294 SCIENCE PROGRESS. 



SOLUTIONS. 



An interesting- lecture (87) on the origin of the new 

 theory of dilute solutions, and dealing also with the present 

 position of the theory, was delivered by van't Hoff to the 

 German Chemical Society early in 1894. In it he makes 

 clear how he was able by means of the work of Pfeffer and 

 De Vries, that is, by means of the conceptions of osmotic 

 pressure and semi-permeable partitions, to correlate the 

 different properties of dilute solutions by the application of 

 thermodynamical principles, and how the^a^ of the case 

 showed that solutions obeyed laws identical with those of 

 gases. It is also clearly pointed out that these laws do not 

 constitute a theory of solutions. The theory of solutions 

 begins, according to van't Hoff, when the exceptions to the 

 above simple laws, those, say, of electrolytic solutions, are 

 interpreted as being of a secondary nature. This is 

 obviously the rational view of the matter, for it is only 

 when attempts are made to fix the molecular state of dis- 

 solved substances that anything of a hypothetical nature is 

 introduced. An important paper bearing on this question 

 of molecular state has been brought forward by Duhem 

 (88), who gives a formal proof of the theorem that every 

 formula obtained by the application of thermodynamical 

 considerations alone to a mixture or solution remains the 

 same, no matter what assumptions be made regarding the 

 molecular condition of the substances. It follows, there- 

 fore, that thermodynamics alone cannot decide whether 

 dissolution is attended by change chemical or otherwise in 

 the molecular state of the dissolved substance. 



Osmotic pressure. — An attempt to throw light upon the 

 action of semi-permeable membranes has been made by 

 Ramsay (89). A platinum vessel connected with a 

 manometer, and containing nitrogen, carbon dioxide, car- 

 bon monoxide, or cyanogen, was closed by a palladium 

 plate and immersed in hydrogen, or hydrogen mixed with 

 other gases, the temperature being kept constant, generally 

 at about 280 . Since palladium acts as semi-permeable 

 membrane, allowing hydrogen to traverse it, the question 

 was : Would the pressure of the hydrogen inside and outside 



