For long it had been noted that certain elements displayed a marked similarity with each other. Thus the metals sodium and potassium, discovered by Sir Humphry Davy, are both white, soft, easily oxidisable metals, forming soluble salts with almost all acids ; these salts resemble each other in colour, in crystalline form, and in other properties. The subsequently discovered metals, lithium, rubidium, and caesium, have also a strong resemblance to potassium and sodium. Their atomic weights also increase progressively ; thus we have the series, Li = 7, Na=23, K = 39-i, Rb = 85, and Cs = 133. Similar series had been noticed with calcium, strontium, and barium ; magnesium, zinc, and cadmium ; and so on. It was not until 1863 that John Newlands called attention to the fact that if the elements be arranged in the order of their atomic weights a curious fact becomes noticeable. It is that, omitting hydrogen, the first, the eighth, the fifteenth, and, in short, all elements may be so arranged that the " difference between the number of the lowest member of a group and that immediately above it is 7 ; in other words, the eighth element starting from a given one is a kind of repetition of the first, like the eighth note of an octave in music." This idea was subsequently discovered independently and elaborated by Lothar Meyer and by D. Mendele'eff, and it has now been adopted, in spite of some difficulties, as the ground-work of classification of chemical substances.

The table may be given in the following form, although there are many ways of representing the order in which the elements lie:—

The Atomic Weights of the Elements arranged according to the Periodic System

¹ It is a matter of indifference which element is placed first on the list. The most convenient form to give the diagram is that of two cylinders, on which the elements follow spiral lines, so that oxygen and fluorine, sulphur and chlorine, follow each other round the smaller cylinder, while selenium and bromine, tellurium and iodine, <xc., are conspicuous round the larger cylinder.

It will be noticed that the number of elements in the first two horizontal rows is not seven, but eight, and that, consequently, every ninth element, and not every eighth, presents similarity with its predecessor in the vertical columns. This is owing to the recent discovery of the elements in the second vertical column. It will also be seen that it is possible, by folding the projecting slip to one side or other, to bring new sets of elements in the third and succeeding horizontal rows beneath the elements in the first and second. The first and second rows are termed " short periods," the others, " long periods." It appears that by so arranging the elements, analogies are brought out more striking than if such long and short periods were not adopted.