grammes of white well-crystallized salt (A) in 380 cc. of hot water. On cooling 6.5 grms. of the salt (I.) separated, and by successive filtration, evaporation, and cooling I obtained the fractions (II.) 8.9 grms., (III.) 6.9 grms., and (IV.) 4.8 grms., the remaining 2.9 grms. being lost in the filter-papers upon which the successive portions were dried. An analysis of these air-dried salts gave the following results : — A. 0.5669 grm. substance gave 0.2239 grm. BaS0 4 . I. 0.5713 grm. substance gave 0.2276 grm. BaS0 4 . II. 0.5139 grm. substance gave 0.2036 grm. BaS0 4 . III. 0.5665 grm. substance gave 0.2231 grm. BaS0 4 . IV. 0.5003 grm. substance gave 0.1941 grm. BaS0 4 . Calculated for Calculated for Found. Ba(C 3 HBr 2 0 2 ) 2 . Ba(C 3 HBr 2 0 2 ) 2 .H 2 0. A. I. H. III. IV. Ba 23.03 22.35 23.21 23 43 23.29 23.15 22.80 From these results it was evident that the barium salt contained a persistent impurity which could not be removed by fractional crys- tallization, and that the constancy of its composition was accidental. I next tried the beautifully crystalline acid potash salt which I have already described,* and found that it gave results which were all that could be desired. This salt can easily be made by neutralizing a weighed amount of the ordinary acid melting at 82—84° with potassic * Loc. cit. p. 194. OF ARTS AND SCIENCES. 127 carbonate, and adding to the hot solution an equal amount of the acid dissolved in a little hot water. As the solution cools the salt separ- ates in long silky needles, which after a few recrystallizations from hot water yield an acid whose melting-point is constant. The loss in recrystallization is comparatively small, since the salt is but sparingly soluble in cold water and dissolves very freely in hot. The analyses which I have made of the acid prepared in this way show its perfect purity. I. 0.2183 grm. substance gave by the method of Carius 0.3573 grm. AgBr. II. 0.2127 grm. substance gave 0.3476 grm. AgBr. III. 0.2530 grm. substance gave 0.4137 grm. AgBr. Calculated for C3H 2 Br 2 02. Found. I. II. III. Br. 69.56 69.66 69.56 69.60 The pure acid melts at 85.5-86°, but in other respects does not differ essentially in its physical properties from the product which I have already described. The solubility of the acid was determined by neutralizing with baric carbonate an aqueous solution of the acid prepared according to the method of V. Meyer, and precipitating with sulphuric acid the barium dissolved. I. 12.7854 grms. of a solution saturated at 17°. 5 gave 0.3107 grm. BaS0 4 . II. 13.5723 grms. of a solution saturated at 17°.5 gave 0.3303 grm. BaS0 4 . III. 10.8488 grms. of a solution saturated at 18° gave 0.2695 grm. BaS0 4 . IV. 9.7341 grms. of a solution saturated at 18° gave 0.2434 grm. BaS0 4 . According to these determinations the aqueous solution saturated at 17°.5 and 18° contains the following percentages : — 17 °. 5 . 18 °. I. II. III. IV. 4.80 4.81 4.90 4.94 Baric Dibromacrylate , Ba(C 3 HBr 2 0 2 ) 2 . H 2 0. The barium salt prepared by neutralizing a solution of the pure acid with baric car- 128 PROCEEDINGS OF THE AMERICAN ACADEMY bonate did not differ in outward appearance from the salt made directly from mucobromic acid by the action of baric hydrate, but proved on analysis to contain a percentage of barium materially lower than that which O. R. Jackson and I had previously obtained, and closely agree- ing with that required by one molecule of water of crystallization. When dried by exposure to the air the salt lost nothing over sulphuric acid or when heated to 85°, but by long-continued heat at 100° (75 to 100 hours) or more rapidly at 120° it gave up its crystal water with- out showing any signs of decomposition. I. 0.6642 grm. of the air-dried salt gave by precipitation 0.2540 grm. BaS0 4 . II. 0.6392 grm. of the air-dried salt gave on ignition with H 2 S0 4 0.2434 grm. BaS0 4 . III. 0.5300 grm. of the air-dried salt lost at 120° 0.0161 grm. H 2 0. IV. 1.9253 grm. of the air-dried salt lost at 100° 0.0548 grm. H 2 0. V. 1.1582 grm. of the air-dried salt lost at 100° 0.0351 grm. H 2 0, and gave by precipitation with II 2 S0 4 0.4408 grm. BaS0 4 . Calculated for Ba(C 3 HBr 2 0 2 ) o • H z O. Found. I. II. III. IV. Ba 22.35 22.48 22.39 II 2 0 2.94 3 04 2.85 The solubility of the salt was also determined. I. 9.4276 grm. of an aqueous solution saturated at 18° gave by precipitation 0.2214 grm. BaS0 4 . II. 8.4088 grm. of a solution saturated at 18° gave 0.1917 grm. BaS0 4 . According to these determinations the aqueous solution of the salt saturated at 18° contains the following percentages of the anhydrous salt : — I. II. 6.00 5.82 Plumbic Dibrom acrylate, Pb(C 3 HBr 2 0 2 ) 2 . H 2 0. The lead salt which, according to the analyses made of former preparations, was anhydrous when made from the pure acid by neutralization with plum- bic carbonate, or from the acid potassium salt by precipitation with plumbic acetate, likewise proved to contain one molecule of water of crystallization. V. 22.38 3.03 C OF ARTS AND SCIENCES. 129 ■ I. 0.5972 grm. of the air-dried salt lost at 100° 0.0161 grm. H 2 0, and gave by ignition with H 2 S0 4 0.2655 grm. PbS0 4 . II. 1.0438 grm. substance dried over II 2 S0 4 lost at 100° 0.0283 grm. H 2 0, and gave by ignition with H 2 S0 4 0.4622 grm. PbS0 4 . Calculated for Pb(C 3 HBr 2 0 2 ).> . IPO. . Found. I. II. Pb 30.31 30.37 30.25 H 2 0 2.64 2.70 2.71 Calcic Dibromacrylate , Ca(C 3 HBr 2 0 2 ) 2 . 3H 2 0. The calcium salt made from the pure acid crystallized in long clustered needles, which gave on analysis results identical with those which O. R. Jackson and I formerly obtained. I. 2.1502 grm. of the air-dried salt lost at 80-85° 0.2034 grm. H 2 0. II. 1.2264 grm. of the air-dried salt lost at 95-100° 0.1171 grm. H 2 0. III. 1.8124 grm. of the air-dried salt lost at 100° 0.1755 grm. H 2 0. Calculated for Ca(C 3 HBr 2 0 2 ) . 