The atom is now being studied along a newer line, that of radio-activity, and it is becoming apparent in many cases there is an active radiation going on. Just where this discovery is going to lead it is impossible to tell, for the study of radio-active substances is as yet in its infancy, and little is actually known. Much of the earlier teaching of physical science has been revolutionised by the discovery of radium, and the more scientists find out, the more it becomes apparent (as they themselves realise), that we are standing on the threshold of very great discoveries, and are on the eve of profound revelations.