Acids. 165 



tar in the greatest quantity : when old Rhenish 

 ' wine is thus depurated it is called old hock. 

 Tartar undergoes an operation to make it fit for 

 medical purposes. It was customary in our la- 

 boratories to purify it by continuing the evapo- 

 ration till the tartar concreted in a thin saline 

 crust at the top, which was skimmed off as fast 

 as it appeared, and was called cream of tartar. 

 Lees of wine contain tartar : they are deposited 

 in the form of a muddy liquor, and are separated 

 by decantation. They yield, by being dried, 

 &c., a large proportion ofcendresgravellees, as it 

 is called in France : it is used in the preparation 

 of ink for copper-plates, under the name of 

 Frankfort black. 



Cream of tartar, or acidulous tartrat of potass, 

 is a compound salt, from which, by a more 

 powerful acid, the alkali may be separated, leav- 

 ing the tartaric acid free and crystallizable by 

 itself. 



Tartaric acid consists of oxygen, carbon, and 

 hydrogen, in the following proportions : 



70-5 parts oxygen. 



19- carbon. 



10-5 hydrogen. 



100- 



Citric acid mixed occasionally with tartaric 

 constitues the native acid of most of our fruits. 

 Great quantities of it are now obtained in a 

 crystallized state from the juice of lemons and 

 limes, which, together with the tartaric, are ap- 



