76 THE PROGRESS OF SCIEXCE n 



equally similar or dissimilar, but that some of 

 them, at any rate, constituted groups, the several 

 members of which were as much like one another 

 as they were unlike the rest. Chlorine, iodine, 

 bromine, and fluorine 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 natural groups, such as those 

 into which animals and plants fall. 



Recently this subject has been taken up afresh, 

 with a result which may be stated roughly in the 

 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 heaviest, as 240 the 

 series does not exhibit one continuous progressive 

 modification in the physical and chemical charac- 

 ters of its several terms, but breaks up into a num- 

 ber of sections, in each of which the several terms 

 present analogies with tin- corresponding terms i.f 

 the other scries. 



Thus, the \\lmle series does not run 



''. '', <l, e,f, g, h, i, k, &c., 

 but 



a, , ', <>. A. B, < . l. a, tf, 7, 8, &C. ; 



