GENESIS OF THE ELEMENTS 159 



methods of analytical chemistry will be so far improved as to 

 enable the chemist to discriminate between the several assumed 

 varieties of such an element as lead. 



Whether that will come about or not it is clear that the 

 supposed " isotopes," 1 as they have been called by Professor 

 Soddy, are much more like each other than any others of the 

 elements standing equally close together in respect to atomic 

 weight. Take cobalt and nickel, for example, with atomic 

 weights differing from each other by little more than a quarter 

 of a unit ; Co = 58-97 ; Ni = 58-68 ; difference - 0-29. These 

 two metals resemble each other very closely, and if it had not' 

 been for the fact that the salts of cobalt are generally red, while 

 those of nickel are green, it is quite possible that if they had 

 both given compounds of the same colour they might have been 

 for a long time confused together. They are not distinguished 

 by any great differences in properties which do not involve 

 colour or degree of solubility, etc., till they are put through 

 some of the less common transformations, e.g. the production 

 by cobalt of a complicated series of ammoniacal compounds for 

 which there is no analogy among the compounds of nickel. If 

 the final products of radio-active disintegration are ever obtained 

 in appreciable quantities, it may turn out that they do present 

 differences of character though not recognisable by the instru- 

 ments or agencies at present at our disposal. 



It will have been noticed by the reader that the radio-active 

 elements thus far referred to are found among the last few 

 members of the series in the periodic table possessing the highest 

 atomic weights known. 2 A very minute activity is exhibited by 

 various ordinary materials, but this appears to be attributable 

 to the wide distribution of such substances as radium and 

 thorium in extremely attenuated quantities. The only elements 

 of relatively small atomic weight which have been found to 

 exhibit an appreciable activity, though weak, are potassium and 

 rubidium. According to Professor Rutherford no a rays are 

 emitted but only /3 rays, and " the activity due to a potassium 

 salt is not more than x^Vir of the activity of the ft rays due to a 

 equal weight of uranium." 



1 From Greek tcros equal, TOTTOS a place. 



2 On the other hand, it may be remarked that the commonest materials of 

 the earth's crust, the ocean, arid the atmosphere are formed of elements of 

 relatively small atomic weight. 



