The subdivision, produced by folding the loose page, is intended to show that the elements represented on it have a double set of resemblances. But there are various anomalous and inexplicable phenomena still attached to this arrangement of elements. For example, copper, although it replaces one atom of hydrogen in some of its compounds, and is thus a monad, forms more numerous and more stable compounds in acting as a dyad and replacing two atoms of hydrogen. Gold, which belongs to the same column, is at once univalent and tervalent; mercury, both univalent and bivalent; thallium, univalent and tervalent; tin and lead, bivalent and quadrivalent, and so on. It is as if some elements had a tendency to enter a column not their own. Again, on comparing the atomic weights of the elements, it is seen that the differences are far from being regular. As a rule, the difference in the vertical columns between any single element and the one following it is approximately 16, or some multiple of 16. Thus we have lithium, sodium, and potassium; beryllium, magnesium, and calcium; boron, aluminium, and scandium; carbon and silicon; oxygen and sulphur; fluorine and chlorine--all with a difference of 16 approximately. But here we come to a break: silicon and titanium, phosphorus and vanadium, sulphur and chromium, chlorine and manganese, each show a difference of about 20. Passing on, between the atomic weights of potassium, rubidium, and caesium there is a difference of about 16 × 3; a similar difference between calcium, strontium, and barium; between scandium and yttrium; between titanium, zirconium, and cerium, and so on; but with wider and wider divergence from the supposed constant, 48 = 16 × 3. In short, we have a seeming regularity, but only a very approximate one--a regularity, in fact, in which a vivid imagination must play a conspicuous part in order to detect it. Now, up to the present, no reason has been suggested to account for the divergence from this irregular regularity, which a little expenditure of time will enable any one to trace through all these numbers. But one thing has been remarked; there is the same seeming regularity between certain physical properties of elements and their compounds: their specific volumes, their melting-points, their refractive indices, and other properties vary from member to member of the same column in a manner bearing more or less similarity to the periodic variation of the atomic weights. It happens that among compounds of carbon we are acquainted with series of compounds which, in variation of molecular weights and gradation of properties, bear a striking resemblance to the elements thus arranged. Thus we have the series:-- CH_{4} Methane 16 C_{2}H_{6} Ethane 30 C_{3}H_{8} Propane 44 C_{4}H_{10} Butane 58 C_{5}H_{12} Pentane 72