3H 2 0. Found. I. II. III. I4 2 0 9.78 9.46 9.55 9.68 I. 0.4393 grm. of the salt dried at 100° gave on ignition with H 2 S0 4 0.1199 grm. CaS0 4 . II. 0.6161 grm. of the salt dried at 100° gave on ignition with H 2 S0 4 0.1674 grm. CaS0 4 . Calculated for Ca(C 3 HBr 2 0 2 ) 2 . Found. I. II. Ca 8.03 8.03 7.99 Fotassic Dibromacrylate , KC 3 HBr 2 0 2 . The analysis of the potas- sum salt made by neutralizing the pure acid with potassic carbonate showed that it was anhydrous, as it had previously been described. I. 0.7334 grm. of the salt dried over II 2 S0 4 gave on ignition with Ii 2 S0 4 0.2373 grm. K 2 S0 4 . II. 0.7507 grm. of the salt dried over H 2 S0 4 gave 0.2427 grm. k 2 so 4 . VOL. XVII. (n. S. IX.) 9 130 PROCEEDINGS OF THE AMERICAN ACADEMY Calculated for KC 3 HBr 2 0 2 Found. I. II. K 14.58 14.53 14.51 Although many unsuccessful attempts had previously been made to prepare brompropiolic acid in a pure state, it seemed to me not im- possible that the prolonged action of alkalies in the cold might remove from the dibromacrylic acid a molecule of hydrobromic acid, and that a purer product might thus be obtained. I found, however, that the reaction was extremely slow, at least with baric hydrate. An aqueous solution which contained one molecule of baric hydrate to each mole- cule of dibromacrylic acid was strongly alkaline even after standing for fifteen days, and gave on acidification and extraction with ether the compound of brompropiolic and dibromacrylic acids which has already been described. After recrystallization from ligroin the sub- stance melted at 103°, and gave on analysis the following result : — 0.2535 grm. substance gave 0.3766 grm. AgBr. Calculated for C 6 H 3 Br 3 0 4 . Found. Br. 63.33 63.22 Since I had before noticed that this intermediate product could be obtained from dibromacrylic acid by the action of baric hydrate in the course of a few hours, and that malonic acid was formed even in the cold after the lapse of several months, it was evident that further attempts in this direction were useless. OP ARTS AND SCIENCES, 131 II. On the Crystalline Form op a Dichloracrylic Acid. By W. H. Melville. (Communicated by H. B. Hill.) Although the physical properties of the dichloracrylic acid of Wallach * are decidedly different from those which W. Z. Bennett and I found to be characteristic of the dichloracrylic acid made from mucochloric acid,f a difference which appeared to be fully confirmed by a comparison of the salts of the two acids, still it seemed to me desirable to prove with a little more precision the difference between the two. Since Wallach had made no determinations of the solubility of his acid or its salts, and moreover thought $ that little weight should be attached to the determinations of crystal-water which he had as yet published, there remained no definite, well-established points of difference except the melting-points (86° and 77°) and a difference in the crystalline form of the potassium salt : one crystal- lizing in needles, the other in hexagonal plates. Since the |S acid had been obtained in measurable crystals and fully described in Wallach’s first paper, although we had previously been unable to get measurable crystals of our acid, I made fresh attempts with larger quantities of material than had then been at our disposal. I found that by the slow cooling of a warm, moderately dilute solution in chloroform well- developed crystals could be obtained, although the determination of the crystals was rendered difficult on account of the rapid roughening of their faces when exposed to the air. Dr. W. H. Melville succeeded, however, in making the necessary measurements, and to his kindness I am indebted for the following description. The purity of the material used was determined by analysis. 0.2061 grm. of the substance gave 0.4198 grm. AgCl. Calculated for C 3 H 2 CI 2 O 2 . Cl. 50.36 Found. 50.35 * Ann. Chem. u. Pharm. vol. cxciii. 19. t These Proceedings, Vol. XVI. (n. s. viii.) p. 206. J Ann. Chem. u. Pharm. cciii. 83. 132 PROCEEDINGS OF THE AMERICAN ACADEMY Crystalline form of Dichloracrylic Acid. * Orthodiagonal b = 1 Vertical Axis c = 0.3637 Angle of Axes XZ = 87° 32' Angles between Normals. Observed. Ill and III = 37° 54') Til “ 001 = 25° 8' l Til “ TOO = 71° 49 ' ) 001 “ TOO = 87° 32' TOO “ 110 = 49° 56' T10 “ 110 = 80° 6' Calculated. Fundamental angles. 87° 33' 49° 51' 80° 18' Although Wallach’s dichloracrylic acid also crystallizes in the mono- clinic system, the forms are totally unlike, and the difference between the two acids is therefore established. OF ARTS AND SCIENCES. 133 III. On the Relation between Dibromacrylic Acid and Tribrompropionic Acids. By Henry B. Hill and Clement W. Andrews. Nearly two years ago Michael and Norton * * * § published a description of the tribrompropionic acid melting at 92° which was first mentioned by Linnemann and Penl,t and which they obtained by the addition of bromine to the so-called ft monobromacrylic acid of Tollens and Wagner. In this paper they remarked that potassic hydrate attacked the acid readily in alcoholic solution, but they attempted no isolation of the dibromacrylic acid which was thus formed. They soon after- wards offered to relinquish the farther study of this, acid, in case we felt interested to undertake its preparation and comparison with the dibromacrylic acid which one of us had already described. This kind offer was accepted, and we began the investigation at once. Although we had no difficulty in the isolation of a dibromacrylic acid which closely resembled that which had been made from mucobromic acid, still it was impossible to establish the identity of the two until the latter had been somewhat more carefully studied. In the mean time Mauthner and Suida, $ in an article upon substituted acrylic and pro- pionic acids, described again the preparation of the tribrompropionic acid melting at 92°, without having seen more than a brief notice of Michael and Norton’s work. In this article they further showed that it might be converted by the action of potassic hydrate into a dibrom- acrylic acid, which, as they asserted, was identical with that which O. R. Jackson and one of us had obtained from mucobromic acid.