;fl^ ffl i l , f fj» »FUiJi |ifltinh i i^WiniWM11WnWffiTy^ | imiiMa i an i « i i iii iiiiii i iH imrmnnmiMi LI B RARY OF THE UNIVERSITY Of ILLINOIS 666 tie. 16-23 CENTRAL CIRCULATION AND BOOKSTACKS The person borrowing this material is re- sponsible for its renewal or return before the Latest Date stamped below. You may be charged a minimum fee of $75.00 for each non-returned or lost item. n»«ff, mufilotlon, or defacement of library materiaU can be cautet for tfudenf ditcipllnary action. All materials owned by th« Unlvenify of IllinoU Library ore the property of the State of Illinois and are protected by Article 16B of tlllnoi% Criminal Law and Procedure. TO RENEW, CALL (217) 333-8400. University of Illinois Library at Urbana-Champaign c;f P 1 S ?00t MAY 2 9 2001 When renewing by phone, write new due date below previous due date. L162 UNIVERSITY OF ILLINOIS BULLETIN Vol. X. 5EPTEMBE.R 16, 1912. No.3 [Lntered February 14, 1902, at Urbana, Illinois, as second-class matter under Act of Congress of July 16, 1894.] BULLETIN No. 17 DEPARTMENT OF CERAMICS A. V. BLLININGER. Director THE EFFECT OF ACID5 AND ALKALIE5 UPON CLAY IN THE PLA5TIC 5TATE BY A. V. BLLININGER AND C. E. FULTON NOTE ON THE DISSOCIATION OF CALCIUM HYDRATE BY R. K. HUR5H NOTE ON THE RELATION BETWEEN PREHEAT- ING TEMPERATURE AND VOLUME SHRINKAGE BY R. K. HUR5H 1911-1912 PUBL15HLD FORTNIGHTLY BY THE UNIVERSITY [Reprinted from Transactions American Ceramic Society. \'<>I. XIV. BY Permission] THE EFFECT OF ACIDS AND ALKALIES UPON CLAY IN THE PLASTIC STATE. A. V. Bleixinger and C. E. Filton, Urbana, 111. INTRODUCTION. The effect of acids, alkalies and salts upon clay suspensions (slips) has been discussed frequently, and the work of Simonis, Mellor, Rieke, Boettcher, Ashley, Foerster and Bollenbach deals mth the viscosity and other phenomena of systems in this state. But little is known concerning the effect of such reagents upon clays in the plastic condition which differs from that of a sus- pension, due to the cohesive inlluence of the particles upon each other. It has been realized for some time that the properties of clays in the wet state are influenced by the presence of alkalies and acids. Seger explains the increase in the plasticity of clay upon storing by the assumption that the fermentation of organic sub- stances results in acids which neutralize the alkalinity due to the decomposed feldspar, and in addition bring about the "sour" condition which accompanies the improvement in working qualities. Rohland' discusses this subject from the theoretical standpoint and makes quite definite statements with reference to the principles underlying the effect of various reagents upon clays in the plastic state. He arrives at the conclusion that the plasticity of clays is increased by the presence of H"*" ions, while, on the other hand, the OH' ions are active in the opposite direc- tion. According to Rohland, the plasticity is likewise increased by the addition of colloids like tannin, dextrine, etc., as has been shown by the work of Acheson, fine grinding and the storage of the clay in cool and moist places. It is supposed that the in- crease in plasticity is coincident with the coagulation which is primarily due to the presence of the hydrogen ions ; it is retarded by the hydroxyl ions. The salts of strong bases and weak acids which dissociate OH' ions hydrolytically produce an effect similar to that of the hydroxyl ions. Neutral salts, Rohland goes on to say, with but few exceptions, are indifferent in their > "Die Tone," pp. 35-19. 4 EFFECT OF ACIDS AND ALKALIES UPON CLAY. effect, though some appear to show a contradictory behavior, which has not yet been explained. "The effect of the hydroxyl ions may be weakened, compensated or strengthened by the action of the salt in question. Thus borax is an example of the first class and sodium carbonate of the second." The same writer further says that with some clays the addi- tion of NajCOa brings about an improvement in plasticity, while ordinarily the same reagent behaves in the opposite sense, due to the hydrolytic dissociation of OH' ions. It is possible that the effect of hydroxyl ions might be neutralized by the CO3" ions. DRYING SHRINKAGE. A decided lack