and a host of others up to a compound of the formula C_{30}H_{62}; in each case there is a constant difference of 14 between the molecular weight of any one hydrocarbon and that immediately preceding or succeeding it in the column. Such a series is termed a homologous series. The analogy is very tempting; to suppose that a similar constant difference should exist in the relations of the atomic weights of the elements, and that they too are undecomposable compounds of two unknown elements, is an attractive hypothesis, but one for which there exists no proof; indeed, it is rendered improbable by the irregularities just pointed out. But there is one noticeable feature in the periodic arrangement of the elements. It is, that although the differences are irregular (_e.g._ between B = 11 and C = 12 the difference is 1, while between O = 16 and F = 19 the difference is 3), yet there is no marked displacement in the _order_ of arrangements of the elements, inasmuch as no element has an atomic weight _lower_ than that preceding it in the horizontal line. It was for some time supposed that tellurium and iodine were thus misplaced; and indeed it is even now not quite established that they are not, but the balance of evidence is in favour of tellurium having a lower atomic weight than iodine. Argon, however, is a marked exception. With an atomic weight of 39·88, its natural position would lie between those of potassium and calcium; but there is no room for it. And for this reason considerable doubts have been thrown on the validity of the conclusion to be drawn from the found ratio of its specific heats, 1⅔, viz. that its molecule and its atom are identical. If it were a diatomic gas, like chlorine or hydrogen, its atomic weight would be 19·94, and it would find a fitting position after fluorine and before sodium. And the difference between its atomic weight and that of helium, to which the atomic weight 2·1 would for the same reasons then attach, would be 17·84, one not incomparable with 16. But, as before remarked, it is difficult, if not altogether impossible, to conceive of a diatomic structure to which all energy imparted in the form of heat should result in translational motion, and as a matter of fact none such is known. There are two methods of escape from this dilemma. If the gases termed argon and helium are not single elements, but mixtures of monatomic elements, then what has been termed their atomic weights will represent the mean of the atomic weights of two or more elements, taken in the proportion in which they occur. For example, supposing that argon is a mixture of an element of atomic weight 37 with one of atomic weight 82, the found atomic weight, nearly 40, would imply a mixture of 93·3 per cent of the lighter, with 6·7 per cent of the heavier element. We must therefore carefully examine all evidence for or against the supposition that argon is a mixture of elements.