§ The only facts which they brought forward in support of this assertion were, the melting-point, 85°, the ready formation of malonic acid by the action of baric hydrate, and the anhydrous form of the lead salt. Since neither the melting-point nor the action of baric hydrate will discriminate between the two isomeric forms of dibromacrylic acid already known, and moreover since the lead salt of one of these two acids has never been described and of the other is not anhydrous, as one of us has recently shown, it is evident that these facts were wholly insufficient to characterize the acid in question. Our investigation of * Amer. Chem. Journ. ii. 18. t Berichte der deutsch. chem. Gesellseh., viii. 1098. f Sitzungsberichte der kk. Akademie, Wien, lxxxiii. 273. § These Proceedings, Vol. XVI. (n. s. viii.) p. 192. 134 PROCEEDINGS OF THE AMERICAN'* ACADEMY the same acid has shown us that their assertion, though unsupported by evidence, was accidentally correct. Dibromacrylic Acid , C 3 H 2 Br 2 0 2 . In the preparation of the tri- brompropionic acid necessary for this research we followed quite closely the method of Michael and Norton, although we did not con- sider it necessary to purify the dibrompropyl alcohol by distillation under diminished pressure before oxidation. For the conversion of the tribrompropionic acid into the corresponding dibromacrylic acid we have found it most advantageous to dissolve it in the calculated amount of a titrated solution of baric hydrate, and to allow the reaction to proceed at ordinary temperatures. After standing for several days the neutral or at most feebly alkaline solution was evap- orated, and the acid extracted from the recrystallized barium salt thus obtained. Since this acid was found by preliminary trial to give a sparingly soluble acid potassium salt which crystallized in long silky needles, for further purification it was converted into this salt. After several recrystallizations from hot water, the acid was set free by the addition of hydrochloric acid and extracted with ether. The acid thus obtained crystallized in small oblique prisms readily soluble in alcohol, ether, and chloroform, more sparingly in benzol or carbonic disulphide. Under water the crystals melted at about 20° to a colorless oil which dissolved readily on heating. The acid dried over sulphuric acid melted at 85-86°, and gave on analysis percentages corresponding to the formula C 3 H 2 Br 2 0 2 . I. 0.7497 grm. substance gave on combustion 0.4341 grm. C0 2 and 0.0673 grm. H 2 0 . II. 0.2863 grm. substance gave 0.4691 grm. AgBr. III. 0.2093 grm. substance gave 0.3432 grm. AgBr. Calculated for C 3 EL>Br 2 02 . Found. I. II. III. c 15.65 15.79 H 0.87 1.00 Br 69.56 69.72 69.84 The solubility of the acid we determined by neutralizing with baric carbonate an aqueous solution prepared according to the method of V. Meyer and determining by precipitation the barium dissolved. I. 12.4640 grms. of a solution saturated at 18° gave 0.3124 grm. BaS0 4 . II. 12.2745 grms. of a solution saturated at 18° gave 0.3091 grm. BaS0 4 . OF ARTS AND SCIENCES. 135 According to these determinations the aqueous solution saturated at 18° contained the following percentages: — I. II. 4.95 4.97 Baric Dibromacrylate , Ba(C 3 HBr 2 0 2 ) 2 . H 2 0. The barium salt made by neutralizing a solution of the acid with baric carbonate crystallized in rhombic plates more or less irregular in form, which when dried by exposure to the air contained one molecule of water. I. 1.3641 grm. of salt dried over H 2 S0 4 gave by precipitation 0.5195 grm. BaS0 4 . II. 3.1482 grms. of the air-dried salt lost at 110° 0.0933 grm. H 2 0; 0.8202 grm. of the same air-dried salt gave by precipitation 0.3147 grm. BaS0 4 . III. 1.7219 grm. of salt dried over H 2 S0 4 lost at 120° 0.0520 grm. H 2 0 ; 0.6769 of the same salt gave by precipitation 0.2555 grm. BaSO. . Calculated for Ba( C 3 HBr 2 0 2 ) 2 . II 2 0 . I. Ba 22.35 22.40 H 2 0 2.94 Found. II. 22.56 2.96 III. 22.19 3.02 For its further identification we determined its solubility in water at 18°. 9.2373 grm. of a solution saturated at 18° gave by precipitation 0.2131 grm. BaS0 4 . From this determination it follows that the aqueous solution satu- rated at 18° contained 5.89% of the anhydrous salt. Calcic Dibromacrylate, Ca(C 3 HBr 2 0 2 ) 2 . 3H 2 0. The calcium salt crystallized in clustered needles which contained three molecules of water of crystallization when dried by exposure to the air. I. 0.5761 grm. of the air-dried salt lost at 80° 0.0535 grm. II 2 0. II. 0.5101 grm. of the air-dried salt lost at 80° 0.0491 grm. H 2 0. Calculated for Ca(C 3 HBr 2 0 2 ) 2 . 3H 2 0. Found. 9.78 H 2 0 I. 9.29 II. 9.63 0.4608 grm. of the salt dried at 80° gave on ignition with H 2 S0 4 0.1262 grm. CaS0 4 . 136 PROCEEDINGS OF THE AMERICAN ACADEMY Calculated for Ca(C 3 HBr 2 0 2 ) 2 . Found. Ca 8.03 8.06 Potassic Dibromacrylate , KC 3 HBr 2 0 2 . The potassium salt was made from the acid by neutralization with potassic carbonate. It crystallized in leafy plates which were anhydrous. 0.6842 grm. of the air-dried salt gave on evaporation with H 2 S0 4 and ignition 0.2229 grm. K 2 S0 4 . Calculated for KC 3 HBr 2 0 2 Found. K 14.58 14.62. A comparison of these results with those which one of us has pre- sented in the preceding paper will be facilitated by the following table, which gives the mean of each series of results : — Dibromacrylic Acid from Mucobromic Tribrompropionic Melting-point Aqueous solution 18°, % acid Barium salt, % water Barium salt solubility 18° Calcium salt, % water 85.5-86° 4.92 2.97 5.91 9.56 85-86° 4.96 2.99 5.89 9.46 Since the identity of the dibromacrylic acid formed by the sub- traction of hydrobromic acid from the tribrompropionic acid melting at 92° with that derived from mucobromic acid was thus established with precision, it seemed to us of interest to study a little more closely the tribrompropionic acid which this same dibromacrylic acid forms by the addition of hydrobromic acid. Mr. C F. Mabery * had with one of us already proved that such an addition product could be formed, but it had been prepared solely from the impure acid melting at 83- 84° and very little studied. We therefore at first undertook its prep- aration in larger quantity from pure acid melting at 85-86°. Tribrompropionic Acid , C 3 H 3 Br 3 0 2 . When dibromacrylic acid made by the action of baric hydrate upon mucobromic acid is heated with three or four times its weight of hydrobromic acid saturated at 0° for eight or ten hours, at 100° the needle-like prisms disappear and are replaced by rectangular plates of the new tribrompropionic acid. With the pure acid no carbonization such as had been noticed in work- ing with the impure acid was observed even at 120°, and we therefore * These Proceedings, Vol. XVI. (n. s. vru.) p. 197. OF ARTS AND SCIENCES. 