SCIENCE IN THE LAST HALF CENTURY. 73 



has reduced those of the infinitely great world, to questions of me- 

 chanics.* 



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

 the scaffolding 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 indivisibility of atoms, or in ' 

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

 mere names for physico-chemical units which have uot yet been sub- 

 divided, and "molecules" for physico-chemical uuits 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 aether. 

 If this tether 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 iu 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 iu natural groups, such as those into which 

 animals and plants fall. 



PERIODIC SERIES OF ELEMENTS. 



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 recognized "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 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,f/, h, i, k, etc., 

 but 



a, b, c, fZ, A, B, c, D, a, /?, ;/, (y, etc. ; 



so that it IS said to express n periodic law of recurrent similarities. Of 



* la the preface to bis M6cauique, Chimique M. Berthelot declares his object to 

 be 'Tamener la cbimie tout entifere - - - aux Amines principes m^caniques qui 

 r^gissent ddja les diverses braucbes de la pbysiqae." 