of data exists with reference to the deter- mination of the effect of reagents upon the plasticity of clays. It was thought advisable for this reason to begin work along this line without reference to any theoretical speculations. The most obvious criterion to be used in this connection is the drying shrinkage, which, from what we know of the properties of clays, is a function of plasticity. It is evident that any effect caused by the addition of reagents will at once be indicated by the shrinkage of the clay. In this series of experiments Georgia kaolin was used. This clay was found to show an acid reaction when tested with phenol- phthalein. This would indicate that the addition of acid should bring about no decided change in the clay, a fact which was verified by experiment. The reagents employed were HCl, H2SO4, NaOH and Na2C03. In carrying out the work a thoroughly mixed sample was first prepared so that variations due to differences in composition were reduced to a minimum. The test specimens were in the shape of bars 3V16 x i x Vs inches. Even the most careful linear shrinkage measurements by means of the vernier caliper were found to be unsuitable for the work. A volumenometer permitting of readings to 0.05 cc. was then employed. The measuring liquid used was petroleum from which the lighter oils had been expelled by heating. The bars were at once weighed and allowed to dry at the laboratory temperature for three days, after which they were heated at 110° to constant EFFECT OF ACIDS AND ALKALIES UPON CLAY. 5 weight, and their shrinkage determined. For each concentration of reagent three bars were made and measured. Clay and Water. — A study was first made of the drying shrinkage of the clay with different amounts of water, ranging from the soft state in which the clay could be barely molded to the condition of minimum water content when molding was likewise difficult for the opposite reason. The shrinkage rela- tions to the various contents of water are sho\vn in Fig. i. The third point on the curve, shomng a shrinkage of 10.45 per cent, with a water content of 32.8 per cent., represents the most work- able state. An)- increase in water above this point is at once ob- served by the rapid softening of the mass. The clay hence is well suited for the work at hand, owing to the ease with which the condition of best working behavior is recognized in distinction from many other plastic clays which possess a long working range. Effect of Acid. — Upon adding from 0.025 to 0.525 gram of hy- drochloric acid to 100 grams of clay, we observe from Fig. 2 that the shrinkage is not materially aflfected by this reagent. While two maxima of somewhat greater contraction are noted, the principal result seems to be a reduction in shrinkage, contrary to what might be expected from Rohland's statements. The fact re- mains, however, that conditions are more complex than they seem, due to the probable solution of various salts in the clay as well as the formation of some chlorides by the acid. It was thought that further insight into the effect of the acid might be obtained by calculating the total and the shrinkage water in terms of the true clay volume, /. c, weight divided by the density of the powdered substance, according to the rela- tion: 100 (Vi — V2) ■w = per cent, (by volume) shrinkage water. 1 Where x\ = volume of wet brickette, t'2 = volume of dry brickette, w = weight of brickette, dried at 110° C, d = density of the dry and powdered clay. EFFECT OF ACIDS AND ALKALIES UPON CLAY. r/?A/V5. AM C£/? SOC /<2?^ X/l^ F-/0./. Bl£/A//^G£/?d/^UL 72W /tSc? /JO /4i:> 1 \ 1 / ■o / V //o —J /ao ^ 9.a a9 J i» per ce/7/ ter/ns a %j- ~s 1/ £ %/SA \ p aozf iP/J ass£ ^jk' a.Jzf a^^ as^f EFFECT OF ACIDS AND ALKALIES UPON CLAY. ■r^JA^S ^A¥C£-^S^X^ /^Z A-Zi^ ^/M^ <:r^f>r' soc Ki>^ -r/*/ 0^.£^w//vi}S/t' A /=^c//.rz?/v c^y ^ i?