(,(> BOILING AND FREEZING POINT. chap.
these axe due either to accident or to the characteristic pecu- liarities possessed by every method; in this respect the large number of possible methods of determining the mole- cular weight is of great importance.
Review of the Results obtained. — These various methods opened up to the investigator a new world which was formerly regarded as quite unattainable. Up till the time of the discovery of these methods the molecular weight was only known for a limited number of substances, namely, those which could be gasified. On account of the great theoretical importance of the molecular weight, a scheme was drawn up from these few results which was supposed to cover the whole field of chemistry. The funda- mental doctrine of this was that free valencies of the atoms cannot occur. This was beKeved to be a reason why two hydrogen atoms always combine to form a molecule; for if the molecule of hydrogen consisted of a single atom it would possess an unsaturated, valency. It is true that the gas densities of mercury and cadmium show that the molecules of these elements consist of single atoms, but as they are divalent the difficulty was got over by assuming that the two valencies of an atom saturated each other. It was, however, later found that the molecule of certain monatomic metals also consisted of a single atom ; and the same was found to be true for bromine and iodine at high temperature. It was then considered as satisfactory to say that at high temperature the doctrine of valency lost its validity, and little importance was attached to the so-called exceptions.
By the newer methods of determining the molecular weight it has been proved that also at low temperature — for instance, at the melting point of mercury — the molecules of the metals, monovalent as well as polyvalent, are as a rule monatomic. It has already been pointed out that the atoms of sulphur and phosphorus form molecules of the same magnitude, namely, Sg and P4, both in the gaseous and in the dissolved state. It would, therefore, seem as if the
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