The most familiar use of this compound is indicated by its common names “baking-soda” and “bread-soda.” It is mixed with dough or other similar material in order to keep this from settling down to a hard solid mass in baking. The way in which bicarbonate of soda prevents this will be readily understood when it is remembered that an ounce of this substance liberates more than 2,300 cu. in. of carbon dioxide when it is heated. When the bicarbonate of soda is well mixed with the ingredients of the cake or loaf and disseminated throughout the mass, each particle will furnish (let us say) its bubble of gas. Since these cannot escape, a honey-combed structure is produced. [Illustration: Fig. 14. THE SOLVAY PROCESS] Baking powder is a mixture of bicarbonate of soda and ground rice; the latter substance is merely a solid diluent. The Solvay Process. Soda ash is one of the principal forms of mild alkali used in commerce. Large quantities of this substance are made by heating bicarbonate of soda. We shall now consider another alkali process in which this substance is the primary product. For the greater part of the first century of its existence, the Leblanc soda process had no rival, although another method, known as the ammonia-soda process, was patented as early as 1838. In this case, however, as in many others, expectations based on the experiments carried out in the laboratory were not realized when the method came to be tried under manufacturing conditions. It was not until 1872 that Ernest Solvay, a Belgian chemist, had so far solved the difficulties, that a new start could be made. In that year, about 3,000 tons of soda were produced by the ammonia-soda or Solvay process, as it has now come to be known. Since then, however, the quantity produced annually has been steadily increasing, until at the present time it amounts to more than half of the world’s supply. The Solvay process is very simple in theory. Purified brine is saturated first with ammonia gas and then with carbon dioxide. Water, ammonia, and carbon dioxide combine, forming ammonium bicarbonate, which reacts with salt (sodium chloride), producing sodium bicarbonate and ammonium chloride.