137 usually allowed the addition to proceed at this higher temperature, since the reaction was then completed in a shorter time. The tubes opened without marked pressure, and the crystalline product separated from the acid mother-liquors by filtration upon a perforated platinum cone was dried upon porous tiles. When treated in this way the dibromacrylic acid gave about its own weight of crude tribrompro- pionic acid. The acid can readily be purified by recrystallizing it successively from ligroin and carbonic disulphide. The use of car- bonic disulphide causes considerable loss, but with ligroin alone we failed to obtain as high a melting-point. After several recrystalliza- tions the acid showed a constant melting-point, and gave on analysis the required percentages. I. 1.0329 grm. substance dried over H 2 S0 4 gave on combustion 0.4446 grm. C0 2 and 0.0994 grm. H 2 0. II. 0.2184 grm. substance gave 0.3963 grm. AgBr. III. 0.1938 grm. substance gave 0.3525 grm. AgBr. c Calculated for C 3 H 3 Br 3 0 2 . 11.57 I. 11.74 Found. II. H 0.96 1.07 Br 77.17 77.29 This tribrompropionic acid is very soluble in alcohol or ether, some- what less soluble in chloroform, carbonic disulphide, benzol or ligroin. It dissolves freely in hot water, but is quite rapidly decomposed on boiling with the formation of hydrobromic acid. From the hot aque- ous solution the acid crystallizes on cooling in pearly scales. Repeat- edly recrystallized from carbonic disulphide, the acid melts at 118°. Argentic Tribrompropioncite, AgC 3 H 2 Br 3 0 2 . Argentic nitrate added to a cold aqueous solution of the acid precipitates the silver salt in small clustered rhombic plates. On warming it with water argentic bromide is rapidly formed, but it may be dried over sulphuric acid without essential decomposition. 0.2901 grm. of the salt dried over H 2 S0 4 gave by precipitation with HBr 0.1316 grm. AgBr. Calculated for AgC 3 H 2 Br 3 0 2 . Found. Ag 25.83 26.05 The barium and calcium salts were readily soluble in water, and their solutions could not be warmed without the instantaneous forma- 138 PROCEEDINGS OF THE AMERICAN ACADEMY tion of bromide. Even on evaporating their solutions at ordinary temperatures over sulphuric acid in vacuo the barium salt was almost wholly decomposed ; the calcium salt was apparently somewhat more stable, for it was thus obtained in dendritic needles, although the mother- liquor contained calcic bromide. Since the air-dried salt lost nothing over sulphuric acid and was decomposed by heat, the water of crys- tallization could not be directly determined. It gave, however, a per- centage of calcium agreeing closely with that required by two molecules of water. I. 1.1087 grm. of the air-dried salt gave on ignition with H 2 S0 4 0.2179 grm. CaS0 4 . II. 0.7279 grm. of the air-dried salt gave on ignition with H 2 S0 4 0.1425 grm. CaS0 4 . Calculated for Ca(C 3 H 2 Br 3 0 2 ).2. 2H 2 0. Ca 5.75 We were unable to prepare other salts. Dibromacrylic Acid. The ready decomposition of the tribrompro- pionic acid made it seem desirable to isolate and identify the dibrom- acrylic acid which was thus formed. For this purpose we dissolved pure tribrompropionic acid, melting at 118°, in water and added from a burette a titrated solution of baric hydrate. So rapid was the action that an alkaline reaction could not be maintained until nearly one molecule of baric l^drate had been added for each molecule of the acid. When the calculated amount of baric hydrate had been added the solution was allowed to stand for half an hour, and then but a trace of baric carbonate could be precipitated with carbonic dioxide. Ether extracted from the acidified solution a crystalline acid melting at 85-86°, which gave on analysis the percentage of bromine required by the formula C 3 II 2 Br 2 0 2 . 0.1979 grm. substance gave 0.3240 grm. AgBr. Calculated for C 3 H 2 Br 2 0 2 . Found. Br. 69.57 ~ ** 69.66. The solubility of the acid in cold water was determined by the method of V. Meyer. I. 10.7793 grms. of a solution saturated at 18° gave on neutralization with baric carbonate and precipitation 0.2908 grm. BaS0 4 . II. 7.7354 grms. of a solution saturated at 18° gave 0.1940 grm. BaS0 4 . Found. I. II. 5.78 5.76 OF ARTS AND SCIENCES. 139 > According to these determinations the aqueous solution of the acid saturated at 18° contained the percentages : — I. II. 5.32 4.95 Baric Dibromacrylate, Ba(C 3 HBr 2 0 2 ) 2 . H 2 0. By neutralizing the acid with baric carbonate, or more conveniently by the direct evapo- ration of the solution obtained by the action of baric hydrate upon tribrompropionic acid, we obtained the barium salt in leafy rhombic plates which, when dried over sulphuric acid, contained one molecule of water. I. 1.1489 grm. of the salt dried over II 2 S0 4 lost at 100-105° 0.0335 grm. H 2 0. II. 1.5158 grm. of the salt dried over H 2 S0 4 lost at 105-110° 0.0457 grm. !I 2 0, and gave on precipitation 0.5790 grm. BaS0 4 . Calculated for Ba(C 3 IIBr 2 0 2 ) 2 . H 2 0 . Found. I. II. Ba 22.35 22.46 H 2 0 2.94 2.92 3.02 Calcic Dibromacrylate, Ca(C 3 HBr 2 0 2 ) 2 . 3 H 2 0. The calcium salt crystallized in radiating needles which contained when air-dried three molecules of water. 