The second constituent, Caseine, is an organic principle somewhat analogous to Albumen in composition and properties. Its aqueous solution however does not, like Albumen, coagulate on boiling, unless an acid be present, which probably removes a small portion of alkali with which the Caseine was previously combined. The substance termed "rennet," which is the dried stomach of the calf, possesses the property of coagulating Caseine, but the exact mode of its action is unknown. Sherry-wine is also commonly employed to curdle Milk; but brandy and other spirituous liquids, when free from acid and astringent matter, have no effect.
In all these cases a portion of the Caseine usually remains in a soluble form in the whey; but when the Milk is coagulated by the addition of acids, the quantity so left is very small, and hence the use of the rennet is to be preferred, since the presence of Caseine facilitates the reduction of the sensitive Silver salts.
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, NO5. 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 the 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.