56 THE ADVAXCE OF SCIENCE 



like one another as they were unlike the 

 rest. Chlorine, iodine, bromine, and flu- 

 orine thus formed a very distinct group ; 

 sulphur and selenium another ; boron and 

 silicon another ; potassium, sodium, and 

 lithium another ; and so on. In some 

 cases, the atomic weights of such allied 

 bodies were nearly the same, or could be 

 arranged in series, with like differences 

 between the several terms. In fact, the 

 elements afforded indications that they 

 were susceptible of a classification in nat- 

 ural groups, such as those into which ani- 

 mals and plants fall. 

 fall into Recently this subject has been taken 

 series. up afresh, with a result which may be 

 stated roughly in tho following terms : If 

 the sixty-five or sixty-eight recognised 

 'elements' are arranged in the order of 

 their atomic weights — from hydrogen, the 

 lightest, as unity, to uranium, the heavi- 

 est, as 240 — the series does not exhibit 

 one continuous progressive modification 



