=Effects of Cold.= Some foods are better for being cold; for example, butter, honey, salads, and ice-cream. Sweet dishes as a rule are improved by a low temperature. The flavor of butter is very different and very much finer when cold than when warm. It is absolutely necessary to keep it cool in order to preserve the flavor. CHEMICAL AND PHYSICAL CHANGES =Chemical Changes.= Since many of the changes which cooking produces in the different food materials are of a chemical nature, it is well to consider what constitutes a chemical process. This idea may perhaps be best conveyed by a few experiments and illustrations, the materials for which may be easily obtained. =Exp. with Cream of Tartar and Bicarbonate of Soda.= Mix two teaspoons of cream of tartar with one of bicarbonate of soda, in a little warm water. A union of the two substances follows and they neutralize each other; that is, the cream of tartar is no longer acid, and the soda is no longer alkaline. Owing to the power of chemical affinities a separation or breaking up of these compounds takes place, and new substances, _carbonic acid_ and _rochelle salts_, are formed out of their constituents. The effervescence which is seen is caused by the escape of the carbonic acid. =Exp. with Hydrochloric Acid and Soda.= Put a few drops of chemically pure hydrochloric acid into a little water; then add soda. A violent effervescence will follow. Continue putting in soda until this ceases, when the reaction should be neutral. Test it with litmus-paper. If it turns blue litmus-paper red, it is acid; if red litmus-paper blue, it is alkaline. Add acid or soda, whichever is required, until there is no change produced in either kind of litmus-paper. The results of this experiment are similar to those in the first one, namely, carbonic acid and a salt. In this case the salt is _sodium chlorid_ or _common salt_, which is in solution in the liquid. Evaporate the water, when salt crystals will be found.[3] =Oxid of Iron.= A piece of iron when exposed to the weather becomes covered with a brownish-yellow coating, which does not look at all like the original metal. If left long enough it will wholly disappear, being completely changed into the yellowish substance, which is _oxid of iron_, a compound of oxygen and iron, commonly called _iron rust_. =Burning of Coal.= A piece of coal burns in the grate and is apparently destroyed, leaving no residue except a little ashes. The carbon and hydrogen of the coal have united with the oxygen of the air, the result of which is largely the invisible gas, _carbonic acid_, which escapes through the chimney. =Formation of Water.= Water is formed by the union of two invisible gases, hydrogen and oxygen. It bears no resemblance whatever to either of them. Its symbol is H_{2}O. All these are examples of chemical changes.