The molecular weights of some of the elements have been determined by Raoult's method, either by the lowering of the vapour-pressure of mercury, or by the depression in the freezing-point of some other metal or solvent in which the element has been dissolved. Lithium, sodium, potassium, calcium, barium, magnesium, cadmium, gallium, thallium, manganese, silver, and gold appear to be mono-atomic, while tin, lead, aluminium, antimony, and bismuth show tendency in concentrated solution to associate to di-atomic molecules. These results were obtained by measuring the lowering of vapour-pressure of mercury produced by known weight of the metals named. By measurement of the depression in the freezing-point of tin, in which metals were dissolved, zinc, copper, silver, cadmium, lead, and mercury appeared to be mono-atomic, while aluminium was found to be diatomic. These results, however, are not to be regarded with the same confidence as those obtained by means of measurements of the vapour-density, for it is not certain whether the molecular weight of the solvent should be taken as identical with its atomic weight. All that can be certainly affirmed is, that the molecular weights of the elements which have been placed in the same class above correspond to formulas with the same number of atoms in the molecule. Thus, if zinc is mono-atomic, so is cadmium ; if di-atomic, cadmium has also a di-atomic molecule; and similarly with the rest.

A method has also been devised, depending on the capillary rise of liquids in narrow tubes, by means of which it is possible to estimate the molecular complexity of liquids. This method is applicable to only a few elements; but by its use it has been found that in the liquid state bromine consists chiefly of di-atomic molecules mixed with a few tetra-tomic molecules ; and that phosphorus in the liquid, as in the gaseous condition, forms molecules corresponding to the formula P₄.