464 InteUige7ice and Miscellaneous Articles. 



sively described under the names oi fatty matter of the brain, cMbrote 

 and cerebric acid. — Comptes Rendus, Avril 12, 1847. 



ON PLASTER OF PARIS. BY M. PLESSY. 



All chemists have agreed with M. Gay-Lussac, that sulphate of lime 

 which had lost the whole of its water of crystallization by the action 

 of heat, amounting- to 21 per cent, or two equivalents, might be re- 

 hydrated, and the setting of plaster was explained by it. M. Millon 

 has, however, communicated to the Academ)' the following remarks : 

 " Sulphate of lime heated to 230° F. loses 1^ equivalent of water ; at 

 284° to 293° F., mere traces of water are separated; lastly, when 

 heated to 572°, the plaster is rendered anhydrous and does not then 

 set." 



These results, opposed to those obtained by RI. Gay-Lussac, induced 

 M. Plessy to occupy himself with a question which chemists had 

 supposed to have been long settled, the consideration of which M. 

 Millon has resumed. 



The facts stated by M. Plessy are, — 1st, that gypsum and crystal- 

 lized sulphate of lime, artificially prepai'ed, lose 21 percent, of water 

 at 230° to 239° F. in a current of dry hydrogen, a gas which is well 

 known to exert no action on the salt under the circumstances in 

 which M. Plessy operated ; 2nd, plaster heated in the open air from 

 230° to 239° F. readily lost 15 per cent, of water, and there then 

 appeared a stop to occur; but at 266° to 284° F., the limit at which 

 plaster is dehydrated, according to Mr. Graham, M. Plessy separated, 

 not traces of water, but 2 per cent., and this was effected in a very 

 short space of time ; lastly, at 392° to 482° F., the plaster operated 

 on suffered a total loss of 21 per cent. The author satisfied himself 

 that it was anhydrous, by heating it strongly with a spirit-lamp, and 

 yet it recombined with the whole of its water of crystallization so 

 perfectly, that after having set, it lost 21 per cent., or two equivalents 

 of water. 



It appears from the above statement that plaster does not retain 

 very precisely half an equivalent of water ; for at 270° to 284° F., 

 the loss is 17 per cent., and the half equivalent of water is greatly 

 endangered; the compound S-0'^2CaO,HO of M. Millon is de- 

 stroyed, and yet it is formed at 230° to 239° F. ; at this temperature, 

 moreover, it does not exist in a current of dry gas ; this combination 

 has, notwithstanding, been obtained. M. Johnston found in the 

 boiler of a steam-engine, prismatic crystals of sulphate of lime which 

 contained half an equivalent of water. 



It results from the experiments now detailed, that sulphate of lime 

 which has lost the whole of its water of crystallization, may regain 

 it when they are brought into contact. 



In concluding, M. Plessy remarks that his results are opposed to 

 those of M. Rlillon, but that they have been previously announced 

 by M. Gay-Lussac and several other chemists — Comptes Rendus, 

 Avril 12, 1847. 



