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XYI. Some Applications of Dynamical Principles to Physical Phenomena. —Part II. 
By J. J. Thomson, M.A., F.P.S., Fellow of Trinity College, and Cavendish Professor 
of Experimental Physics in the University of Cambridge. 
Received March 31,—Read April 21, 1887. 
§ 1. The two laws of Thermodynamics have proved by far the most powerful, indeed 
almost the only, means we possess of connecting the phenomena in one branch of 
Physics with those in another. Though the two laws are usually grouped together, it 
should not be forgotten that they differ essentially in character. The First Law is a 
direct application to Physics of one of the most important dynamical principles, that 
of the Conservation of Energy ; while the Second Law, which for the purpose of 
connecting various physical phenomena is even more important than the first, is not, 
strictly speaking, a dynamical principle at all, since its statement involves a reference 
to quantities which never occur in abstract Dynamics. 
Clausius and Sir William Thomson, the two physicists to whom the Second Law 
owes its importance, have connected it with other principles which seem more 
axiomatic. 
Thus Clausius bases the Second Law on the principle that “ heat cannot by itself 
pass from a colder to a hotter body.” In this statement too much depends upon the 
meaning to be attached to the words “ by itself ” for it to be regarded as axiomatic, 
and the following extract from Clausius seems to show that in his view it is the 
statement of a new physical principle, and not the necessary consequence of previously 
recognised ones. He says (‘Mechanical Theory of Heat,’ English translation, by 
W. R. Browne, p. 342), “If, however complicated the processes may be, it is main¬ 
tained that without some other permanent change, which may be looked upon as a 
compensation, heat can never pass from a colder to a hotter body, it would seem that 
this principle ought not to be treated as one altogether self-evident, but rather as a 
newly propounded fundamental principle on whose acceptance or non-acceptance the 
validity of the proof depends.” 
Again Sir William Thomson has connected the Second Law with the principle that 
“ it is impossible, by means of inanimate material agency, to derive mechanical effect 
from any portion of matter by cooling it below the temperature of the coldest of the 
surrounding objects.” To follow out the connection between this principle and the 
Second Law, it is convenient to divide the energy of a body into two kinds, the one 
kind depending upon circumstances over which we have complete control, the other 
15.12.87 
