November 10, 1911] 



SCIENCE 



625 



only an amateur of thermodynamics, but 

 the results achieved by him in that field 

 are of lasting importance, and his work and 

 the conception of osmotic pressure have 

 given a great stimulus to the cultivation of 

 thermodynamics to chemistry. 



And here we trench on a question on 

 which a certain confusion of thought often 

 exists. To the investigator it is open to 

 choose that one of several equivalent meth- 

 ods or conceptions which best suits his per- 

 sonal idiosyncrasy. To the teacher such a 

 choice is not open. He must choose the 

 method or conception which is most clearly 

 intelligible to students, and is at the same 

 time least likely to lead to misconception. 

 Osmotic pressure is a conception which the 

 chemical student of mediocre mathematical 

 attainments can grasp, and it is not diffi- 

 cult to teach the general elementary theory 

 of dilute solutions by means of it and of 

 reversible cycles without liability to radical 

 error or misconception. I should be sorry 

 on the other hand to try to teach the theory 

 of solutions to ordinary chemical students 

 by means of any thermodynamic function. 

 The two methods are thermodynamically 

 equivalent, and the second is mathematically 

 more elegant and in a way simpler, but it 

 affords less opportunity than the first for 

 the student to submit his methods to any 

 practical cheek or test, and in nine cases 

 out of ten would lead to error and confu- 

 sion. The difficulty of the student is not 

 the mathematical one; with the excellent 

 teaching of mathematics now afforded to 

 students of physics and chemistry the 

 mathematical difficulty has practically dis- 

 appeared — the difficulty lies in critically 

 scrutinizing the conditions under which 

 each equation used is applicable. 



Of the mechanism of osmotic pressure we 

 still know nothing, but with the practical 

 measurement of osmotic pressure great 

 advances have been made in recent years. 



In particular the admirable work of Morse 

 and Frazer is of the first importance in 

 establishing for solutions up to normal 

 concentration the relationship between 

 osmotic pressure and composition, and its 

 variation with the temperature. Much may 

 be anticipated from the continuation of 

 these accurate and valuable researches, the 

 experimental difficulties of which are enor- 

 mous. 



We are indebted to America not only for 

 these researches, and for the voluminous 

 material of H. C. Jones and his collabor- 

 ators dealing with hydrates in solution, but 

 also to A. A. Noyes and his school for 

 accurate experimental work and for syste- 

 matic treatment of solutions on the theore- 

 tical side. They, and also van Laar, have 

 shown how solutions not coming within the 

 ordinary range of dilute solutions to which 

 van't Hoff's simple law is applicable, may 

 in some cases at least be made amenable to 

 mathematical treatment. Van't Hoff chose 

 one simplification of the general theory by 

 considering only very dilute solutions, for 

 which very simple laws hold good, just as 

 they do for dilute gases. Even a single gas 

 in the concentrated or compressed form 

 diverges widely from the simple gas laws; 

 much more then may concentrated solu- 

 tions diverge from the simple osmotic 

 pressure law. The other simplification is 

 to consider solutions of which the compo- 

 nents are miscible in all proportions and 

 are without action on each other; and this 

 method has been developed with marked 

 success from the point of view of osmotic 

 pressure and other colligative properties. 



The outstanding practical problem in the 

 domain of electrolytic solutions is to show 

 why the strong electrolytes are not subser- 

 vient to the same laws as govern weak elec- 

 trolytes. If we apply the general mass-action 

 law of chemistry to the electrically active 

 and inactive parts of a dissolved substance 



