Mercury, Bichloride of
Symbol, HgCl2. Atomic weight, 274.
This salt, also called Corrosive Sublimate, and sometimes Chloride of Mercury (the atomic weight of Mercury being halved), may be formed by heating Mercury in excess of Chlorine, or more economically, by subliming a mixture of Persulphate of Mercury and Chloride of Sodium.
Properties.—A very corrosive and poisonous salt, usually sold in semi-transparent, crystalline masses, or in the state of powder. Soluble in 16 parts of cold, and in 3 of hot water; more abundantly so in Alcohol, and also in Ether. The solubility in water may be increased by the addition of free Hydrochloric Acid, or of Chloride of Ammonium.
The Protochloride of Mercury is an insoluble white powder, commonly known under the name of Calomel.
METHYLIC ALCOHOL.
This liquid, known also by the names of wood naphtha and pyroxylic spirit, is one of the volatile products of the destructive distillation of wood. It is very volatile and limpid, with a pungent odour.
By a recent excise regulation, ordinary Spirit mixed with ten per cent, of wood naphtha is sold free of duty, under the name of “Methylated Spirit.”
The Milk of herbivorous animals contains three principal constituents—Fatty matter, Caseine, and Sugar; in addition to these, small quantities of the Chloride of Potassium, and of Phosphates of Lime and Magnesia, are present.
The fatty matter is contained in small cells, and forms the greater part of the cream which rises to the surface of the milk on standing; hence shimmed milk is to be preferred for Photographic use.
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.
The concentrated liquid Nitric Acid contains 1 atom of water, and has a sp. gr. of about 1·5; if perfectly pure, it is colourless, 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 Pyroxyline. The yellow Nitrous Acid, so called, is a strong Nitric Acid partially saturated with the brown vapours 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 colour disappears, a compound being formed which has been termed a Sulphate of Nitrous Acid.
In the Appendix a Table is given which exhibits the quantity of real anhydrous Nitric Acid contained in samples of different densities.
Chemical Properties.—Nitric Acid is a powerful oxidizing agent (see page 13); 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 colour, 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 reagent, in the same manner as that of Hydrochloric and Sulphuric Acid.
Impurities of Commercial Nitric Acid.—These are principally Chlorine and Sulphuric Acid; also Peroxide of Nitrogen, which tinges the acid yellow, as already described. Chlorine is detected by diluting the acid with an equal bulk of distilled water, and adding a few drops of Nitrate of Silver,—a milkiness, which, is Chloride of Silver in suspension, indicates the presence of Chlorine. In testing for Sulphuric Acid, dilute the Nitric Acid as before, and drop in a single drop of solution of Chloride of Barium; if Sulphuric Acid be present, an insoluble precipitate of Sulphate of Baryta will be formed.
NITROUS ACID. See Silver, Nitrite of.
NITRATE OF POTASH.
Symbol, KO NO5. Atomic weight, 102.
This salt, also termed Nitre, or Saltpetre, is an abundant natural product, found effloresced upon the soil in certain parts of the East Indies. It is also produced artificially in what are called Nitre-beds.
The properties of Nitrate of Potash are described as far as necessary at page 190.