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 m.ost 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 

 Avorld 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 o9,000 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 



