It will be noticed that the angle of dolomite lies between those of calcite and magnesite. It will not be necessary to go further into this subject of isomorphism for our present purpose. Vapour-Density Determinations. We must not conclude this chapter without mentioning the value of vapour-density determinations in the fixing of atomic weights. The method cannot be used for the actual determination of atomic weights owing to the errors of experiment involved by the use of the high temperatures required in some cases for the volatilisation of the compounds. However, it affords an excellent means of ascertaining the maximum atomic weight of an element, when its equivalent has been found by analysis. The compounds most frequently employed are the halides or hydrides, these being usually the most easily volatile. Of the three methods of determination available, namely, those of Dumas, of Hofmann, and of Victor Meyer, the first and last are the most frequently used. In cases where the substance does not volatilise below a high temperature, Victor Meyer's method is always used. A description of these various methods and a discussion of their respective advantages and disadvantages would be superfluous, as these are generally well known to most students of general chemistry, and further, they are all well described in the different text-books on the subject. As an example of the many cases in which the knowledge of the vapour density of a compound has been the means of settling the atomic weight of an element, beryllium chloride may be cited. The formula assigned to this body was BeCl3, and the equivalent of beryllium being 4.6, its atomic weight would thus be 3×46=138. This was disputed because such an element would not fit in with the periodic classification. A determination of the vapour density of the compound gave 4042. Hence the molecular weight must be 80.84. Obviously, therefore, there cannot be more than two atoms of chlorine in the molecule. Assuming the formula to be BeCl2, beryllium would have an atomic weight of 9.2, and the vapour density of the chloride would be (9.2+70-92)=4006, which is sufficiently near to the observed vapour density to justify the assumption of the formula chosen. This is only one of many such cases, but it serves to show the method of applying the result of a vapour-density determination to the settling of atomic weights. The following papers on the subject may be consulted: Dumas, Ann. Chim. Phys. (2), 33 (1826), p. 341. Do., do., 9 (1868), p. 1304. Victor Meyer, Berichte, 10 (1868), p. 2068. Victor Meyer, Berichte, 11 (1869), pp. 1867 and 2253. Demuth and V. Meyer, Berichte, 23 (1875), p. 311. The various cases to which these physical laws have been applied will be found in the next chapter in that portion dealing with the selection of the atomic weights. Their great value will be quite obvious, and this is probably a sufficient excuse for their introduction here, although, of course, strictly speaking, they do not come under the category of the Periodic Law. CHAPTER III A DISCUSSION ON THE PERIODIC LAW (A) The Relations existing between the THE ideas of Mendeléeff and Lothar Meyer were at first difficult to carry through, owing to the fact that there was no systematic way of selecting the true atomic weight from the possible values, and further, that several determinations of this important constant had proved to be false. Mendeléeff was most energetic in the task of setting these points right, for he thoroughly believed in his discovery, and we shall see later how justified he was in so doing. Some of the values had to be doubled to make them fit into the table and others had to be halved, whereas again several were either a few units too high or too low. However, it was found that the determinations, and not the law, were at fault. The only exception was tellurium, which will be found discussed below. 1 Lothar Meyer's table and Mendeléeff's first table are practically identical. They are, however, both given here for the sake of completeness. The table here reproduced is not the original one, but a slight modification of it. The latter was given |