/ ^^ cs cs ^ as ^7 as ^ , lO EFFECT OF ACIDS AND ALKALIES UPON CLAY. Similarly, the volume of the total water in terms of the true clay volume is calculated. In the diagram of Fig. 3, the respective volumes of total and shrinkage water are shown. The boundary between the volumes of water and that of clay is, of course, the line representing zero water and 100 volume per cent, of clay. It is shown in Fig. 3 that the content of pore water has been decreased, that of the shrinkage water having been increased both at the expense of the pore water and due to the rise in the total water content at the two max. points. The addition of sulphuric acid likewise tends to decrease the shrinkage as is shown in the diagram of Fig. 4. Effect of Alkalies. — The influence of NaOH is illustrated in the diagram of Fig. 5. It is at once noted that with 0.2 per cent, of this reagent a striking max. point is reached, indicating a marked increase in shrinkage, contrary to what we should ex- pect according to Rohland's views. Only after adding larger amounts does the contraction descend towards the normal value. Here again, according to Fig. 6, the increased shrinkage is due in part to the specific effect of the reagent in increasing the distance between the particles in the plastic state and, in part, to the denser structure of the clay upon drying. Beyond the max. point this condition changes, since the pore water line rises above the normal level. Since, at the same time, the total water line descends, the shrinkage is gradually decreased. The structure of the dried clay is thus more open with the higher contents of NaOH than with the smaller additions. The growth in shrinkage is still more pronounced in the case of NajCOj, Fig. 7, a phenomenon contrary again to Rohland's statements, although, of course, in this case the effect of the CO3 ion might have proven a factor, especially if absorption has taken place to any appreciable extent. However, even under this assumption, it is somewhat improbable that the carbonic acid could have brought about such a change where other acids failed to accomplish anything like the same result. In this diagram the maximum occurs with 0.7 per cent, of the reagent. With larger concentrations the shrinkage is again reduced, but appears to gain once more with amounts beyond 1.2 per cent. EFFECT OF ACIDS AND ALKAI.IICS UPOX CUAY. I I As may be observed from the diagram of Fig. 8, the pore water volume is diminished throughout this series with a gradually increasing total water content up to the maximum. DEFLOCCULATION SERIES. It was thought desirable to study the effect of the acids and alkalies upon the clays as regards dellocculation, using solutions of the same concentration present in the plastic clay, as sho\\Ti by the preceding curves. To illustrate: If to loo grams of clay, requiring 34.9 per cent, of water, 0.025 gram Na^COj was added, this would represent a solution carrying 0.025 "^ 34-9 = 0.000716 gram NaoCOj per cubic centimeter of water. Such solutions Fig. 9. No. Wt. clay. Grams wt. NajCOa, Grains Water, cc. Volume of sediment, cc. Condition of turbidity of supernatant liquid 5 98 18.0 Clear I 5 0.0713 98 19-5 " 2 5 0.1423 98 21.8 " 3 5 0.2296 98 24 3 » 4 5 0.4529 98 28.0 u 5 5 . 6803 98 -^7-4 n 6 5 0.8913 98 28.0 « 7 5 1.0659 98 28.0 » 8 5 1-3549 98 28.0 « 9 5 I 55" 98 28.0 u 12 EFFECT OF ACIDS AND ALKALIES UPON CLAY. were made up of concentrations corresponding to the various points in the preceding curves. In each case to 5 grams of clay 98 cc. of the solution were added in a graduated tube. The tubes were placed in a shaking machine for 90 minutes and allowed to stand. It was found that the clay itself, without any reagent, settled well, showing a clear, supernatant liquid and a sediment occupying 18 cc. It was shown that the addition of acid produced no change, excepting in the volume of the sediment, which was finally in- creased from 18 to 28 cc, as is observed from Fig. 9. The sodium carbonate solutions, on the other hand, started with conditions of complete deflocculation (Fig. 10). The sediment volumes are shown in the table accompanying each figure. m <^' ^^1 flii Fig. 10. No. Wt. clay, grams wt. NaaCOs, Water, Volume of sediment, cc. Condition of turbidity grams cc. supernatant liquid 5 98 18 Clear 5 0.0702 98 4-5 Very turbid 5 0. 1426 98 19 Very turbid 5 . 2090 98 24 Slightly less turbid 5 0.2822 98 24 Less turbid 5 0-5713 98 20 Slightly turbid 5 0.8388 98 20 Slightly turbid 5 1.3642 98 21 Almost clear 5 1-8571 98 19 Clear 5 2-5059 98 18 Clear 5 3 . 4006 98 18 Clear 5 4.0709 98 18 Clear o. I . 