36 
spencer’s 
FIRST PRINCIPLES. 
Feb., 1889. 
t ( 
5 5 
ready to turn into heat, or kinetic or chemical energy in the 
electric circuit, latent since we have no electric or magnetic 
senses to detect it. 
All these results lead us to believe in the truth of the 
principle of the continuity or identity of energy, a principle 
evidently founded on the identity postulate, since what we 
observe is that energy passes from one form and that 
simultaneously energy appears in another form, and that 
when it passes from this latter form we can obtain energy 
again in the original form. But this continnitv does not 
necessarily imply constancy in quantity. That is another 
principle founded on experiment. Determinations like those 
of Joule tend to show that when energy changes from one 
form to another there is a fixed rate of exchange. If then, 
using the known rates of exchange, we suppose all energy 
converted into one form, experiment leads us to suppose that 
the sum total is constant. 
We can now see in what sense it is true that energy must 
show itself either as kinetic or as strain. It is only true if 
we assume that light, heat, and the rest are either kinetic or 
strain energies or mixtures of the two. 
This brings me to the consideration of another part of 
the work of the physicist. 
His main work, as I have said, is the determination of 
resemblances or similarities, and he groups phenomena 
according to these. In the course of scientific work many 
of these groups are shown to resemble each other—one set of 
phenomena is shown to be a mere combination of phenomena 
already known, and the phenomena are then said to be 
explained. Thus Wells showed that in the deposition of 
dew there is a cooling of the earth’s surface, cooling there¬ 
fore of the moisture-laden atmosphere in contact with it, 
and deposition of some of the moisture. In other words, 
he showed that the deposition of dew resembled other 
depositions of water, and so he explained it. Or again, 
Faraday explained the formation of electricity by the jet of 
steam in the hydro-electric machine when he showed that 
there was friction between the drops of water carried by the 
steam jet, and the sides of the orifice past which they rushed, 
that the two were oppositely electrified, and that it was 
therefore similar to other known cases of electrification bv 
•/ 
friction. And numberless other instances might be given. 
But the physicist is not content with explanations which 
he can prove. He is an inveterate builder up of hypotheses 
for the most part unverifiable, but that hardly troubles him. 
His hypotheses are always attempts to imagine such a 