0.9444 grm. of the air-dried salt lost at 100-105° 0.0906 grm. H 2 0, and gave on ignition with H 2 S0 4 0.2332 grm. CaS0 4 . Calculated for Ca(C 3 HBr 2 0 2 ) 2 - 3 H 2 0 . Found. Ca 7.25 ~ 7.26 H 2 0 9.78 9.59 These results are sufficient to prove that this dibromacrylic acid is identical with the one already studied, as a comparison of the mean results given in the following table will show. Dibromacrylic Acid from Mucobromic Acid. Tribrompropionic. Melting-point 85.5-86° 4.92 2.97 9.56 85-86° 5.12 2.97 9.59 Aqueous solution saturated at 18°, acid Barium salt, % water Calcium salt, % water 140 PROCEEDINGS OF THE AMERICAN ACADEMY IV. On Certain Tetrasubstituted Propionic Acids. By Henry B. Hill and Charles E. Mabery. In a previous communication * one of us has already mentioned the fact that the dibromacrylic from mucobromic acid, unlike the dichlor- acrylic acid of Wallach, takes up a molecule of bromine with readiness and forms a tetrabrompropionic acid. We were led to undertake a more complete study of the tetrasubstituted propionic acids, especially with the hope that the study of those containing two different halo- gens might throw some light upon the position of the halogen atoms in the disubstituted acrylic acids formed from mucobromic and muco- chloric acids. After our investigations were concluded Mauthner and Suida t published in the Proceedings of the Vienna Academy a paper entitled “ Ueber gebromte Propionsauren und Acrylsauren,” in which they anticipate us in the publication of a portion of our work. Since they were perfectly well aware that, in studying derivatives of a sub- stituted acrylic acid which they asserted, although without adequate proof, was identical with that obtained from mucobromic acid, they were trespassing upon ground which one of us had already fully re- served, we think it advisable, although it involves a certain amount of repetition, to give our results in full, more especially since we are able to correct their work in several important particulars. Tetrabrompropionic Acid . Tetrabrompropionic acid can readily be made by the addition of bromine to dibromacrylic acid at ordinary temperatures. $ We have prepared it by adding to a solution in chloroform the calculated amount of bromine. On standing, the addition product gradually separates, often in large, well -formed prisms. The amount of the product thus obtained was about 90% of the theoretical yield. After recrystalliza- tion from chloroform the substance was dried over sulphuric acid. * These Proceedings, Vol. XVI. (n. s. viii.) p. 197. t Sitzungsberichte der kk. Akademie, Wien, lxxxiii. 273. X Mauthner and Suida assert that the dibromacrylic acid will take up no bromine in the cold. They prepared tetrabrompropionic acid by heating to 100° with undiluted bromine. That the bromine is very readily taken up one of us first mentioned several years ago (Berichte der deutsch. chem. Gesellsch., 1. xii. 657). OF ARTS AND SCIENCES. 141 I. 0.5480 grra. substance gave 0.1837 grm. C0 2 and 0.0286 grm. H 2 0. II. 0.1775 grm. substance gave 0.3432 grm. AgBr. Calculated for C 3 H 2 Br 4 0 2 . Found. I. II. ' c 9.23 9.14 4 H 0.51 0.58 Br 82.04 82.28 Tetrabrompropionic acid crystallizes in prisms of the triclinic sys- tem which melt at 125-126°. It is very soluble in alcohol or ether, readily soluble in hot chloroform, carbonic disulphide or benzol, and separates in crystals as these solutions cool. In ligroin it is sparingly soluble. Under water it melts at a very low temperature to a color- less oil which dissolves freely on heating. For a crystallographic study of the substance we are indebted to Dr. W. H. Melville. Crystalline Form of Tetrabrompropionic Acid. Triclinic System. Forms, $100} $010} {001} {011} $1T0}. Elements : Macrodiagonal a = 1.507 Brachydiagonal b = 1 Vertical Axis c = 0.934 Angles of Axes XY == 94° 59' XZ = 104° 28' YZ = 74°20 / 142 PROCEEDINGS OF THE AMERICAN ACADEMY Angles between Normals. Observed. 100 and 010 = 88° 48' 010 “ Oil = 56° 3' 011 “ 001 = 48° 51^ ► 100 “ Oil = 77° 21J r 100 “ 1T0 = 57° 25 ' 100 “ 001 = 77° 1' Calculated. Fundamental angles. 76° 21' Argentic Tetrabrompropionate , AgC 3 HBr 4 0 2 . Argentic nitrate added to a solution of the acid in dilute alcohol precipitates the silver salt in clustered needles, which may further be increased in quantity by the cautious addition of ammonic hydrate. The salt is extremely unstable, forms argentic bromide on warming, and blackens rapidly in diffused light. 1.2182 grm. of the salt dried over H 2 S0 4 gave 0.4744 grm. AgBr. Calculated for AgC 3 HBr 4 0 2 . Found. Ag 21.78 22.38 Baric Tetrabrompropionate , Ba(C 3 HBr 4 0 2 ) 2 . 2H a O. An aqueous solution of the acid dissolved baric carbonate readily in the cold, and if the solution was not warmed there was no noticeable decomposition. On spontaneous evaporation at ordinary temperatures the barium salt was left in clusters of flattened needles. When dried by exposure to the air they contained two molecules of water which they lost over sulphuric acid. I. 0.7239 grm. of the air-dried salt lost over H 2 S0 4 0.0272 grm. H 2 0. II. 0.7087 grm. of the air-dried salt lost over H 2 S0 4 0.0259 grm. H 2 0. Calculated for Ba( C 3 HBr 4 0 2 ) 2 . 2H 2 0. Found. I. II. H 2 0 3.79 3.76 3.66 0.6756 grm. of the salt dried over H 2 S0 4 gave on ignition with H 2 S0 4 0.1742 grm. BaS0 4 . Calculated for Ba(C 3 HBr 4 0 2 ) 2 . 14.97 Ba Found. 15.16 OF ARTS AND SCIENCES. 143 Calcic Tetrabrompropionate , Ca(C 3 HBr 4 0 2 ) 2 . The calcium salt, made by neutralizing an aqueous solution of the acid with calcic car- bonate and allowing the solution to evaporate spontaneously, crys- tallized in needles which proved to be anhydrous. The salt freed from moisture by pressure did not materially lose in weight when exposed to the air, and when air-dried lost nothing over sulphuric acid. I. 0.5065 grm. of the salt dried over H 2 S0 4 gave on ignition with H 2 S0 4 0.0888 grm. CaS0 4 . II. 1.0886 grm. of the salt dried over H 2 S0 4 gave 0.1850 grm. CaS0 4 . Calculated for Ca(C 3 H 2 Br 4 0 2 ) 2 . Ca 4.89 Found. I. II. 5.16 5.00 When a solution of baric tetrabrompropionate was heated baric bromide was formed, carbonate dioxide escaped, and at the same time the liquid became turbid through the separation of a colorless oil.* On distilling the liquid the oil passed readily over with steam, and since from the method of its formation there could be little doubt that it was tribromethylen, for identification it was immediately converted into its dibromide by the addition of bromine. The crystalline addi- tion product which was thus obtained when purified by recrystallization melted at 53° and gave on analysis a percentage of bromine which showed it to be pentabromethan. 0.1696 grm. substance gave 0.3766 grm. AgBr. Calculated for C 2 HBr 5 . Found. 