~Stannous Chloride~, SnCl_{2}.2H_{2}O.--The crystals are best purchased. If kept dry and free from air they are fairly permanent. A solution is made by dissolving 20 grams in 10 c.c. of hydrochloric acid and diluting to 1 litre. The solution is not permanent. It is a strong reducing agent, and is chiefly used in solution for this purpose. ~Tin~, Sn.--Grain tin should be purchased. It is not pure, but contains 99.5 per cent. of the metal. The chief impurity is copper. It can be used as a standard. When acted on with hot hydrochloric acid it slowly dissolves (more rapidly in contact with platinum) and forms stannous chloride. ~Uranium Acetate~, UO_{2}(C_{2}H_{3}O_{2})_{2}.H_{2}O.--It is best purchased in crystals. The solution is used for the determination of phosphates and arsenates. A solution of 3 per cent. strength is occasionally used as an indicator. ~Uranium Nitrate~, UO_{2}(NO_{3})_{2}.6H_{2}O.--This salt is very soluble in water and is sometimes used instead of the acetate, which is somewhat difficult to dissolve. "~Water~," H_{2}O.--Spring or well water is sufficiently pure for most purposes, 100 c.c. will leave a residue of from 10 to 30 milligrams, so that where a salt has to be dissolved out, evaporated, and weighed it should be replaced by distilled water. Rain water, melted snow, &c., always leave less residue than spring water; but in other respects they are often dirtier. Distilled water is best prepared in the office, a glass or tin condenser being used. ~Zinc~, Zn.--It is sold in a granulated form or in sticks. It generally contains over 1 per cent. of lead, with a little iron and arsenic. It is used for separating metals from their solutions, and generally as a reducing agent. For the preparation of hydrogen, and in most other cases, scrap sheet zinc may be used. ~Zinc Oxide~, ZnO.--The commercial oxide sometimes contains carbonate. ~Zinc Sulphate~, ZnSO_{4}.7H_{2}O.--It is occasionally used as a standard, and can be purchased nearly pure. FOOTNOTES: [6] 3HCl + HNO_{3} = Cl_{2} + NOCl + 2H_{2}O. [7] NaC_{2}H_{3}O_{2} + HCl = H_{4}C_{2}O_{2} + NaCl. CHAPTER VII. FORMULÆ, EQUATIONS, ETC. Formulæ and equations are a kind of short hand for expressing briefly and in the language of the atomic theory the facts of chemical composition and reaction. The convenience of this method of expressing the facts justifies a short description of it here. On comparing the percentage composition of a series of compounds the proportions in which the elements combine appears to be regulated by no simple law. For example: Realgar. Orpiment. Mispickel. Pyrites. Arsenic 71.4 60.9 46.0 -- Sulphur 28.6 39.1 19.6 53.3 Iron -- -- 34.4 46.7 ------ ------ ------ ------ 100.0 100.0 100.0 100.0