xciv REPOKT— 1871. 



instalment in Clausius and Maxwell's work, is complete, wo are but brought 

 face to face with a superlatively grand question, what is the inner me- 

 chanism of the atom ? 



In the answer to this question we must find the explanation not only 

 of the atomic elasticity, by which the atom is a chronometi-ic vibrator ac- 

 cording to Stokes's discovery, but of chemical affinity and of the differences 

 of quality of different chemical elements, at present a mere mystery in 

 science. Helmholtz's exquisite theory of vortex-motion in au incompressible 

 frictionlcss liquid has been suggested as a finger-post, pointing a way 

 which may possibly lead to a fall understanding of the properties of atoms, 

 carrying out the grand conception of Lucretius, who " admits no subtle 

 " ethers, no variety of elements with fiery, or watery, or light, or heavy 

 " principles ; nor supposes light to be one thing, fire another, electricity a 

 " fluid, magnetism a vital principle, but treats all phenomena as mere pro- 

 " perties or accidents of simple matter." This statement I take from 

 an admirable paper on the atomic theory of Lucretius, which appeared in 

 the ' North British Eeview ' for March 1868, containing a most interesting 

 and instructive summary of ancient and modern doctrine regarding atoms. 

 Allow me to read from that article one other short passage finely describing 

 the present aspect of atomic theory : — " The existence of the chemical 

 " atom, already quite a complex little world, seems very probable ; and 

 " the description of the Lucretian atom is wonderfully applicable to it. "VVe 

 <' are not wholly without hope that the real weight of each such atom may 

 " some day be known — not merely the relative weight of the several atoms, 

 <' but the number in a given volume of any material ; that the form and 

 " motion of the parts of each atom and the distances by which they are 

 " separated may be calculated ; that the motions by which they produce heat, 

 " electricity, and light may be illustrated by exact geometrical diagrams ; and 

 " that the fundamental properties of the intermediate and possibly constituent 

 " medium may be arrived at. Then the motion of planets and music of the 

 " spheres will be neglected for a while in admiration of the maze in Avhich 

 " the tiny atoms run." 



Even before this was written some of the anticipated results had been par- 

 tially attained. Loschmidt in Vienna had sho^wn, and not much latter Stoncy 

 independently in England showed, how to deduce from Clausius and Max- 

 well's kinetic theory of gases a superior limit to the number of atoms in a 

 given measurable space. I was unfortunately quite unaware of what Loschmidt 

 and Stoney had done when I made a similar estimate on the same founda- 

 tion, and communicated it to ' Nature ' in an article on " The Size of 

 Atoms." But questions of personal priority, however interesting they may be 

 to the persons concerned, sink into insignificance in the prospect of any gain 

 of deeper insight into the secrets of nature. The triple coincidence of inde- 

 pendent reasoning in this ease is valuable as confirmation of a conclusion 

 violently contravening ideas and opinions which had been almost universally 

 held reo-arding the dimensions of the molecular structure of matter. Che- 

 mists and other naturalists had been in the habit of evading questions as to 

 the hardness or indivisibility of atoms by virtually assuming them to bo in- 

 finitely small and infinitely numerous. We must now no longer look upon 

 the atom, with Boscovich, as a mystic point endowed with inertia and the 

 attribute of attracting or repelhng other such centres with forces depending 

 upon the intervening distances (a supposition only tolerated with the tacit 

 assumption that the inertia and attraction of each atom is infinitely small and 

 the number of atoms infinitely great), nor can wo agree with those who have 



