The history of science may be approached from two points of view. It may be regarded purely as growth of knowledge of natural objects and phenomena along the corridors of time, or it may be viewed selectively according to its influence upon social and economic conditions. Knowledge becomes power only when it is used. To trace the development of any particular aspect of it in scientific fields is the work of the specialist, but it is not until discoveries and the inventions to which they lead can be recognised as transforming factors in civilisation that the historian of human society is much concerned with them.