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observation increased, some of these ideas led to consider- 
ably forced hypotheses, as for example the Newtonian 
hypothesis of the material nature of light. When the 
peculiar phenomena of diffraction and interference became 
known, it was evident that the hypothesis of the ma- 
terial nature of light required some peculiar modification, 
and this strange modification was suggested by Newton 
himself, that the particles or molecules of light were subject 
to periodical changes of condition, and the hypothesis was 
formed, with reference to them, that they might be probably 
egg-shaped, perpetually turning over endwise in their progress ; 
and that if they impinged endwise on a medium they would 
enter it, while if they came sideways they would rebound 
from it. This was an exceedingly strained and forced hypo- 
thesis, formed to meet those changes of condition which are 
identical with what are now known as the different phases of 
the waves or undulations of light. With regard to the caloric 
theory, that again led to some still further hypotheses, when it 
became known, from an experiment of Leslie, that if a ball of 
ice were placed in the focus of a reflector, and the bulb of a 
thermometer were placed in the other focus, the temperature 
of the thermometer was lowered. From that Leslie was led 
to form this hypothesis, that there was a radiation of absolute 
cold as well as of absolute heat. But that is explained now on 
the supposition that heat consists of motion, and on the theory 
of exchanges. I can best explain that by an illustration. 
Suppose on each side of a room there was a harp, both harps 
having their strings tuned exactly in unison. It is known 
very well that if a string belonging to one of them be sounded, 
it will put that string of the other harp, which is in unison with 
it, into vibration ; and that phenomenon is known as the reci- 
procation of sound. In the same way, supposing one of the 
strings to be very lightly sounded and the other to be 
sounded loudly, then the vibrations of each string are com- 
municated to the other, the sound produced by one string 
being increased, while the sound produced by the other is 
diminished. That exactly illustrates the theory of exchanges 
with regard to heat ; namely, that each body is capable of 
radiating the heat it possesses ; and because two bodies are 
capable of imparting their heat to each other, it follows that 
one will lose and the other will gain heat, if their tem- 
peratures were previously unequal. And thus it will be seen 
that the theory now generally received, that heat consists in 
molecular motion, entirely supersedes the hypotheses which 
had been previously formed. It is obvious also, that if any 
one of these various forms of physical energy consists in 
vol. hi. 2 A 