2 . 3- 4 5- 6. 7 ■ 8. 9 10. EFFECT OF ACIDS AND ALKALIES UPON CLAY. I3 The maximum point of the shrinkage curve corresponds to tube No. 8, where the supernatant Hquid is clear for the first time. CONCLUSIONS. The writers do not attempt at this time to explain the phenomena on theoretical grounds. It is evident that the con- ditions are quite complex and in order to explain them still further modes of attack must be sought for. The rules laid down by Rohland do not seem to apply, since in the main the acids cleaily caused shrinkage to decrease while the alkalies produced the reverse effect, which is contrary to his statements. In order to be fair, however, attention must be called to the fact that shrinkage in this work has been considered a meastu-e of plasticity, while Rohland speaks of plasticity itself without attempting to correlate this property with any numerical value. As is well known, there is at the present time no clear conception as to the relation be- tween plasticity and shrinkage excepting the general fact that the plastic clays as a class show a greater dr>dng shrinkage than the leaner ones. DISCUSSION. Mr. R. J. Montgomery: I should like to ask Prof. Bleininger how long those slips in the cylinders had stood when the photo- graphs were taken. Prof. Bleininger: Twenty-four hours. I might add also that in making the volume determinations they were stored twelve hours in a moist chamber in order to bring about some sort of an equilibrium between the clay and the reagent. Mr. Kerr: I should like to raise the question as to what determinations, if any, were made of the electrolytes present in the clay before the acids and alkalies were added. Was any general data obtained upon this point? Prof. Bleininger: No direct determination was, of course, made. However, you have seen the series of tubes which ought to indicate pretty clearly to one familiar with this work whether the initial conditions are acid or alkaline. We are principally endeavoring to get at the experimental facts without much re- gard to theoretical assumptions. The evidence so far obtained along these lines is not sufficient to base upon it any definite O. 0^ {LL Ub. 14 EFFECT OF ACIDS AND ALKALIES UPON CLAY. line of procedure. The work of VeimaTn especially has dis- turbed pievious conclusions by his very startling claims with reference to colloids. We thought it wise to work along the lines which I have indicated. Mr. Kerr: The only point which I wished to bring up was that if one clay contained positive ions in excess and another clay negative, the addition of either acid or alkali to one clay would not correspond to a similar addition to the other clay. Some clays give a strongly acid reaction, others a weakly acid, while still others are somewhat alkaline. Data upon neutralization might be included. Prof. Bleimnger : This is brought out in the deflocculation experiments. At the same time corrections work very well in theors^ but when you come to make them you will find that neutralization does not necessarily follow. I, of course, want to check Mr. Ashley's work in this investigation in a general way. I realize we have learned a good deal from his work and I want to say that he is to be given great credit for having started work of this kind. Mr. Purdy: I would like to ask if any experiment has been made to determine whether, as a rule, trivalent electrolytes coagu- late clays more readily than do the uni- and divaleat salts. Prof. Bleininger: I would say that it has been done with various materials. Mr. Purdy: Has it been done with clays? I would like to see some experiments tried on that and reported, because I have been unable to show that the trivalent salts have any more effect than the other. That is one of the respects in which the clay is different. Prof. Bleininger: Mr. Ashley, of course, has done such work. Mr. Purdy: That is what he did not do, he accused himself ■on that point. Prof. Bleininger: I think he did work with phosphates. Of course, as I said before, this work is being continued and we expect to take representative reagents. Mr. Kerr: What meastuements other than volume shrinkage were made? Prof. Bleininger: We hope to take up various things in EFFECT OF ACIDS