272 Pleasant Ways in Science. 



mirror comes back to its place, the second reflection will be 

 on a different spot to the first, and. the distance between the 

 two reflections will enable an experimenter to tell how much 

 the candle has been moved. If, moreover, the time occupied 

 by the mirror in rotating is known, it will become evident that 

 in that time the candle's motion was effected. 



Let us now suppose a mirror rotating with great velocity, 

 that a ray of light falls upon it, and is reflected by it on a given 

 spot. Let this same ray of light, after traversing a certain 

 number of feet, be a second time thrown upon the mirror, and 

 a second time reflected by it. If daring the time occupied by 

 the ray of light in the journey it made between the first reflec- 

 tion and the second was sufficient to allow the mirror to per- 

 form any appreciable part of its rotation, the light ray must, on 

 its second arrival at the mirror's surface, have struck that sur- 

 face at an angle differing from the first. It is evident that as 

 light moves so quickly, the mirror must be very quick for the 

 faintest difference of position to have occurred ; but by making 

 a rotation of 600 to 800 turns in a second, and by viewing the 

 image through a magnifying eye-piece, M. Foucault obtained 

 a sensible distance between the first and second reflections, 

 although the light only passed through a space of twenty- 

 seven feet.* 



In the present state of science, we seem justified in regard- 

 ing fight, heat, and elasticity as modes of motion, and we may 

 suppose that they all exhibit the two kinds of motion we have 

 described — the oscillations of particles in a limited space, and 

 the indefinite propagation of the wave form. Heat is also a 

 mode of motion, and a continual cause of motion in every sub- 

 stance and particle that it acts upon. Heat performs two 

 functions, which are evidenced in a different manner to our 

 senses ; it expands bodies by forcing their particles further 

 apart, and it makes bodies hot by communicating to their par- 

 ticles a particular kind of motion. If a certain quantity of 

 heat is added to various substances, it will not make them all 

 equally hot ; but the heat which does not make itself cognizant 

 to our senses in the form of augmented warmth is occupied in 

 internal work, and produces a movement of particles that may 

 become known to us in some other way. " To raise a pound 

 of water one degree would require thirty times the amount of 

 heat necessary to raise a pound of mercury one degree. "f 



When chemical attractions operate powerfully, as when a 

 mixture of oxygen and hydrogen is ignited by an electric 

 spark, the atoms of the gases rush together with inconceivable 



* Ganofs FJiysics, already referred to, contains a description, with diagrams, 

 of this experiment. 



f TyndalVs Heat as a Mode of Motion. Second Edition, p. 146. 



