333 
27. The lectures <{ on heat considered as a mode of motion/* 
by this distinguished Professor, are to my mind models of the 
former kind of instruction. In commencing these lectures he 
was careful to describe to his hearers an apparatus which he 
had contrived — a thermo-electric pile, — by means of which the 
smallest amount of heat received was caused to generate an 
electric current. This was rendered perceptible by a needle, 
the motion of which was made clearly visible to every person in 
the room. Thus possessed of a most accurate and delicate test 
of the slightest changes of temperature, he led on his audience 
from one step in demonstration to another, and that on the firm 
basis of actually 'proven science ; for the thermo-electric pile may 
fairly be allowed to take the place of our favourite balance and 
weights. As the Professor observes most truly, “ No chemist 
ever weighed the perfume of a rose, but in radiant heat we have 
a test more refined than the chemist’s balance.” Indeed, the 
chemist can no longer refuse to urge his inquiries amongst the 
imponderables; but in so doing he soon finds that a whole 
world of investigation opens before him, and one in which it 
becomes more and more difficult to secure such determinative 
elements as shall prove to himself and to others that he is not 
mistaken in his theories. 
28. After philosophers had become aware of the manner in 
which sound was produced and transmitted, analogy led some 
of them to suppose that light might be produced and transmitted 
in a somewhat similar manner. And perhaps in the whole 
history of science there was never a question more hotly con- 
tested than this one. Sir Isaac Newton supposed light to con- 
sist of minute particles darted out from luminous bodies. This 
was the celebrated “ Emission Theory,” destined in all proba- 
bility to accompany the theory of “ caloric,” and others of a 
more recent conception, into the limbo of vanity ; for light 
travels at the velocity of 192,900 miles in a second ; and if light 
consisted of ponderable particles, it would indeed be past all 
belief that these could strike the retina of the eye without abso- 
lutely destroying its texture. Professor Tyndall reduces this 
amount to inches, and finds the number to be 12,165,120,000. 
“Now it is found that 39,000 waves of red light placed end to 
end would make up an inch ; multiply the number of inches in 
192,000 miles by 39,000 and we obtain the number of waves 
of red light in 192,000. This number is 474,439,680,000,000. 
All these waves enter the eye in a single second. To produce the 
impression of red in the brain, the retina must be hit at this 
almost incredible rate ! ” 
29. Huyghens, the contemporary of Newton, found great 
difficulty in conceiving of the cannonade of particles rendered 
VOL. vii. 2 A 