AND ALKALIES UPON CLAY. 1 5 time. One of them is a vapor tension investigation, for which a special apparatus is now being designed. Prof. Grout: I would like to ask if the curves which are drawn there, such as the first curve which you show on the screen, were the average of a series of results on one clay or just one series of tests. Prof. Bleininger : Taken as the average of three determina- tions in each case. Prof. Grout: I wondered if that approximation of a maxi- mum was so characteristic that you could report it for publica- tion on one series of tests; whether your area of determination was not such that you might not safely report it. Prof. Bleininger: Well, we were able to get very good checks, also we notice that the two acids are behaving very similarly. We recognize, however, that there are a good many factors involved which it is almost impossible to correlate in a technical investigation of this kind. Of course, if we were to carr}^ on this investigation from a strictly physical chemical standpoint, we would proceed along somewhat different lines. Mr. Potts: I would like to ask Prof. Bleininger just what practical application he expects to make of that treatment. Does he propose to make kaolins plastic? Prof. Bleininger: I haven't any idea as to what this in- formation could be used for and am indifferent in regard to that point. [Reprinted from Transactions American Ceramic Society. Vol. XIV, BY Permission.] NOTE ON THE DISSOCIATION OF CALCIUM HYDRATE. By R. K. HuRSH. INTRODUCTION. The present study, which was intended to be of a techno- logical rather than of physical-chemical nature, was undertaken with the purpose of learning more regarding the properties and behavior of the compound CaCOH),. The work has a practical bearing in demonstrating the value of methods of thermal study upon problems dealing with the dehydration of limes, cements and plasters. A number of values have been given for the dissociation temperature of calcium hydrate. Herzfeld^ says that dissocia- tion evidently begins at 470° to 500° C. He gives the thermal effect of slaking CaO as i .51 cals. per gram of CaCOH), and the maximum temperature of formation as 468°. H. Rose"- found that pure calcium hydrate lost nothing at 100° C, absorbed CO, at 200° and 300°, and began to lose H.p at about 400° C. Le Chatelier^ gives a vapor tension of 100 mm. at 350° C, and 760 mm. at 450° C. Tichborne^ found the precipitate from a heated solution of lime water to show a loss on blasting that corresponded to the formula 3Ca0.2H20. Others using similar methods failed to find such a hydrate. Dr. Johnston,^ whose work is taken up further on, found the dissociation pressure of Ca(OH)., to reach 760 mm. at 547° C. METHODS AVAILABLE. There are several methods of studying the dissociation of hydrates, such as the making use of heating curves, the deter- mination of the aqueous pressure in direct or differential tensim- eters, and the method depending upon the determination of the loss of weight at different temperatures ' llandbuch dcr anorg. Chem., C. Damnier. - Pogg. Ann. du Physik u. Chem.. LXXXVI. 105. <• Handbuch dcr anorg. Chem., 22, Gmelin Kraut. < Chemical Xews. XXIV, 199. 5 Ztschr. phys. Chem.. I.XII, 330. l8 NOTE ON DISSOCIATION OF CALCIUM HYDRATE. HEATING CURVE METHOD. A portion of the substance is placed in a furnace with a thermocouple touching it and another near it. The furnace is heated, and the temperatures of the furnace and substance are noted. At the point where dissociation takes place, a lag may be noted in the heating curve due to the endothermic reaction, /. e., the absorption of heat due to the expulsion of water. It is fre- quently difficult and sometimes impossible to determine the point by this means, owing to the small amount of heat required for the reaction of the slow rate of dissociation. Distinction may be made between mechanically held or dissolved water and chem- ically combined water. In the case of chemical water, the lag will occur abruptly at the temperature of dissociation. Mechan- ical or dissolved water will pass off gradually over a range of tem- perature, and the lag due to these is gradual, showing no abrupt break at a definite temperature. In the use