THE MOLFXULAR WEIGHT OF LIQUIDS. 



THE properties of matter may be conveniently divided 

 into three main classes: i., Additive; ii., Constitutive; 

 iii., Colligative. 



In determining- the molecular weight of a substance, no 

 matter whether it be gaseous or liquid, it is necessary to 

 make use of some colligative property of the matter under 

 investigation. The present article will therefore deal al- 

 most exclusively with colligative properties, or rather 

 with a particular colligative property of liquids. As 

 regards the molecular weight or complexity of solid sub- 

 stances it is well to state, at the outset, that our stoichio- 

 metrical knowledge of the properties of solids is so limited 

 that up to the present time we have absolutely no reliable 

 information on this point. 



If a substance plays a part in a chemical phenomenon in 

 such a way that any particular property of the substance re- 

 mains unaltered or independent of its state of chemical com- 

 bination, then this property is said to be additive. The best 

 known additive property of matter is its mass. Thus, the 

 mass of a substance undergoes no alteration during a change 

 in its state of chemical combination. This fact finds its expres- 

 sion in the well-known law of the " Conservation of Matter ". 

 Properties, such as melting- and boiling-points, molecular 

 volume, etc., which depend to a large extent on the arrange- 

 ment of the atoms in the molecule, have been termed con- 

 stitutive. It very often happens, however, as in the 

 examples just quoted, that a substance exhibits properties 

 which are partially additive and partially constitutive. 



The third group of properties is characterised by the 

 fact that for chemically comparable quantities of the most 

 widely different substances, a colligative property has 

 always the same value. Whilst the mass of a compound is 

 the sum of the masses of the elements of which it is com- 

 posed, the volume of a gas, under the same conditions of 

 temperature and pressure, is a colligative property and is 



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