Identification of Organic Compounds. 39 



760 must then be calculated. If t is the temperature of 

 the air, and /3 the barometric pressure, and T the vapour 

 tension of aqueous vapour at t, then if v is the volume 

 read directly in the eudiometer tube, v the corrected 



273 3 T 



volume = v - 9734.^0 x "T^TT* Now the wei g ht of tne 



corresponding volume of hydrogen is known (22*4 litres 

 weigh 2 grammes). If the weight of substance taken is 

 divided by this amount, the vapour density of the sub- 

 stance (taking that of hydrogen as = 1) is obtained. From 

 Avogadro's hypothesis the molecular weight should be twice 

 the vapour density. 



The Molecular Weight of Substances which cannot be 

 volatilized without Decomposition. When a substance is 

 dissolved in any liquid the freezing points of the latter 

 are lowered and the boiling points raised, and it has been 

 found that the amount of change thus produced is a 

 function of the molecular weight of the dissolved substance. 

 By the determinations, therefore, of these changes pro- 

 duced by solution of a known weight of a given substance 

 in a liquid, the molecular weight of the former can be 

 calculated. It is not possible to enter into the theory of 

 the phenomena (which was worked out fully by van 't 

 HofF) ; it must be noted, however, that according to this 

 theory, correct molecular weight determinations are only 

 to be expected when the depressions in freezing points and 

 raising of boiling points produced by dissolving relatively 

 small amounts of substance in the liquid are measured. 

 In carrying out the measurement of the depression of 

 freezing points, which was first applied to molecular 

 weight determinations by Raoult, and known as the 

 cryoscopic method, an apparatus devised by Beckmann 

 is generally employed. As the actual depression to be 

 measured is, in practice, small, a special form of thermo- 



