i66 POPULAR SCIENCE MONTHLY. 



eighth group proves to be transitional between group seven and group 

 one; iron, cobalt and nickel make a direct gradation from manganeee 

 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 Ea)^leigh 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 Eayleigh 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 differ- 

 ence. Lord Eayleigh and Professor Eamsay, 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 Eamsay 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 Eamsay. 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 nitro- 

 gen. Under the influence of the electric spark oxygen and nitrogen 

 may be made to combine with each other. In Cavendish's experiment 



