arguments may be adduced in favor of either view, although neither can be yet demonstrated.
The idea that a very few true elements, uniting together in a variety of proportions, may give rise to all the bodies which we now look upon as elementary, derives perhaps its strongest support from an analogy pointed out by Prof. Cooke something like twenty years ago. He first called attention to the many serial relations which connect the members of any elementary group, and then showed how much these groups resemble the homologous series of organic chemistry. In such a series we have a number of compounds each differing from its immediate predecessor in a very definite way. Thus, in the series of alcohol radicles, we have first the hydrocarbon methyl. Adding to this an atom of carbon and two of hydrogen, we get the second member of the series; the third is formed by the same addition to the second, the fourth similarly derived from the third, and so on. The difference between the molecular weights of any two successive members in this series is always the same. Just so in some groups of elements, as we have already seen. The atomic weight of lithium is seven, add sixteen and we get that of sodium, while another increase of sixteen gives the value of potassium. Again, the atomic weight of sulphur is that of oxygen plus sixteen; three times sixteen more brings us to selenium, and another forty-eight reaches the equivalent of tellurium. Here certain multiples of sixteen are missing; do they correspond to the atomic weights of undiscovered elements? Such a speculation is curious, but not very profitable.
The analogy, then, between the groups of elements and the homologous series of organic compounds is quite striking, although it may not be very precise. Hence Cooke suggested that, if the elements were compounds, their resemblances might be explained by supposing them to form series like the hydrocarbons, in which bodies of similar constitution are akin in general properties. Now, this conception was certainly very brilliant, and rendered intelligible many important facts which before it were unclassified. It did not, however, suggest the possible unity of matter, but merely put the ultimate question regarding the nature of the elements a step farther back. Instead of many, it gave us the idea of few elementary bodies; why and how these differed were yet to be found out. Prof. Cooke was, fortunately, too cautious a chemist to put forward views of this sort dogmatically; he did not offer a theory even; he only made suggestions to be taken later at their true value, whatever that might be.
The other side of the question, that of the unity of matter, has been worked up by several chemists in a variety of ways. Some have studied the phenomena of crystallization and drawn their conclusions therefrom; others have taken up the subject from a dynamical point of view. Given atoms of one kind only, how to arrange these in different aggregations so as to present all the phenomena offered by our
