688 The Baconian Philosophy of Heat. [November, 
oblivion, but, it is hoped, have now regained for ever their 
ascendency. 
As already indicated, the fundamental notion of the nature 
of heat which it is our intention to follow through its 
successive stages of growth, proceeded from Bacon — the 
Chancellor., not the Friar. In his Novum Organum some- 
thing like a chapter is devoted by way of illustration to an 
<• inquisition into the form of heat ” ; and, whatever may be 
its value considered as an illustration, there can be no doubt 
that in this “ inquisition ” is contained the first, comparatively 
speaking, precise, if not positively well-backed, statement 
of what is now coming to be universally recognised as the 
true definition of the nature of heat. This definition, 
expressed in the somewhat peculiar language belonging to 
Bacon, is that “ heat is a motion, expansive restrained, and 
adting in its strife on the smaller particles of matter ” ; to 
which he adds, by way of caution on what is called 
“ sensible ” heat, that “ heat, as far as regards the touch 
of man, is a thing various and relative, insomuch as tepid 
water, for instance, feels hot if the hand be cold, 
but cold if the hand be hot.” Views very similar to these 
were described somewhat later by Descartes, in his Principia ; 
whence, probably, they were likewise adopted by Boyle, who, 
however, re-stated them in language much more precise and 
philosophical than hadbeen used by either of hispredecessors. 
This happened in the seventeenth century. At the beginning 
of the eighteenth, Newton again propounded the same 
dodtrine, that heat is essentially but a motion of the mole- 
cules ; but, unfortunately, whilst his authority contributed 
so powerfully to spread and sustain false ideas on the nature 
of light, no such overruling influence was allowed to his 
much sounder notions on the subjedt of heat. To some 
opinions of great novelty and importance put forth by 
Newton in connedtion with this matter, we shall have to 
advert to a later stage, but the merit of having advanced the 
Baconian theory beyond the immediate position in which it 
had been left by Bacon himself, by Descartes, and by Boyle, 
belongs to Hermann, the author of Phoronomia. In this 
work heat was for the first time defined, not merely as 
motion in a vague general sense, but as energy or vis viva , 
having a precise mathematical meaning. This same point 
of view was later adopted also by D. Bernoulli, who in his 
Hydrodynamica showed, moreover, how the elascity of gases 
might be accurately accounted for, instead of by the aid of 
an assumed repulsive force or expansive imponderable 
substance, as the result of the impact of the continually 
moving molecules on their collision with the walls of 
