100 Dr. Thomas Thomson [Aug. 



the vacuum of an air pump over sulphuric acid. The 

 weight was reduced to 105-05 grains. This loss amounting 

 to 1*822 per cent, was owing to the escape of water lodged 

 mechanically between the plates of the crystals. 



2. Twenty grains of litharge previously dried in a tem- 

 perature of 300°, were triturated in a porcelain mortar 

 with 10*5 grains of crystals of racemic acid and a little 

 water, till the liquid ceased to affect litmus paper. The 

 whole was then dried for some hours on a sand bath heated 

 to 300°. The weight was reduced to 28*19 grains. So 

 that the ten grains of acid had lost 2*31 grains which 

 must have been water. Of this 0*192 grains was mechani- 

 cally lodged water. Consequently 2*118 grains of water 

 must have been chemically combined in the crystals with 

 pure racemic acid. Thus, the crystals (supposing the 

 mechanically lodged water removed) were composed of 

 Real acid . . . 8*19 or 8*25 

 Water .... 2*118 or 2*133 

 I repeated the experiment, and exposed the litharge and 

 acid to a higher temperature. The white colour of the 

 powder at the bottom of the glass capsule had a decided 

 mixture of yellowish gray, shewing that the acid had begun 

 to undergo decomposition. The weight was reduced to 

 27*90 grains. So that the loss of weight sustained by the acid 

 was 2*51 grains. Of this 0*192 was mechanically lodged 

 water. The remainder amounting to 2*318, obviously 

 rather exceeds the whole chemically combined water con- 

 tained in the crystals. 



According to this experiment the crystals of racemic 

 acid are composed of 



Real acid . . . 7*99 or 8*25 

 Water .... 2*318 or 2*393 

 It is obvious, that in the first of these experiments, the 

 chemically combined water is reckoned below the truth, 

 because the salt had not been sufficiently dried : while in 

 the second experiment it is above the truth, because the 

 salt had been exposed to so high a temperature as to occa- 

 sion an incipient decomposition of the acid. The truth 

 must lie between the two. Now, a mean of the two experi- 

 ments gives us 2*263 for the water chemically united with 

 8*25 of acid. This approaches so near 2*25, the weight of 



