VI 



PREFACE 



discoverers of the facts they must learn ; and, when they do associate 

 names, they are frequently incorrect. Naturally so, since the dis- 

 coverers are, as a rule, mere names to those commencing the study 

 of the subject. For this reason, names, for the most part, have been 

 omitted, with the exception of those of a few outstanding men of 

 genius, such as Faraday, Claude Bernard, or Ludwig, which ought 

 to be familiar to all who claim to be educated. I would therefore 

 ask pardon of my colleagues who may find their discoveries referred 

 to without their names. At the same time, I have not hesitated to 

 refer to any personal or historical fact which might give added 

 interest to a dry description. Similarly, the only references given 

 are to my own larger " Principles," where those desirous of more 

 detail can find what they require, or, at any rate, indication of where 

 to find it. These references have the letter P prefixed to the page 

 indicated. 



It is to be feared that the exposition may be regarded as too 

 dogmatic. This may be so, but it is of set purpose, and the 

 arguments have been carefully weighed before making a statement. 

 Where there is obviously insufficient knowledge, it is preferable to 

 state that this is the case, rather than to confuse the student by 

 conflicting views. In my early student days I was repeatedly pre- 

 sented with arguments on both sides, about as many on one side as 

 on the other, so that a blank remained, not only in the note-book, 

 but in my memory. If only a view is clearly grasped, it can easily 

 be corrected later, if necessary, since the point where it fails can 

 be seen and understood. 



On the whole, it would have been preferable to leave details of 

 experiments to be shown to the student, or made by himself, to the 

 discretion of the individual teacher, in accordance with the resources 

 of his laboratory. But it has been represented to me that the value 

 of the book would be much increased if such details were included. 

 This has accordingly been done. As will be seen, however, the 

 experiments vary a good deal in their simplicity. Some of them 

 require apparatus that a particular laboratory may not possess, and 

 must perforce be omitted. On account of their importance, never- 

 theless, it would have been misleading to omit them on the ground 

 that the necessary means may not always exist. The value of 

 experiments, especially when made by the student himself, is very 

 great. They give a reality to description in words and a belief in 



