The chloride of gold and sodium is prepared by dissolving common salt in a solution of chloride of gold and then evaporating the solution. Sodium chloro-aurate is also another name for this salt. Chloride of sodium is common salt, and the chemical formula is NaCl, meaning that it is composed of one part natrium (the Latin name for sodium) and one part chlorine. The chemical formula for chloride of gold and sodium is NaCl, AuCl_{3}+2H_{2}O, meaning that it is composed of one molecule of chloride of sodium, one molecule of chloride of gold, to which are added (+) two molecules of water. The chemical formula is also written in this way: NaAuCl_{4}+2H_{2}O. When chloride of gold and sodium is used for toning, a larger quantity by weight must be used than when the pure chloride of gold is used. A stock solution may be prepared by adding 15 grains of chloride of gold and sodium to 7-1/2 oz. of water. (By a "stock solution" is meant a solution that keeps for a long time, and may therefore be prepared in a large quantity.) The toning-bath is made by taking 3-1/2 oz. of water and pouring in the gold solution till the mixture will turn blue litmus-paper red. (About half an ounce will be sufficient.) To this mixture add bicarbonate of soda until it turns the red litmus back to blue. This bath should be prepared about half an hour before it is needed for toning. A saturated solution (see first paper on simple chemistry) should be made of bicarbonate of soda, and kept in stock. Bicarbonate of soda is a fine white powder, soluble in ten parts of water. It is used for neutralizing the excess of acid in gold toning-baths. Natural deposits of bicarbonate of soda are found in Africa, where it is called "trona," and in South America, where it is called "urao." Its chemical formula is HNaCO_{3}. Names of chemical elements mentioned and their atomic weight: Atomic Chemical Element. Symbol. Weight. Carbon C 12 Chlorine Cl 35.5 Gold (Latin name Aurum) Au 196 Hydrogen (standard weight) H 11 Oxygen O 12 Sodium (Latin name Natrium) Na 23 Hydrogen is the lightest substance known, and an atom of hydrogen is used as the standard weight by which all other atoms of the chemical elements are weighed.