INTRODUCTION 



IN the following treatment of the physical phenomena of 

 diffusion and osmotic pressure no attempt is made to be 

 exhaustive. Certain aspects only of the present conceptions 

 of these matters among most physicists and chemists 1 are 

 discussed, and every discussion is presented with the sole 

 aim of clearing the way for the physiological discussions 

 which are to follow. Thus, for example, the whole subject 

 of atomic and molecular weights and their experimental 

 determination so important to the chemist, but not pri- 

 marily interesting to the physiologist is entirely omitted. 

 Also it may be added that no attention is given to a his- 

 torical treatment of this part of the subject, the excellent 

 chemical treatises which are now available rendering this 

 unnecessary. 



In the present Part very little is original with the author, 

 excepting the mode of presentation. The various texts and 

 the original papers have been drawn upon wherever it has 

 seemed expedient. Footnotes give the names of the authors 



1 A general confusion among younger students with regard to the way in which 

 these conceptions take the form of theories makes it seem expedient to call atten- 

 tion to the following points : A scientific theory does not pretend to state the truth. 

 It may sometimes state a part of the truth, but this is not primarily its aim. Its 

 aim is to connect the facts together in the most logical and plausible manner pos- 

 sible, and thus to aid the further advancement of our knowledge. Its " employment 

 has its origin in the organization of the human mind, which handles abstract truths 



much less easily by themselves than by the help of an illustrative image A 



hypothesis [or theory] can neither be proved nor disproved. It is merely a tool 



which is rejected when found to be no longer serviceable What the 'real' 



nature of matter is, is to us a matter of complete ignorance, as it is of complete 

 indifference." (OsxwALD-WALKER, Outlines of General Chemistry [London, 1895J, 

 pp. 5, 7.) A principle, on the other hand, does attempt to state the truth ; it is a 

 generalization and induction from a great number of known facts. When a fact is 

 discovered which cannot be included under a principle, then that principle falls to 

 the ground and ceases to be a principle. What was at first a theory may at length, 

 by the accumulation of evidence, come to be a principle. 