94.12 94.48 This decomposition may therefore be expressed by the equation : Ba(C 3 HBr 4 0 2 ) 2 = BaBr 2 + 2 C0 2 + 2 C 2 HBr 3 . By the action of an alcoholic solution of potassic hydrate upon tetra- brompropionic acid a molecule of hydrobromic acid is removed, and there results the tribromacrylic acid melting at 118° which we have * Mauthner and Suida assert that this decomposition also takes place on long standing at ordinary temperatures. They were therefore able to isolate no salts. We have never observed any decomposition in the cold. Loc. cit. p. 284. 144 PROCEEDINGS OF THE AMERICAN ACADEMY already described.* * * § Dr. W. H. Melville kindly made a crystallo- graphic study of the crystals which we obtained, and established their identity with those he had previously examined, which were prepared by the addition of bromine to brompropiolic acid. A description of the crystalline form of tribromacrylic acid Dr. Melville presents in a separate communication. a Dichlorbrompropionic Acid. In studying the dichloracrylic acid made from mucochloric acid W. Z. Bennett and one of us found that even at 100° it would not take up bromine when dissolved in chloroform.f Subsequently it appeared from the experiments of C. W. Andrews that the substituted propionic acid could readily be made by the action of undiluted bro- mine, although circumstances at the time prevented a detailed study on his part. As a precise characterization of this addition product seemed of interest we undertook its preparation and investigation. Pure dichloracrylic acid melting at 85-86° was heated with a mol- ecule of bromine for several hours at 100°. The almost colorless product of the reaction was pressed thoroughly with paper and purified by crystallization, at first from carbonic disulphide, and finally from chloroform. When dried over sulphuric acid this substance gave on analysis percentages agreeing closely with those required by the for- mula CH 2 Cl 2 Br 2 0 2 . In the indirect determination of the halo- gens we used the extremely convenient and accurate method recently described by Mr. L. P. Kinnicutt.l I.§ 0.8124 grm. substance gave 0.3550 grm. C0 2 and 0.0665 grm. h 2 o. II. 0.1715 grm. substance gave 0.3775 grm. AgCl -f- AgBr. III. 0.4790 grm. substance gave 1.0559 grm. AgCl -j- AgBr. From this by reduction was obtained 0.6887 grm. Ag. * These Proceedings, Yol. XVI. (n. s. viii.) p. 216. Mauthner and Suida assign to the barium and calcium salts of this acid, prepared by them from tetra- brompropionic acid, formulae differing greatly from those which we formerly established by our analyses. Since their determinations were made with small quantities of material, we have not thought it necessary to make further analyses in support of our formulae. t These Proceedings, Vol. XVI. (n. s. viii.) p. 211. J These Proceedings, Vol. XVII. (n. s. xi.) p. 91. § These analyses were made by Mr. Andrews. OP ARTS AND SCIENCES. 145 Calculated for C 3 H 2 Cl 2 Br 2 0 2 . Found. I. II. c 11.96 11.91 H 0.67 0.90 Cl 23.59 Br 53.15 76 - 74 76.93 This a dichlordibrompropionic acid crystallizes in well-formed tri- clinic prisms, which melt at 94-95°. It is readily soluble in water, alcohol, or ether ; in carbonic disulphide, chloroform, or benzol it dis- solves less freely. From a solution in carbonic disulphide it could be obtained by slow evaporation in well-formed crystals, whose study was kindly undertaken by Dr. W. H. Melville. Crystalline Form of a Dichlordibrompropionic Acid. Triclinic System. Forms, {100} {010} {001} {101} {011} {10T} {0TT} ; {110} and ^110} often present. * Elements : Macrodiagonal a = 1.023 Brachydiagonal b = 1 Vertical axis c = 1.052 Angles of axes XY = 91° XZ = 76° 31J' YZ = 108° 52' * Through an error the ratios of the fundamental parameters were originally given in the Berichte der deutsch. chem. Gesellsch. xiv. 1680. a : b : c = 1.084 : 1 : 1.062. VOL. XVII. (N. 8. IX.) 10 146 PROCEEDINGS OF THE AMERICAN ACADEMY Angles between Normals. Observed. Calculated. 100 and 010 = 93° 37' 100 “ 101 = 52° 58' 010 “ Oil = 34° 57' 010 “ 101 = 77° 19' 101 “ Oil = 58° 46' 100 “ 10T= 38° 32' 011 “ 001 = 35° 21' 001 “ Oil = 55° 45' 100 “ Oil = 101° 3' 100 “ Oil = 98° 49' 101 “ 101 = 87° 34' 101 “ Oil = 50° 32' y Fundamental angles. 38° 41' 35° 52' 55° 30' 100° 39' 98° 38' 88 ° 20 ' 50° 30' Argentic a Dichlordibrompropionate , AgC 3 HCl 2 Br 2 0 2 . The silver salt falls, on the addition of argentic nitrate to an aqueous solution of the acid, in flattened jagged needles which are readily decomposed by heat. They could, however, be dried over sulphuric acid without any essential decomposition, as is shown by the following analysis : — 0.4885 grm. of substance gave by precipitation with HBr 0.2231 grm. AgBr. Calculated for AgC 3 HCl 2 Br 2 02 . 26.46 Found. 26.23 Ag Baric a Dichlordibrompropionate , Ba(C 3 HCl 2 Br 2 0 2 ) 2 . The barium salt we made by neutralizing a cold aqueous solution of the acid with baric carbonate. On evaporation at ordinary temperatures it crys- tallized in long branching needles, which when dried by exposure to the air did not lose materially in weight over sulphuric acid, and con- tained a percentage of barium corresponding to the anhydrous salt. I. 0.5069 grm. of the air-dried salt gave 0.1606 grm. BaS0 4 . II. 0.5239 grm. of the air-dried salt gave 0.1676 grm. BaS0 4 . Calculated for Ba(C 3 HCl2Br 2 02) 2 . Found. I. II. 18.58 18.81 Ba 18.59 OF ARTS AND SCIENCES. 