
NATURE 
265 

In referring to recent advances in several branches of science, 
T simply choose some of those which have struck me as most 
notable, 
Accurate and minute measurement seems to the non-scientific 
imagination a less lofty and dignified work than looking for 
something new. But nearly all the grandest discoveries of science 
have been but the rewards of accurate measurement and patient 
long-continued labour in the minute sifting of numerical results. 
The popular idea of Newton’s grandest discovery is that the 
theory of gravitation flashed into his mind, and so the discovery 
was made. It was by a lon» train of mathematical calculation, 
founded on results accumulated through prodigious toil of prac- 
tical astronomers, that Newton first demonstrated the forces 
urging the planets towards the sun, determined the magnitude of 
those forces, and discovered that a force following the same law 
of variation with distance urges the moon towards the earth. 
Then first, we may suppose, came to him the idea of the uni- 
versality of gravitation ; but when he attempted to compare the 
magnitude of the force on the moon with the magnitude of the 
force of gravitation of a heavy body of equal mass at the earth’s 
surface, he did not find the agreement which the law he was dis- 
covering required. Not for years after would he publish his dis- 
covery as made. It is recounted that, being present at a meeting 
of the Royal Society, he heard a paper read, describing geodesic 
measurement by Picard, which led to a serious correction of the 
previously accepted estimate of the earth’s radius. This was 
what Newton required. He went home with the result, and 
commenced his calculations, but felt so much agitated that he 
handed over the arithmetical work to a friend; then (and not 
when, sitting in a garden, he saw an apple fall) did he ascertain 
that gravitation keeps the moon in her orbit. 
Faraday’s discovery of specific inductive capacity, which in- 
augurated the new philosophy, tending to discard action at a 
distance, was the result of mmute and accurate measurement of 
forces. 
Joule’s discovery of thermo-dynamic law through the regions 
of electro-chemistry, electro-magnetism, and elasticity of yases 
was based on a delicacy of thermometry which seemed simply 
impossible to some of the most distinguished chemists of the day. 
Andrews’s discovery of the continuity between the gaseous and 
liquid states was worked out by many years of laborious and 
minute measurement of phenomena scarcely sensible to the 
naked eye. 
Great service has been done to science by the British Associa- 
tion in promoting accurate measurement in various subjects. The 
origin of exact science in terrestrial magnetism is traceable to 
Gauss’s invention of methods of finding the magnetic intensity 
in absolute measure. I have spoken of the great work done by 
the British Association in carrying out the application of this 
invention in all parts of the world. Gauss’s colleague in the 
German Magnetic Union, Weber, extended the practice of ab- 
solute measurement to electric currents, the resistance of an 
electric conductor, and the electromotive force of a galvanic 
element. Hé showed the relation between electrostatic and 
electromagnetic units for absolute measurement, and made the 
beautiful discovery that resistance, in absolute electromagnetic 
measure, and the reciprocal of resistance, or, as we call it, ‘‘ con- 
ducting power,” in electrostatic measure, are each of them a 
velocity. He made an elaborate and difficult series of experi- 
ments to measure the velocity which is equal to the conducting 
power, in electrostatic measure, and at the same time to the re- 
sistance in electromagnetic measure, in one and the same con- 
ductor, Maxwell, in making the first advance along a road of 
which Faraday was the pioneer, discovered that this velocity is 
physically related to the velocity of light, and that, on a certain 
hypothesis regarding the elastic medium concerned, it may be 
exactly equal te the velocity of light. Weber’s measurement 
verifies approximately this equality, and stands in science movz- 
mentum ere perennius, celebrated as having suggested this most 
grand theory, and as having afforded the first quantitative test 
of the recondite properties of matter on which the relations 
between electricity and light depend. A re-measurement of 
Weber’s critical velocity on a new plan by Maxwell himself, and 
the important correction of the velocity of light by Foucault’s 
laboratory experiments, verified by astronomical observation, 
seem to show a still closer agreement. The most accurate pos- 
sible determination of Weber's critical velocity is just now a 
primary object of the Association’s Committee on Electric 