of heating curves, close regulation of the tempera- ture is very necessary to get reliable results. There should be no fluctuations in the heating of the furnace. Three general methods may be followed in the heating: Indiscriminate, in which no attention is given to the rate of the furnace curve, and only the lags in the heating curve of the substance are given attention. Constant rate, in which the temperature of the furnace is- raised at a uniform rate. Constant difference, in which a uniform difference between furnace temperature and that of the material is maintained. This method is the best, although the most difficult one of the three. The constant rate method gives good points, but the lag will, in most cases, be sloped instead of horizontal. AQUEOUS PRESSURES METHOD. Van Bemmelen, in studying the dehydration of the silicic acid gel, placed his samples in desiccators containing various concentrations of HjSO^. Constant temperature was maintained, and the samples were kept in the desiccators for sufficient time to reach equilibrium imder the various vapor tensions. By plotting the loss of weight curve for the several concentrations of H2SO4 or the corresponding vapor pressures, he was able to NOTE ON DISSOCIATION OF CAIXIl'M HYDRATE. 19 determine the inversion points and the degrees of hydration in each case. The same method has been appUed by Prof. A. W Bleininger" in studying the moisture in clays. Dr. John Johnston^ studied the dissociation pressures of several metal hydroxides and carbonates, using two experimental methods. The first was applied for hydroxides alone and is sim- ilar to one used by Brill. A small crucible containing a weighed portion (about i . 5 mg.) of the substance was suspended in a small electric furnace through which a current of air free from CO, and of definite vapor pressure was passed. The air was freed from COo by passing through XaOH, then saturated with moisture by bubbling through a Liebig potash bulb, containing water, and rf?^A^3. ^/^. c^/?. soc. /^22/ x/i^ y^/C? ■ /. /H'O'^3/^ 760 '<3iS(P ■4^C <«c5i? ^(P^ ^s^o " Bulletin No. 7, Bureau of Standards. ' Ztschr. phys. Chem., LXII. p. 330. 20 NOTE ON DISSOCIATION OF CALCIUM HYDRATE. was heated before passing to the furnace to prevent any conden- sation. The temperature of the water in the Liebig bulb was regulated by immersing it in a water bath. The furnace was held at constant temperature, and the vapor tension maintained at a definite value for lo minutes by regulation of the water bath temperature. The crucible was then removed from the furnace and weighed on a very fine balance. Conditions of tem- perature and vapor pressure were so regulated that the substance maintained constant weight or gained slightly during the period, and these values were taken as the corresponding temperature and dissociation pressure of the material. The results of this method for Ca(0H)2 are shown in Fig. i. This method was found to be too slow and to frequently give inconsistent results. It was impossible to prevent the ab- sorption of some CO2 while removing the crucible from the fur- nace for weighing. STATIC METHOD. Dr. Johnston then resorted to the "static method," in which the dissociation pressirres are measured directly. A diagram of the apparatus is showTi in Fig. 2. A platinum tube, P, about 5 cm. long and 4 mm. inside diameter, contained the substance. This tube was placed in a small electric furnace with a thermo- couple for determining the temperatures. A piece of glass tube, C, was fused to P and to one arm of a U tube which was connected to the barometer. On each arm of the U tube was a bulb, L, bent to the side and holding enough mercury to fill the U-tube to a depth of about 3 cm. To prevent condensation of the vapor from P, the U-tube and C were enclosed by a glass steam jacket. With the mercury in bulb L, the apparatus was exhausted through A by means of a mercur}' pump. Cock A was then closed, and the mercury run from L into the U-tube by tilting the ap- paratus. Heating was begun, and at the first indication of pres- sure in P, the mercur>^ in the two arms of the U-tube was brought to the same level by admitting some air at B and adjusting by means of the leveling tube R. In his work with calcium hj'droxide. Dr. Johnston slaked pure CaO and absorbed the excess water in a desiccator. An- other portion was made by allowing the CaO to absorb moisture NOTE ON DISSOCIATION OF CALCIUM HYDRATE. r/?