Caseine combines with oxide of silver in the same manner as albumen, forming a white coagulum, which becomes brick-red on exposure to light.
Sugar of milk, the third principal constituent, differs from both cane and grape sugar; it may be obtained by evaporating whey until crystallization begins to take place. It is hard and gritty, and only slightly sweet; slowly soluble, without forming a syrup, in about two and a half parts of boiling, and six of cold water. It does not ferment and form alcohol on the addition of yeast, like grape sugar, but by the action of decomposing animal matter is converted into lactic acid.
When skimmed milk is exposed to the air for some hours it gradually becomes sour, from lactic acid formed in this way; and if then heated to ebullition, the caseine coagulates very perfectly.
Nitric Acid.
Symbol, NO{5}. Atomic weight, 54.
Nitric acid, or aqua-fortis, is prepared by adding sulphuric acid to nitrate of potash, and distilling the mixture in a retort. Sulphate of potash and free nitric acid are formed, the latter of which, being volatile, distils over in combination with one atom of water previously united with sulphuric acid.
Properties.—Anhydrous nitric acid is a solid substance, white and crystalline, but it cannot be prepared except by an expensive and complicated process.
The concentrated liquid nitric acid contains 1 atom of water, and has a sp. gr. of about 1·5: if perfectly pure it is colorless, but usually it has a slight yellow tint, from partial decomposition into peroxide of nitrogen: it fumes strongly in the air.
The strength of commercial nitric acid is subject to much variation. An acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly met with. If the specific gravity is much lower than this (less than 1·36), it will scarcely be adapted for the preparation of peroxyline. The yellow nitrous acid, so called, is a strong nitric acid partially saturated with the brown vapors of peroxide of nitrogen; it has a high specific gravity, but this is somewhat deceptive, being caused in part by the presence of the peroxide. On mixing with sulphuric acid the color disappears, a compound being formed which has been termed a sulphate of nitrous acid.
Chemical properties.—Nitric acid is a powerful oxidizing agent; it dissolves all the common metals, with the exception of gold and platinum. Animal substances, such as the cuticle, nails, etc., are tinged of a permanent yellow color, and deeply corroded by a prolonged application. Nitric acid forms a numerous class of salts, all of which are soluble in water. Hence its presence cannot be determined by any precipitating re-agent, in the same manner as that of hydrochloric and sulphuric acid.