Measurement ; and it isat present premature to speculate as to 
the closeness of the agreement between that velocity and the 
yelocity of light, This leads me to remark how much science, 


even in its most lofty speculations, gains in return for benefits 
conferred by its application to promote the social and material 
welfare of man. Those who perilled and lost their money in 
the original Atlantic Telegraph were impelled and supported by 
a sense of the grandeur of their enterprise, and of the world- 
wide benefits which must flow from its success ; they were at the 
same time not unmoved by the beauty of the scientific problem 
directly presented to them; but they little thought that it was 
to be immediately, through their work, that the scientifie world 
was to be instructed in a long-neglected and discredited funda- 
mental electric discovery of Faraday’s, or that, again, when the 
assi-tance of the British Association was invoked to supply their 
electricians with methods for absolute measurement (which they 
found necessary to secure the best economical return for their ex- 
penditure, and to obviare and detect those faults in their electric 
material which had led to disaster), they were laying the foun- 
dation for accurate electric measurement in every scientific 
laboratory in the world, and initiating a train of investigation 
which now sends up branches into the loftiest regions and 
subtlest ether of natural philosophy. Long may the British As- 
sociation continue a bond of union, and a medium for the inter- 
change of good offices between science and the world. 
The greatest achievement yet made in molecular theory of the 
properties of Matter is the Kinetic theory of Gases, shadowed 
forth by Lucretius, definitely stated by Daniel Bernoulli, largely 
developed by Herapath, made a reality by Joule, and worked 
out to its present advanced state by Clausius and Maxwell, 
Joule, from his dynamical equivalent of heat, and his experi- 
ments upon the heat produced by the condensation of gas. was 
able to estimate the average velocity of the ultimate molecules or 
atoms composing it. His estimate for hydrogen was 6,225 feet 
per second at temperature 60° Fahr., and 6,055 feet per second 
atthe freezing-point. Clausius rook fully into account the impacts 
of molecules on one another, and the kinetic energy of relative 
motions of the matter constituting an individual atom. He in- 
vestigated the relation between their diameters, the number in a 
given space, and the mean length of path from impact to impact, 
and so gave the foundation for estimates of the absolute dimen- 
sions of atoms, to which I shall refer later. He explained the 
slowness of gaseous diffusion by the mutual impacts of the 
atoms, to which I shall refer later. He explained the slowness 
of gaseous diffusion by the mutual impacts of the atoms, and 
laid a secure foundation for a complete theory of the diffusion 
of fluids, previously a most refractory enigma. The deeply 
penetrating genius of Maxwell brought in viscosity and thermal 
conductivity, and thus completed the dynamical explanation of 
all the known properties of gases, except their electric resistance 
and _ brittleness to electric force. 
No such comprehensive molecular theory had ever been even 
imagined before the nineteenth century. Definite and complete 
in its area as it is, it is but a well-drawn part of a great chart, in 
which all physical science will be represented with every property 
of matter shown in dynamical relation to the whole. The 
prospect we now have of an early completion of this chart is 
based on the assumption of atoms. But there can be no perma- 
nent satisfaction to the mind in explaining heat, light, elasticity, 
diffusion, electricity, and magnetism, in gases, liquids, and solids, 
and describing precisely the relations of these different states of 
matter to one another by statistics of great numbers of atoms, 
when the properties of the atom itself are simply assumed. 
When the theory, of which we have the first instalment in 
Clausius and Maxwell’s work, is complete, we are but brought 
face to face with a superlatively grand question, whati is the 
inner mechanism 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 chrono- 
metric vibrator according 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 an incompressible friction- 
less liquid has been suggested as a finger-post, pointing a way 
which may possibly lead to a full 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 properties or acci- 
dents of simple matter.” This statement I take from an ad- 
mirable paper on the atomic theory of Lucretius, which 
appeared in the Worth British Review for March 1868, 
containing a most interesting and instructive summary ef 
