RELATIONS TO OTHER SCIENCES 79 



able and the total energy of a portion of matter. But this discrimi- 

 nation can have no meaning unless it is granted that a portion of the 

 energy of a substance is not available. If we ask why it is not avail- 

 able, the answer may be that when a substance passes from one 

 state to another at constant temperature the work that it can do is 

 less than its total intrinsic energy as a consequence of the laws of 

 thermodynamics. The case must therefore be one to which the second 

 law of thermodynamics can apply. That is, it must involve flow of 

 energy by some such process as heat conduction. 



It might perhaps be successfully argued that the very existence 

 of such a process implies grained structure of some sort to which a 

 statistical law may apply. However this may be, it is certainly diffi- 

 cult to conceive of energy as existing apart from some vehicle, matter 

 or ether or both as you will; but to conceive of this sublimated energy 

 as in part available and in part non-available is surely quite beyond 

 attainment. 



It is with great diffidence that we dissent from the expressed views 

 of one who has done so much for the advance of physical chemistry, 

 and our excuse for entering on the discussion must be that as the 

 latest utterance with regard to matter, and coming from one who has 

 won the right to have his views given a respectful consideration, it 

 seemed more fitting to present this brief and imperfect discussion 

 than to pass them by without comment. 



One of the most important reactions of physics upon the other 

 sciences has resulted from the extension of the thermodynamic laws 

 to chemical problems which has marked the new physical chemistry, 

 a science which has sprung into being within the last seventeen years 

 and has already, under the leadership of van't Hoff, Ostwald, Arrhe- 

 nius, and Nernst, attained a surprising development, and is making 

 itself felt in many other lines of scientific activity, notably in electro- 

 chemistry, geology, and biology. The starting-point in this devel- 

 opment was the idea conceived by van't Hoff that Avogadro's law 

 might be so extended as to apply to the case of substances in solu- 

 tion. Just as a gas expands and fills the containing vessel exerting 

 a pressure against its walls, so a salt dissolved in a liquid diffuses 

 uniformly throughout the liquid and exerts a pressure within the 

 liquid tending to expand it. This osmotic pressure, so called, had 

 been measured in certain cases by Pfeffer and de Vries, but it re- 

 mained for van 't Hoff to show that, as in case of a gas, the pressure 

 was proportional to the absolute temperature and to the number of 

 molecules of the dissolved substance contained in unit volume. 



As has so often happened before, the study of the apparent ex- 

 ceptions to the rule led to a second great advance, the theory of 

 electrolytic dissociation proposed by Arrhenius, to account for the 

 observation that in solutions of electrolytes the osmotic pressure was 