^/v3. y^M. dT^/f? 30C. /^z. x/P' /ycz/F-s/-/ 21 slowly until the composition was about CaO o.SH^O. In using this substance, it was found necessary to heat it slightly during exhaustion of the apparatus since pressures of several cm. ap- peared between 200° and 300° which again disappeared in part on further heating. These abnormal pressures were supposedly due to loosely combined or absorbed moisture, and upon their appearance the test was stopped and the apparatus again ex- hausted. Only such pressures were taken as appeared at definite temperatures on heating and again disappeared on cooling. The "abnormal pressures" disappeared only partly on cooling. Un- der these conditions, it was found advisable to use a mixture of CaO and the hydroxide, although this did not entirely eliminate 22 NOTE ON DISSOCIATION OF CALCIUM HYDRATE. the trouble, which was noted with all of the hydroxides studied. The results of the work on Ca(0H)2 by this method are shown in Fig. 3. By the curve, it is seen that the dissociation pressure eoa TT^^/KS.^/*/ C£'/?..SC>C. l^iCPJ-.^/l^ ^^/G.3 I I ^aA — «-o- ■ "0 ^ ' y ^ i ,eoo 300 <%7^ soo 600 reaches 760 mm. at a temperature of 547° C. Hence this is taken as the dissociation temperature of the substance under atmospheric pressure. In studying zeolites FriedeP heated them at successively higher temperatures in a current of air of approximately constant vapor pressure. This method was adopted by Allen and Clement'' in their study of tremolite, using dry instead of moist air. A crucible containing the material was placed in an electric fur- nace, through which a current of air, dried by concentrated H2SO4, was passed. After heating for some time at a definite temperatm"e, the crucible was quickly removed to a desiccator, cooled and weighed. Heating was continued at each tempera- ture until practically constant weight was obtained. In one case, the experiment was repeated with moist air to determine the effect upon the results obtained by using dry air. EXPERIMENTAL WORK. This method was adopted for the present work. The hy- drate was prepared by calcining pure CaCOg at 1050° C. and slaking the oxide with a slight excess of water. A portion of the hydrate was placed in a platinum crucible and heated in an 8 Ztschr. phys. Chem., XXVI, p. 323. 8 Am. Jour. Sci.. Vol. XXVI. No. 152. NOTE ON DISSOCIATION OF CALCIUM HYDRATE. 23 electric furnace in a bath of dry air, free from CO,, at successive temperatures from 200° to 750° C. at 50° intervals. At 30-min- ute intervals, the crucible was removed from the furnace and cooled in a desiccator over concentrated H^SOj. The heating was con- tinued at each temperature until constant weight was reached. It was impossible to prevent the absorption of some COj during the transfer of the hot crucible from the furnace to the desiccator. The first loss of weight was noted at 400° C. Continued heating at this temperature gave a total loss of weight of 77 per cent, of the water present above 200° C. At 650° another loss of weight took place amounting to 22 per cent. A second trial was made with 10° intervals from 350° to 400° C, and the first loss was found to take place at 380° C. To prevent the absorption of COj by the sample, a method of weighing within the furnace was adopted. A platinum cru- cible was suspended in the furnace by a fine platinum wire from one pan of a balance that was carefully protected from unequal heating from the furnace. A thermocouple was placed with the junction just under the middle of the crucible. A current of air, free from COj and dried by CaClj and P2O5, was circulated through the furnace. A weighed portion of the hydrate was placed in the crucible and dried at 25° C. The temperature was then raised gradually until a loss of weight began at 375° C. It was somewhat surprising that there was no loss of weight be- r/j>j/KS.>»/K cf-zp sec t^OZ. -^/i^ 4W f^J *9^ — .4- — • -3 23 a \ ' ' '7 ' 1 1 -f 1 ' ■ 1 1 1 i \o- %.30- oaa 7a»*r cs/rsoc t/o/..^/i^ /«=•//:> 4C it /5l X=7