426 The Evolution of Scientific Knowledge. [July, 
by the modern view that they result from the displacement 
of the hydrogen in an acid by a metallic or other radical ; 
and when the reader is reminded.of the advances implied in 
the terms atomicity, isomorphism, allotropy, isomerism, 
diffusion, dialysis, crystalloid and colloid, and many others, 
he will admit that, though chemical science yet awaits its 
Newton, it exemplifies a continuous advance in definiteness, 
dependence of parts, orderly complexity, and integration. 
Without staying to point out that our knowledge of the 
physical constitution of matter exemplifies evolution not 
less clearly than that of the chemical constitution of matter, 
attention mav now be very briefly drawn to the conceptions 
of Force. To illustrate the indistinctness of ideas on this 
subject which prevailed in comparatively early times, I may 
give a quotation from Pliny, cited by Whewell " What,” 
he cries, “ is more violent than the sea and the winds ? 
what a greater work of art than a ship ? yet one little fish 
(the Echineis) can hold back all these when they strain the 
same way. The winds may blow, the waves may rage ; but 
this small creature controls their fury, and stops a vessel 
when chains and an anchor would not hold it ; and this it 
does not by hard labour, but merely by adhering to it.” 
“ In our own memory,” he continues, “ one of these animals 
held fast the ship of Caius, the Emperor, when he was 
sailing from Astura to Antium.” Now although it may be 
objected that the Greeks, long before Pliny wrote, had some 
definite mechanical notions, and that this quotation gives an 
over-estimate of the amount of indefiniteness of early 
knowledge on this subject, still the faCt remains that so in- 
definite was this knowledge before the time of Galileo that 
the first law of motion— that a moving body, if entirely left 
to itself, will continue moving in the same direction and 
with the same velocity— had not been distinctly grasped. 
And even when this highly abstract law was at length 
graspe d —when it was seen that under ordinary circum- 
stances ’(on our earth, for example) the onward motion of a 
body is stopped by the resistance afforded by the air, or by 
solid or liquid bodies,— even then for a long time no account 
was taken of the motion thus apparently lost ; and it was 
reserved for modern times to show that the force which 
this motion implies is not destroyed by the resistance, but 
only takes on a new form in the molecular motion of heat, 
developed in the resisting and resisted bodies. And this 
conversion of the motion of a mass of matter into molecular 
motion was subsequently found to be only a particular 
instance of the conservation of energy, the discovery of 
