eighth group proves to be transitional between group seven and group one; iron, cobalt and nickel make a direct gradation from manganese to copper; ruthenium, rhodium and palladium, from molybdenum to silver; osmium, iridium and platinum, from tungsten to gold. Not only do these three triplets stand between these other elements in atomic weight, but their properties also show a similar gradation.
While now we have these transitional elements, the question might very naturally arise whether there are similar transitional elements from fluorin to sodium and from chlorin to potassium. The case here is, however, somewhat different from the former one. Manganese and copper are both metals, and not so widely separated in properties; the transitional elements, iron, cobalt and nickel, partake of the nature of both extremes, and the transition seems a natural one. Hardly any elements can be more unlike than fluorin and sodium, or chlorin and potassium. Chlorin is very electro-negative, potassium as strongly electro-positive. A transitional element would thus probably be inert, that is, lacking in both electro-positiveness and in electro-negativeness, and up to a few years ago such an element could hardly have been conceived of. At that time Lord Rayleigh was engaged in determining with all possible accuracy the density of nitrogen. In this work he prepared nitrogen by several different methods. Some specimens were obtained by the decomposition of chemical compounds, such as urea and ammonium nitrite, others from the air by removing the oxygen. To his surprise. Lord Rayleigh found that in every case the nitrogen obtained from the atmosphere was slightly heavier than that prepared from chemical compounds. In searching for the cause of this difference. Lord Rayleigh and Professor Ramsay, who had been associated with him in this work, found that there is present in the atmosphere a new gas, much like nitrogen in its properties, whose existence, although it is present to the extent of nearly one per cent, had been unsuspected. This gas, christened argon from its inertness, is nearly three times as heavy as nitrogen, and it is this that increases the weight of atmospheric nitrogen slightly above the weight of pure nitrogen, obtained from chemical compounds. Stimulated by this discovery it was not long before Ramsay had isolated from the atmosphere at least two other gases, both characterized by an inertness similar to that of argon. These are helium, whose spectrum had long been known from the fact that this gas is plentiful in the corona of the sun, and neon. It is probable that there are several other similar gases in the atmosphere, and one, xenon, has been recently isolated by Ramsay. It is not uninteresting to note that argon had been in the hands of chemists from the time of Cavendish down, but all had supposed it to be nitrogen. Under the influence of the electric spark oxygen and nitrogen may be made to combine with each other. In Cavendish's experiment