147 The barium salt is decomposed by warming, its solution giving products similar to those obtained in the same way from tetrabrom- propionic acid. This decomposition, however, we have not as yet studied further. Dichlordibrompropionic Acid, Although dibromacrylic acid assumes a molecule of bromine so readily, we found at first great difficulty in preparing the corresponding addition product with chlorine. If chlorine gas is passed into melted dibromacrylic acid it is gradually taken up and the melting-point of the acid is slowly changed. After long-continued action a considerable quantity of the dichlordibrompropionic acid is formed, but so contam- inated with oily by-products that its purification is a matter of some difficulty. This addition of chlorine is, however, rapidly and neatly accomplished if the reaction is allowed to proceed in direct sunlight at 100°. When at this temperature the melted acid becomes nearly solid with separating crystals of the addition product, the chlorination is interrupted. The product which we obtained in this way from pure dibromacrylic acid melting at 85-86° was purified by crystalliza- tion first from carbonic disulphide and finally from chloroform. On analysis it gave the following results : — I. 0.5183 grm. substance gave 0.2335 grm. C0 2 and 0.0431 grm. H 2 0. II. 0.1547 grm. substance gave 0.3400 grm. AgCl -f- AgBr. From this by reduction was obtained 0.2214 grm. A g. Calculated for (C 3 H 2 Cl 2 Br 2 0 2 ). Found. I. II. c 11.96 12.13 H 0.66 0.92 Cl 23.59 23.37 Br 53.15 53.21 This ft dichlordibrompropionic acid crystallizes in oblique prisms which melt at 118-120°, and in its behavior with solvents closely resembles the a acid. In water, alcohol, or ether it dissolves very easily, but with somewhat more difficulty in carbonic disulphide, chlo- roform, or benzol. The solution in carbonic disulphide gave by slow evaporation well-developed crystals whose elements .Dr. W. H. Mel- ville kindly determined. 148 PROCEEDINGS OF THE AMERICAN ACADEMY Crystalline Form of p Dichlordibrompropionic Acid. Monoclinic System . Forms, {100} {110} Jill} {102} Elements : Clinodiagonal a = 2.393 Orthodiagonal b = 1 Vertical axis c = 1.731 Angle of axes XZ = 46° 9 Angles between Normals. Observed. Calculated. 110 and 110 = 60° IF \ 111 u 110 = 42° 24J' [ ■ Fundamental Angles. 111 u 110 = 40° 33 J ' ) TOO a T10 = 59° 53' 59° 54J' 111 a 111 ~ 120° V 119° 56' 100 u 111 = 00 00 o Or 88° 47' 102 u T10 = 102° 21' 102° 23 ; 102 u 110 = 78° 5' 77° 37' 102 a 111 = 63° 44' 63° 42J' The difference between the a and ft acids was further confirmed by a study of the silver and barium salts. Argentic ft Dichlordibrompropionate , AgC 3 HCl 2 Br 2 0 2 . The silver salt is precipitated in the form of short, thick, pointed prisms when argentic nitrate is added to an aqueous solution of the acid. It is OF ARTS AND SCIENCES. 149 readily decomposed by heat, but may be dried for analysis over sul- phuric acid. 0.4950 grm. of the salt gave on precipitation with HC1 0.1731 grm. AgCl. Calculated for AgC 3 HCl 2 Br 2 0 2 . Found. Ag 26.46 26.31 Baric ft Dichlordibrompropionate , Ba(C 3 HCl 2 Br 2 0 2 ) 2 . 2H 2 0. The barium salt which we made by neutralizing an aqueous solution of the acid with baric hydrate crystallized on spontaneous evaporation of its solution in long radiating needles which were very soluble in cold water. When dried by exposure to the air the salt proved to contain two molecules of water which it lost over sulphuric acid. I. 1.6201 grm. of the air-dried salt lost over H 2 S0 4 0.0705 grm. H 2 0. II. 1.5443 grm. of the air-dried salt lost over H 2 S0 4 0.0731 grm. H 2 0. Calculated for Ba(C 3 HCl 2 Br 2 0 2 ) 2 . 2 H 2 0. Found.^ H 2 0 4.66 4.35 4.74 0.8236 grm. of the salt dried over H 2 S0 4 gave 0.2619 grm. BaS0 4 . Calculated for Ba(C 3 HCl 2 Br 2 0 2 ) 2 . Found. Ba 18.59 18.69 These results prove that the a and § dichlordibrompropionic acids described are essentially different. Dichlordibrompropionic Acid. a P System of Crystallization Melting-point Barium salt Triclinic. 94-95° Ba(C 3 HBr 2 Cl 2 0 2 ) Monoclinic. 118-120° Ba(C 3 HBr 2 Cl 2 0 2 ) 2 . 2 H 2 0. The barium salt was readily decomposed by warming its aqueous solution. Baric chloride and carbonic dioxide were formed together with a colorless oil which undoubtedly was a dibromchlorethylen. With bromine this oil gave a solid addition product, which, however, we have not as yet prepared in quantity sufficient for complete purifi- cation and identification. 150 PROCEEDINGS OF THE AMERICAN ACADEMY V. On the Constitution of the Substituted Acrylic and Propionic Acids. By Henky B. Hill. Within a few years the number of substituted acrylic and pro- pionic acids known has been largely increased, and yet the constitution of but few of these can be said to be satisfactorily established. In a previous communication I was led to adopt provisionally for muco- bromic acid the formula, — CBr 2 — CHO i — C — COOH which explained its connection with maleic acid, in whose molecule the researches of Fittig had shown the provable existence of a dyad carbon atom. The structure of the related dibromacrylic acid was then naturally expressed by the formula, — CBr 2 H I C = i COOH against which at the time nothing more convincing than a belief in its improbability could be urged. The relations which Andrews and I have shown to exist between this same acid and two different tribrom- propionic acids prove, however, that this formula is incorrect. An acid with this structure could be formed from but a single tribrompropionic acid, and must of necessity give this same tribrompropionic acid by the addition of hydrobromic acid. The same objection also applies with equal force to the other two conceivable formulas for dibromacrylic acid which contain dyad carbon, — = CH = CBr i CBr 2 and CHBr COOH COOH OF ARTS AND SCIENCES. 151 and these must consequently be rejected. There remain therefore for the acid in question but two possible formulae, — CBr 2 CHBr II II CH and CBr l i COOH COOH The formation of two isomeric dichlordibrompropionic acids by the addition of chlorine to the dibromacrylic acid and of bromine to the analogous dichloracrylic acid, as Mabery and I have shown, would seem again to be decisive in favor of the first of these formulae, since its adoption would give, — CBr 2 Cl CCl 2 Br CHC1 and CHBr COOH COOH as the structure of the two isomeric dichlordibrompropionic acids, while the second formula would give in either case the same com- pound, — CHBrCl I CBrCl i COOH Although it was by no means impossible that a molecular rearrange- ment had taken place in one of these two reactions, still it seemed improbable, since the reactions were apparently neat, and in the treat- ment with chlorine, where such a change would be more naturally expected, no bromine could be detected in the escaping chlorine. On the other hand the adoption of the formulae, — CBr 2 CC1 2 II II CIi •and CH COOH COOH for the derivatives of mucobromic and mucochloric acids presented difficulties quite as serious. In the first place the dichloracrylic acid 152 PROCEEDINGS OF THE AMERICAN ACADEMY of Wallach had been proved beyond all doubt to be different from that which Bennett and I had described by a comparison of the crys- talline form of the two acids, and it would follow then that Wallach’s acid could not have the formula he assigned to it. The formation of an acid with different structure from chloralid could, however, be explained upon the assumption that the chlorpropiolic acid was the first product of the reduction, and that this passed, on the one hand by the addition of hydrogen, into the 0 chloracrylic acid, and on the other hand formed dichloracrylic acid by addition in the subsequent treatment with strong hydrochloric acid which Wallach prescribes.