The newer the science, the more wary must be the steps. In the newest of all the sciences, therefore, which enables us to regard matter wheresoever situated from an entirely new point of view, the investigator's caution much be redoubled, and those who would follow him must be careful to secure firm foothold at every step. Fortunately, the laws and phenomena of nature have such a oneness with their diversity, and are so exquisitely intertwined, that it is possible for us in the consideration of any new branch of science to aid our conceptions by mental images derived from the older sciences or ordinary phenomena, and this is especially true for that science now under consideration.