Scientific Intelligence. — Chemistry. 185 



atmospheric air, at 60° Fahr., and 30 inches of the barometer, 

 weigh at least ^1 grains. The estimate of ^.5 grains, deduced 

 from the observations of Schuckburgh, is therefore incorrect. — 

 Turner's Chemistry^ p. 808. 



CHEMISTRY. 



4. On Efflcyrescence. — The following observations are by M. 

 Gay Lussac. Many salts, when exposed to the air, are well 

 known to effloresce ; that is, they fall to powder, and lose their 

 water of crystallization ; and it is generally supposed that salts, 

 after efflorescence, are perfectly anhydrous. Having been long 

 convinced that this opinion is not correct, I have made some ex- 

 periments upon the principal salts, which are efflorescent in a 

 high degree. Crystallised sulphate of soda, exposed to the air, 

 even when it is not very dry, readily loses all its water of crystal- 

 lization. Phosphate of soda becomes readily opaque, without 

 losing its form. After three months exposure to the air, it con- 

 tained, on the 18th of July, 7.4 of the 12 proportions of water, 

 which it contains in its usual state. Reduced to powder, and 

 thinly spread upon paper, it contained, on the 26th of July, 

 5.65 proportions of water ; again exposed to the air during a hot 

 and dry period, it contained, on the 31st of July, only 5.Qo pro- 

 portions ; afterwards exposed till the 21st of October, the weather 

 having become colder and more damp, it was found to contain 

 7.2 proportions of water ; some phosphate, which had been cal- 

 cined, absorbed, in five days'* exposure to the air, nearly half a 

 proportion of water. Carbonate of soda, on exposure, exhibits 

 nearly the same phenomena as the phosphate ; it becomes opaque, 

 loses much water without altering its form ; but I have never 

 found it anhydrous after exposure. It results from these obser- 

 vations, that some salts completely lose their water of crystalli- 

 zation by efflorescence ; but that others retain variable quantities, 

 according to the hygrometric state of the air. I do not assert, 

 however, that the water may not remain in definite proportions; 

 it merely appears that, in the phosphate and carbonate of soda, 

 which retains a proportion of water of a certain number, — the 

 seventh for example,^-differs but little from that which unites 

 the proportion immediately above or below. — Ann. de Chein. 

 xxxvi. 335. 



