112 COMBUSTIBLE ACIDS. 



crystals, which are not altered by exposure to 

 the air. Tartaric acid is much more easily de- 

 composed by heat than oxalic acid ; hence, it 

 is apt to acquire a brown tinge, unle ss great care 

 be taken to crystallize it in rather a low tempera- 

 ture. It is manufactured in the neighbourhood 

 of London, and sold for the use of the calico 

 printers in the state of most beautiful crystals. 



1. In the experiments which I formerly pub- 

 lished to determine the atomic weight of tartaric 

 acid, 5 * I employed crystals of tartaric acid. 

 These were dissolved in water, and saturated 

 with ammonia, and the atomic weight of the 

 acid was determined by the quantity of nitrate 

 of lead, or carbonate of lime necessary to throw 

 down the whole of the acid ; but happening 

 soon after to examine somewhat particularly a 

 quantity of very fine tartaric acid in uncommon- 

 ly large crystals, prepared I believe by Mr. 

 Luke Howard in the neighbourhood of London, 

 I was struck with an appearance, on breaking 

 one of the crystals, that led me to suspect that 

 water might be mechanically lodged between 

 the plates. The possibility of this having been 

 the case with the crystals which I had employed 

 in my experiments (though much smaller, and 

 carefully pressed between folds of blotting pa- 

 per) rendered it proper to repeat the whole in 



* Annals of Philosophy, (second series) II. 138. 



