Oxalic Acid. Oxalic acid and its salts, the oxalates, are very widely distributed in the vegetable kingdom. These compounds are present in wood sorrel (_Oxalis acetosella_), in rhubarb, in dock, and in many other plants. The acid is made on a large scale by mixing pine sawdust to a stiff paste with a solution containing caustic soda and potash. The paste is spread out on iron plates and heated, care being taken not to heat the mixture to the point at which it chars. The mass is then allowed to cool, and is mixed with a small quantity of water to dissolve out the excess of alkali. This is recovered and used again. Sodium oxalate, which is the main product of the reaction described above, is dissolved in water and treated with milk of lime, whereby insoluble calcium oxalate is obtained, which is subsequently decomposed with sulphuric acid, yielding oxalic acid. Potassium hydrogen oxalate is sometimes called _salts of sorrel_, and potassium quadroxalate, _salts of lemon_. The most familiar use of the latter substance is in the removal of ink stains. Oxalic acid and its salts are poisonous. The free acid has sometimes been mistaken for sugar with fatal results. Formic Acid (_L. formica_, an ant) is found both in the vegetable and in the animal kingdom. If the leaf of a stinging nettle is examined with a microscope, it is seen to be covered with long pointed hairs having a gland at the base. This gland contains formic acid. When the nettle is touched lightly, the fine point of the hair punctures the skin, and a subcutaneous injection of formic acid is made, which quickly raises a blister. The inconvenience which arises from the stings of bees and wasps, also from the fluid ejected by ants when irritated, is due to formic acid. The remedy in each case is the same; the acid must be neutralized as quickly as possible with mild alkali, such as washing soda. Formic acid was first made by distilling an infusion of red ants. It is now made from glycerine and oxalic acid. The Fatty Acids. Animal fats and vegetable oils are similarly constituted bodies. They are composed mainly of three chemical compounds known as stearin, palmitin, and oleïn. Of these, stearin and palmitin are solids at ordinary temperatures, while oleïn is a liquid. Hard fats like those of mutton and beef are composed mainly of stearin; fats of medium hardness contain stearin, palmitin, and some oleïn; while oils such as cod-liver oil and olive oil are nearly pure oleïn. Stearin, palmitin, and oleïn are analogous in composition to salts. Their proximate constituents are glycerine and certain organic acids, stearic, palmitic, and oleïc respectively. In order to obtain the fat free from tissue which it contains in its natural state, it is tied up in a muslin bag and heated in boiling water. The fat is squeezed out through the meshes of the fabric and floats on the surface of the water as an oil which solidifies on cooling. This clarified fat is called tallow.