PERIODIC ARRANGEMENT OF ELEMENTS 165 



uranium ; but there are certain gaps unfilled, denoted by 

 the sign ?, which, it is believed, represent the places of 

 still undiscovered elements. Indeed, Meyer's original 

 diagram contained a larger number of these; and Men- 

 deleeff, averaging the properties of the elements surround- 

 ing such gaps, prophesied the discovery of scandium, 

 gallium, and germanium, made at a much later date by 

 Cleve, by Lecoq de Boisbaudran, and by Winckler. 



There are many other ways of representing these rela- 

 tions; but except perhaps in convenience (and questionably 

 even in that), they present no particular advantage, and 

 convey no new knowledge. Only one point must be 

 emphasised. The elements, as arranged above, divide 

 themselves into two 'periods' long periods and short 

 periods. Thus the seventh member after lithium, sodium, 

 is in its character very like lithium; and, again, potassium, 

 the seventh after sodium, presents strong analogies with 

 the two elements named ; but it is then necessary to pass 

 over fifteen elements before rubidium is reached, which 

 again closely resembles lithium, sodium, and potassium ; 

 and csBsium, the seventeenth element after rubidium, forms 

 the first term of another long period. Copper, silver, and 

 gold are also separated by long periods ; and so with the 

 elements in the other columns. To distinguish these in 

 the table, the symbols of the elements in the middle of 

 the long periods are printed towards the left, and those at 

 the beginning towards the right, of the figures denoting 

 the atomic weights. 



One other point requires mention. Several instances 

 occur in which the elements appear to occupy a reversed 

 position. Thus, nickel, with the atomic weight 587, 

 follows cobalt, to which a higher atomic weight is ascribed; 

 tellurium precedes, instead of following iodine ; and it will 

 be seen that argon precedes potassium. The differences 

 between the various consecutive atomic weights are 



