SCIENCE IN THE LAST HALF CENTURY. 7o 



has reduced those of the iufiuitely great world, to qnestious of me- 

 chanics.* 



lu the meanwhile, the primitive atomic theory, which has served as 

 the scaffbldiug for the edifice of modern physics and chemistry, has 

 been quietly dismissed. 1 can not discover that any contemporary^., 

 physicist or chemist believes in the real indivisibilitj' of atoms, or in ' 

 an inter-atomic matterless vacuum. "Atoms" appear to be used as 

 mere names for physico-chemical units which have not yet been sub- 

 divided, and "molecules" for physico-chemical units which are aggre- 

 gates of the former. And these individualized particles are supposed 

 to move in an endless ocean of a vastly more subtle matter — the aither. 

 If this ajther is a continuous substance, therefore, we have got back 

 from the hypothesis of Dalton to that of Descartes. But there is much 

 reason to believe that science is going to make a still further journey, 

 and in form, if not altogether in substance, to return to the point of 

 view of Aristotle. 



The greater number of the so-called "elementary" bodies, now known, 

 had been discovered before the commencement of our epoch; and it had 

 become apparent that they were by no means equally similar or dis- 

 similar, but that some of them, at any rate, constituted groups, the sev- 

 eral 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 sus- 

 ceptible of a classification in natural groups, such as those into which 

 animals and plants fall. 



PERIODIC SERIES OF ELEMENTS. 



Eecently 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 recognized "elements" are arranged in the order of their atomic 

 weights — from hydrogen, the lightest, as unity, to uranium, the heavi- 

 est, as 210 — the series does not exhibit one continuous progressive 

 modification in the physical and chemical characters of its several 

 terms, but breaks up into a number of sections, in each of which the 

 several terms present analogies with the corresponding terms of the 

 other scries. 



Thus the whole series does not run 



• a, b, c, d, e, f, (j, h, /, /.-, etc., 

 but 



a, h, c, r?, A, B, c, D, a, /?, ;/, d, etc. ; 



so that it IS said to expi'ess 'Aperiodic law of recurrent similarities. Or 



* In the preface to,bis Mdcanique, Chimique M. Berthclot declares his object to 

 be "raniener la chiiiiie tout enti^re - - - aux emuies principes mdcaniques qui 

 rdgisseut d(5j;\ les iliverses branches de la pbysique." 