* Although this hypothesis was far from satisfactory, it seemed to me hardly more improbable than that a similar molecular rearrangement had taken place in the reaction which had come under my own obser- vation. Still another difficulty was to be found in the formation of the dibromacrylic acid in question from the tribrompropionic acid melting at 92°, which, if the ordinarily accepted formula for the latter, — CH 2 Br CBr 2 i COOH were correct, would prove the incorrectness of the formula assumed. I was at first unable to attach any great importance to this argument, inasmuch as the constitution of the monobromacrylic acids was ex- tremely uncertain. For although the discovery by Wallach of the ft monobromacrylic acid melting at 115° had rendered the constitution of the a and monobromacrylic acids of Tollens extremely doubtful, it by no means proved their identity. Erlenmeyer has, however, recently shownf that the a and ft acids of Tollens, as well as their potassium salts, crystallize in identical forms, and the structure of the tribrompropionic acid melting at 92° is there- fore put upon quite another footing. Since the same (a) monobrom- acrylic acids can be made from a dibrompropionic and also from its isomer, the a ft dibrompropionic, it follows that this acid must have the structure, — * Ann. Chem. u. Pharm., xcxiii. 7. t Berichte der deutsch. chem. Gesellsch., xiv. 1867. OF ARTS AND SCIENCES. 153 CH ii CBr i COOH and the tribrompropionic acid made from it by the addition of bromine must of course have the corresponding form, — CH 2 Br CBr 2 i COOH Since the formulae with dyad carbon are in this case excluded, there remains for the dibromacrylic acid in question onlv the structure, — CHBr II CBr l COOH Although this conclusion is directly at variance with the results which Mabery and I obtained in the study of the dichlordibrompro- pionic acids, it must be confessed, I think, that it is probably correct. Still, since its adoption presents undoubted difficulties, I shall endeavor to bring more direct experimental evidence as to its correctness. The dibromacrylic acid of Fittig and Petri, which, as Mabery and I have shown, can be made from brompropiolic acid, would naturally have the form, — CBr 2 M CH i COOH and the acids made in the same way containing two halogens the corresponding, — CBrI CBr Cl II II CH CH COOH COOH 154 PROCEEDINGS OF THE AMERICAN ACADEMY The tribrompropionic acid melting at 118° would be written, — CHBr 2 I CHBr COOH and the tetrabrompropionic acid would have the form, — CHBr 2 CBr 2 i COOH VI. Crystalline Form of Tribromacrylic Acid. By W. H. Melville. Monoclinic System . Forms, {010} {110} {011} {101} fTOl} {021} ; the last three forms being rarely observed. Elements : Clinodiagonal a = 0.502 Orthodiagonal b = 1. Vertical axis c = 0.559 Angle of axes XZ = 64° 29 J 7 \ OF ARTS AND SCIENCES. 155 Angles between Normals, Observed. Calculated. 010 and 110 = 65° 38/) 010 “ Oil = 63° 14' > Fundamental angles, 011 “ 110 = 57° 35') T10 “ TIO = 48° 42' 48° 44' 010 “ 021 = 44° 41' 44° 45' 021 “ Oil = 18° 33' 18° 29' 011 “ 011 =53° 33' 53° 32' Oil “ T01 = 42° 25' 42° 23 Since* Dr. F. Becke’s results of the determination of the same crystals do not appear to be reconcilable with the above, a comparison is important. The forms r (110) l (1T0) when referred to the crys- tals which I have examined, correspond to {021}, and o (111), u (III) would, if present, occur as the prism {120}. Among the many sets of crystals submitted to me I have never observed the prism {120}. The following table shows a few of the angles both actual and hypothetical, assuming that the crystals are, monoclinic in form, when compared with Becke’s corresponding measurements. Monoclinic. (010) A (021) = 44° 41' (021) A (021) = 90° 38' (021) A (010) = 44° 41' (010) A (120) = 47° 45 1' (120) A (120) == 84° 28 1' (IE0)a(0T0) = 47° 45 1' (010) A (021) == 44° 41' (010) A (120) = 47° 45|' Becke. 44° 42' = hr 91° 11' = rl 44° 13' = bl 47° 19' = bu 84° 55' = ou 48° 10' = bo 44° 27 J' = mean of br and bl 47° 441' „ „ bu „ bo The following measurements, which were obtained from a single crystal, appear to establish conclusively the system of crystallization as monoclinic. * Sitzungsberichte der kaiser. Akad. der Wissensch., Band lxxxiii. 286-287, Wien, 1881. 156 PROCEEDINGS OF THE AMERICAN ACADEMY Zone [010, 110] Zone [010, Oil] (110) A (010) = 65° 33' (OIO)A(HO) = 65° 44' (110) A (TO) = 48° 28' (TTO)a(OTO) = 65° 52J' (010) A (HO) = 65° 35' (110) A (HO) — 48° 44' 359° 561' (010) A (021) == 44° 41' ) (021) A (Oil) = 18° 30' J 11 (011) a (Oil) = 54° 5' (OII)a(OTO) — 62° 56' 180° 12' (110) A (Oil) = 122° 32 J' (Oll)A(TlO) = 57° 18|' 179° 51' (OTl) A (HO) = 57° 21' The crystal face (OTl) was somewhat imperfect, so that the reflected image was extended in width. Hence, the angles between (OTl) and adjacent planes are rendered uncertain, but only by the small va’ae of 7' or 8'. All the other faces gave exceedingly sharp reflections. From these considerations upon the system in which tribromacrylic acid crystallizes, it will appear that we have to deal with a question of small differences, and that in consequence of the very prominent monoclinic habit, we are justified in making these crystals monoclinic, and not triclinic, as Becke has determined them.