1' ' H K A R V 



I VKKs IT v OF 



PREFACE. 



IHROUGHOUT the following 



pages it has been the Author's 

 aim to present the subjects of 

 which they treat in a manner 

 entirely compatible with the re- 

 quirements of a book intended 

 for popular use. Necessarily, 

 therefore, the work has features 

 very different from those which might be expected in a history 

 of graver pretensions. The general reader is here supplied 

 with simple illustrations of laws and principles, and with 

 ample details of experiments and observations, which illustra- 

 tions and details would in no wise be needed by the accom- 

 plished student. An expert in this or that branch of science 

 might perhaps complain that we have not always brought 

 into view those highly generalized conceptions of modern 

 thought with whose value and importance he is familiar. 

 But the highly general and abstract character which gives 

 philosophic value to such conceptions render them unsuitable 

 for popular presentation, for they must ever remain unmean- 

 ing and unintelligible to minds not prepared for their recep- 

 tion by profound and systematic study. Without, however, 

 assuming the possession on the part of the reader of other 

 attainments than those possessed by the average schoolboy or 

 schoolgirl, who has had the ordinary opportunities of witnessing 

 experiments at popular lectures, we believe that it is possible 



