28 MOLECULAR WEIGHT AND POLYMERISM 



thing possible for these latter substances. We will therefore 

 next develop that theory, and then, in connexion with it, 

 give the methods for determination of molecular weight, 

 with their results and applications. 



A. The Theory of Dilute Solutions. 



Whilst for the determination of molecular weight in 

 dilute gas Avogadro's law can be applied directly, for dilute 

 solutions the most varied methods, based on measurements 

 of freezing points, boiling points, and vapour pressures, are 

 in use. All these methods, however, may be brought into 

 connexion with, and regarded as deductions from a law 

 exactly corresponding to that of Avogadro, but which refers 

 to osmotic instead of gas pressure; the law states, in fact, that 

 solutions that exert the same osmotic pressure at the same 

 temperature contain equal numbers of dissolved molecules in 

 unit volume. This law may be arrived at in various ways. 

 We shall here start from the molecular properties of known 

 gases, and follow the deduction through gaseous solutions. 



i.. Henry's Law and the Constitution of the Dissolved Gas. 



Take any gas of known molecular constitution, e. g. 



nitrogen (N. ? ) and a liquid, water, in which it is capable of 

 solution, and consider the question whether 

 the dissolved nitrogen corresponds to the 

 formula N 2 or possibly is present as N or 

 N 4 or as a hydrate. We have then to con- 

 sider the equilibrium that is set up when the 

 water B (Fig. 3) is saturated with the nitrogen 

 Fig. 3. A under the existing conditions of tempera- 



ture and pressure. Kinetically considered, 



this equilibrium is based on the fact that, in unit time, as 



many gaseous molecules enter the solution B as leave the 



latter l . 



1 In the following considerations the vapour of the solvent, present 

 in A, may be supposed absent, by covering B with a membrane permeable 



