Chemistry and Physios. 221 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. On the Determination of molecular mass by means of vapor 

 pressure. — As early as 1878, Raoult had observed a relation 

 between the diminution of the vapor pressure of a saline solution, 

 the lowering of its freezing point and the molecular mass of 

 the substance dissolved. The relation between the molecular 

 mass and the depression of the solidifying point was developed in 

 1886 (Ann. Chim. Phys., VI, viii, 3 1 V, July, 1886). He has now 

 investigated the relation between the molecular mass of a sub- 

 stance and the lessening of the vapor pressure of a liquid in 

 which it is dissolved, taking ether as the particular solvent em- 

 ployed. He finds that for substances whose vapor pressures are 

 very low compared with that of the ether, there is a relation 



f 

 expressible by the formula 100^-=100— KN; in which f and f 



are the vapor pressures of the ether alone and of the etherial solu- 

 tion respectively, at the temperature of the experiment, N the 

 number of molecules of the substance in 100 molecules of the 

 solution and K a constant depending on the character of the dis- 

 solved substance, whose value is not far from unity. Thus for 

 turpentine, aniline and ethyl benzoate it was found to be 0*90, for 

 methyl salicylate - 82 and for nitrobenzene 0*70. Since these 

 values do not greatly vary, K may be taken as equal to unity for 

 all substances, provided that the solutions be dilute, the value of 

 N not being greater than 15. Where N is less than 3, the rela- 

 tion is more complex, arising probably from the fact that the 

 condition of the substance in solution changes with the degree 

 of concentration. If in the above formula N be replaced by 

 100 — N', in which N' represents the number of ether molecules 

 contained in 100 molecules of the solution, the above expression 



f 



(taking K as unity) becomes 100 *^-=N'; or, in etherial solutions 



of moderate concentration, the partial pressure of the ether vapor 



is proportional to N', the number of ether molecules existing in 



100 molecules of the mixture, and is independent of the nature of 



the dissolved substance. Direct experiment showed the ratio of 



f':f to be independent of temperature between 0° and 21°. 



f 

 The first expression above given 100 — = 100— KN may be put 



f—f KN / 



into the form / = . The first member of this equation 



/ 100 H 



is called the relative diminution of vapor pressure, for the given 

 solution; and hence the law that for all etherial solutions of the 

 same character, the relative diminution of vapor pressure is pro- 

 portional to the number of molecules of substance which are dis- 

 solved in 100 molecules of the solution. Moreover, since K is 



