The Romance of Modern Chemistry

HIGH TEMPERATURES These cases of dissociation are of great interest, and there are many common substances which undergo this change on heating. Carbonate of lime in its various forms—limestone, chalk, and marble—is one of them. When heated it breaks up into quicklime (calcium oxide) and carbon dioxide. If the latter were left in contact with the quicklime, then, on cooling, re-combina- tion would take place, and the carbonate of lime would be regenerated. This being so, the reader may ask how it is possible to convert limestone into lime by heating or “ burning ” in kilns. The explanation is quite simple, for in the lime-kilns the carbon dioxide is constantly being removed by the draught, so that when the lime begins to cool, the carbon dioxide with which it would gladly have combined is not there. This interesting phenomenon of a chemical change taking place in one direction at a particular tempera- ture, and in the opposite direction at another tempera- ture, is very well illustrated by one of the common methods for obtaining oxygen from the atmosphere on the large scale. There is a solid compound, somewhat similar to quicklime, known as barium oxide, which at a temperature of 1100° Fahrenheit or thereabout readily takes in more oxygen, forming a new substance —barium dioxide. The grip of the latter, however, on the extra atom of oxygen is not very secure, and by raising the temperature to 1560° it can be so weakened that the gas is released and may be collected. In the actual manufacturing process a current of air is pumped into retorts heated to 1300° Fahrenheit and contain- ing barium oxide, which takes up the oxygen and allows the nitrogen to pass on. When the charging is com- pleted, the current of air is shut off, and the retorts, 195