In all these cases a proportion 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, 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_.