The main purpose of our existence on earth-aside from the sacred and paramount duty of securing our salvation-is undoubtedly to make ourselves masters of the tangible world around us, as it stands revealed to our senses, and as it was expressly made subject to our will by the Creator. We are, however, at the same time, not left without information about the existence of certain laws and the occurrence of certain phenomena, which belong to a world not accessible to us by means of our ordinary senses, and which yet affect seriously our intercourse with Nature and our personal welfare. This...