It is believed that the development of our knowledge during the last few years fully justifies this course. The theory of ionization has also been freely used, as the only means we have by which a large class of phenomena can be clearly presented and understood. It seems desirable to give some material which it is not possible to emphasize or teach thoroughly in a brief course and some things which are rather for reference than to be learned. To aid teachers and students in distinguishing such paragraphs, they are indicated by an asterisk. This device, which was, I think, first used by...