64 SCIENCE PRIMERS. [MATERIAL 



surface of the water in a tumbler at 100 is just as un- 

 ruffled as that of the same water at 32. What, then, 

 is meant by saying that heat is a kind of motion, and 

 that the greater the heat in any body the greater 

 the amount of motion in that body ? 



The answer to this question is that the motion which 

 causes the phenomena of heat, is not a visible mo- 

 tion of the whole mass of the hot body, but a motion 

 of the individual particles of which it is composed. 

 And each particle moves, not straight forward, but 

 backwards and forwards in the same space, so that its 

 motion may be roughly compared to that of a pendu- 

 lum, or to that of the balance-wheel of a watch. It is 

 in fact a sort of vibratory movement ; each vibration 

 taking place through a very short distance and with 

 extreme rapidity. The sensation of heat is caused by 

 the vibratory movements of the particles of matter, 

 just as sound is so caused. The prongs of a tuning- 

 fork which has been struck, certainly vibrate, for 

 you can see them do so if the note is low. If you 

 now put your ear at one end of -a long piece of timber 

 and the handle of the vibrating tuning-fork is placed 

 upon the other end, the vibratory motion of the 

 tuning-fork will be communicated to the particles 

 of the wood and will be loudly heard. All the time 

 the sound is heard the particles of the wood are 

 vibrating. Nevertheless, the wood as a whole does 

 not move, but its particles swing backwards and for- 

 wards through such a minute space that their motion 

 is imperceptible. 



But what are these particles of matter which by 

 their vibration give rise to the phenomena of heat ? 



