(3) Extended application of machinery. New industrial arts owing their origin to scientific inventions and their practice to machinery arise for utilising waste products. Under "waste products" we may include (_a_) natural materials, the services of which were not recognised or could not be utilised without machinery--_e.g._, nitrates and other "waste" products of the soil; (_b_) the refuse of manufacturing processes which figured as "waste" until some unsuspected use was found for it. Conspicuous examples of this economy are found in many trades. During the interval between great new inventions in machinery or in the application of power many of the principal improvements are of this order. Gas tar, formerly thrown into rivers so as to pollute them, or mixed with coal and burnt as fuel, is now "raw material for producing beautiful dyes, some of our most valued medicines, a saccharine substance three hundred times sweeter than sugar, and the best disinfectants for the destruction of germs of disease." "The whole of the great industries of dyeing and calico-printing have been revolutionised by the new colouring matters obtained from the old waste material gas tar."[67] These economies both in fuel and in the utilisation of waste material are largely due to the increased scale of production which comes with the development of machine industry. Many waste products can only be utilised where they exist in large quantities. § 4. If we trace historically the growth of modern capitalist economies in the several industries we shall find that they fall generally into three periods-- 1. The period of earlier mechanical inventions, marking the displacement of domestic by factory industry. 2. The evolution of the new motor in manufacture. The application of steam to the manufacturing processes. 3. The evolution of steam locomotion, with its bearing on industry. As these periods are not materially exclusive, so also there are close economic relations subsisting between the development of machinery and motor, and between the improvements in manufacture and in the transport industry. But in order to understand the nature of the irregularity which is discernible in the history of the development of machinery, it is essential to consider these factors both separately and in the historical and economic relation they stand to each other. For this purpose we will examine two large staple industries, the textile and the iron industries of England, in order that we may trace in the chief steps of their progress the laws of the evolution of modern machinery.