SULPHURIC ACID. 327 



tint to it ; so are selenium and tellurium. Charcoal also ap- 

 pears to be slightly soluble in this acid, imparting to it a pink 

 tint, which afterwards becomes reddish brown. The concen- 

 trated acid has a great affinity for water, which it absorbs from 

 the atmosphere ; and is usefully employed to dry substances 

 placed near it in vacuo. Considerable heat is evolved in its 

 combination with water ; when 4 parts by weight of the concen- 

 trated acid were suddenly mixed with 1 part of water, the tem- 

 perature was observed by Dr. Ure to rise to 300. The 

 density of this acid becomes less in proportion to its dilu- 

 tion. 



Acid of sp. gr. 1.78 is a second hydrate, containing two 

 atoms of water to one of acid. This hydrate forms large and 

 regular crystals, even above the freezing point of water, and 

 remains solid, according to Mr. Keir, till the temperature rises 

 to 45. If the dilute acid is evaporated at a heat not exceeding 

 400 its water is reduced to the proportion of this hydrate. 

 This second atom of water is expelled by a higher temperature, 

 but the first atom can only be separated from the acid by a 

 stronger base. Sulphuric acid forms still a third hydrate, of sp. 

 gr. 1.632, containing three atoms of water, the proportion to 

 which the water of a more dilute acid is reduced, by evaporation 

 in vacuo at 2 1 2. It is also in the proportions of this hydrate, 

 that the acid and water undergo the greatest condensation, or 

 reduction of volume, in combining. The following then are the 

 formulae of the definite hydrates of this acid, including that 

 derived by Mitscherlich from the Nordhausen acid : 



HYDRATES OF SULPHURIC ACID. 



Hydrate in the Nordhausen acid. . HO,2SO 3 



Oil of vitriol (sp. gr. 1.850). . . HO, SO 3 



Acidofsp.gr. 1.78 . . . HO, SO 3 + HO 



Acid of sp. gr. 1.632 . . . HO, SO 3 + <2HO 



Sulphuric acid acts in two different modes upon metals, dis- 

 solving some, such as copper and mercury, with the evolution 

 of sulphurous acid, and others, such as zinc and iron, with 

 the evolution of hydrogen gas. The metal is oxidated at the 

 expense of the acid itself in the one case, and of the water in 

 combination with the acid in the other. The acid acts with 



