THERMOELECTRICITY. 281 



Certain remarkable results follow from the difference in power 

 possessed by different substances as regards the passage of heat 

 along them by conduction, one of which is that when heat passes 

 through, or is evolved at or near the junction of the surfaces of 

 two substances of different conducting power, a portion of the 

 heat becomes transformed into electricity, whence the term 

 tlierjnoelectricity applied to this kind of action. 



CHAPTER XXI. 

 MEASUREMENT OF HEAT SPECIFIC AND LATENT HEAT. 



In the complete measurement of most natural kinds of mani- 

 festations made known to us by the senses, there are two distinct 

 qualities, both of which are involved in the phenomenon ex- 

 amined, and both of which require to be measured before a 

 complete knowledge of the matter is gained ; thus, in the case of 

 light and bodies rendered visible thereby, two different impres- 

 sions are made on the eye, one of which is termed colour, and the 

 other brightness, brilliancy, or illumination ; thus a candle and a 

 powerful oil lamp give out light of much the same colour in each 

 case, but of very different degrees of brilliancy ; whilst by holding 

 various kinds of glass between the eye and the candle, &c., a long 

 series of different colours may be obtained, not differing greatly in 

 their relative brightness. In the same way, in reference to audible 

 sounds and more especially musical notes, there are the two 

 corresponding qualities, viz., the Pitch of the note (whether high 

 or low, treble or bass) and the Loudness of the sound. * Similarly, 

 in the consideration of a projectile force, such as a stone thrown 

 by the hand, an arrow propelled by a bow, &c., there are the two 

 separate ideas involved, viz., the direction in which the body is 

 made to move, and the rate at which it moves, or the velocity 

 imparted to it. In the case of the power or energy possessed by 

 a moving body of water, such as a hydraulic engine supplied by a 

 stream of water in a pipe, there are again two measurements 

 requisite, viz., the degree of pressure exerted by the water, 

 dependent on the height of the cistern or reservoir out of which 



* In the case of musical notes, a third quality also comes into play, called 

 timbre or quality of sound : the same note played on a violin, cornet, or 

 clarionet, &c., has a different effect on the ear in each case, although the 

 pitch may be the same throughout and the loudness not widely different. 



