This is neither a technical manual, nor a treatise dealing with the history of a particular branch of applied science, but it partakes somewhat of the character of both. It is an attempt—perhaps somewhat bold—to present in a popular form an account of the great industry which has arisen out of the waste from the gas-works.
ture of certain chemical products, such as the alkalies. We have therefore in coal a substance which supplies us with the power of doing work, either mechanical, chemical, or some other form, and anything which does this is said to be a source of energy. It is a familiar doctrine of modern science that energy, like matter, is indestructible. The different forms of energy can be converted into one another, such, for example, as chemical energy into heat or electricity, heat into mechanical work or electricity, electricity into heat, and so forth, but the relationship between these convertible forms is fixed and invariable. From a given quantity of chemical energy represented, let us say, by a certain weight of coal, we can get a certain fixed amount of heat and no more. We can employ that heat to work a steam-engine, which we can in turn use as a source of electricity by causing it to drive a dynamo-machine. Then this doctrine of science teaches us that our given weight of coal in burning evolves a quantity of