44 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



vapour as compared with the weight of an equal volume of 

 hydrogen. 1 The number of molecules being the same in the two 

 equal volumes according to Avogadro's law, then, assuming 

 the weight of a molecule of hydrogen to be 2, the molecular 

 weight of the substance is twice the vapour density, and the 

 molecular formula can therefore now be readily deduced from 

 the empirical formula. Thus, supposing that the vapour 

 density of the substance, whose empirical formula was calcu- 

 lated above, was found to be 44, this gives a molecular 

 weight of 44 X 2 = 88. The molecular formula will be 

 CoHiOa as the atoms are in the same ratio as in the empirical 

 formula, and the sum of their atomic weights equals 88. 



There are of course a great many substances which cannot 

 be vaporised without decomposition ; in such cases it is 

 impossible to determine their molecular weights, and conse- 

 quently their molecular formula), by measurement of their 

 vapour density as compared with hydrogen. It has, however, 

 been shown by the experiments of van't Hoff, Raoult and 

 others, that in dilute solutions the molecules of the dissolved 

 substance behave as if they were in the gaseous state, and a 

 specific effect is produced on the melting and boiling-point of 

 the solvent, proportional to the molecular weight of the dis- 

 solved substance. By determining the rise of boiling-point, or 

 the lowering of the melting-point of a solvent, produced by a 

 known weight of the dissolved substance, and comparing these 

 values with those obtained when an equal weight of a substance 

 of known molecular weight is dissolved, the molecular weight 

 of the first substance can be deduced. 



Various other means of a somewhat indirect character are 

 made use of in certain special cases ; e.g., the determination of 

 the osmotic pressure of a solution of known concentration may 

 be employed as indicated in Chapter I. 



1 In practice a given weight of liquid is converted into vapour and 

 the volume of this vapour measured by allowing it to replace an equal 

 volume of air. 



