This text-book contains, in plain and simple language the results of a series of experiments carried out during a period of seven years. With the exception of a few conducted by that master scientist Sir William Crookes, President of the Royal Society, London, and referred to in Chapter III. (to which mine were in the nature of a sequel), all were carried out in my presence; and the reports on these experiments have been collated from the official records kept by my three research societies in London.