PRESIDENTS ADDRESS—SECTION A. 27 
19. Wenzel and Le Sage, and Atomism.—As further illustrat- 
ing how the atomistic conception was becoming productive, it 
ought to be mentioned that thirty-nine years later Wenzel (v), 
in discussing the doctrine of chemical affinity, argued that the 
properties of bodies depend upon the configuration of the smallest 
particles (w). 
In 1782 George Louis Le Sage, in a remarkable memoir, en- 
titled Lucréce Newtonien, and in his Traité de Physique 
Mécanique (x) proposed a theory of gravitation which accounted 
for that action on the assumption of the mutual screening of two 
material bodies of each other, from the impact of “ ultramundane 
corpuscles” on the larger atoms or molecules of the bodies. 
20. Vortex-atom Theory.—Passing for the present the story, 
commencing more particularly towards the end of last century, 
of the development of the atomic theory through chemistry, and 
restricting our consideration for the. present to ultimate concep- 
tions only, the next in historical sequence is the vortex atom 
theory of Sir William Thompson (now Lord Kelvin), published 
in 1867. In 1858 Helmholtz (y) had published in Crelle’s 
Journal (z) his memorable investigation of vortex-motion in a 
fluid conceived as continuous, homogeneous, incompressible, hay- 
ing density or mass, but absolutely non-viscous. Vortices in 
such a fluid, which, of course, is a purely conceptional one, 
possess three fundamental properties of importance in connection 
with Lord Kelvin’s theory, viz.: (a) A portion of the fluid at any 
moment constituting a vortex must, whatever the movement of 
the vortex as a whole, do so continually ; (4) the strength of a 
vortex-filament [or thread of fluid rotating about its longitudinal 
axis|, defined as the product of the area of its cross section into 
the spin of that section, is constant; (c) a vortex filament must 
form either a closed curve, or terminate in the boundaries of the 
fluid, so that in an indefinitely extended fluid the filament must 
be either of indefinite length or must return into itself. 
Lord Kelvin’s atom theory is that the ether is [sensibly ?] sucn 
a fluid; that atoms are minute vortex filaments therein; that 
since such filaments must be permanent, atoms when once exis: 
tent will continue to exist; that as no part of the fluid not 
originally in rotational or vortex-motion can ever enter into such 
motion, new atoms cannot come into existence. If ring-shaped 
filaments be linked together, or if a closed filament be in any 
way knotted, they or it must for ever so remain, since the condi- 
(v) [1740—1793]. 
(w) Vorlesungen iiber die chemische Verwandtschaft der Kirper. 1777. Kopp claimed 
for Wenzel the discovery of the law of reciprocal proportions [Gesch. d. Chem.], but it 
had already been shown by Hess [1840] that Wenzel’s researches led to no definite con- 
clusion of this character. Kopp later acknowledged his mistake [1573]. 
(xz) Afterwards published with a similar treatise of his own by Pierre Prévost in the 
Abhandlungen of the Berlin Academy, 1818. 
(y) (1821—1894]. 
(z) Ueber Integrale der hydrodynamischen Gleichungen welche den Wirbelbewegungen 
entsprechen. Crelle. 55. pp. 25-55. 1858. 
