THE THERMOMETER. 



10. When heat is communicated to any part of a body, the 

 temperature of that part 1s momentarily raised above the general 

 temperature of the body. This excessive heat, however, is 

 gradually transmitted from particle to particle throughout the 

 entire volume, .until it becomes uniformly diffused, and the 

 temperature of the body becomes equalised. 



This quality, in virtue of which heat is transmitted from 

 particle to particle throughout the volume of a body, is called 

 conductibility : 



Bodies have the quality of conductibility in different degrees ; 

 those being called good conductors in which any inequality of 

 temperature is quickly equalised, the excess of heat being trans- 

 mitted with great promptitude and facility from particle to 

 particle. Those in which it passes more slowly and imperfectly 

 through the dimensions of a body, and in which, therefore, the 

 equilibrium of temperature is more slowly established, are called 

 imperfect conductors. Bodies, in which the excess of heat fails 

 to be transmitted from particle to particle before it has been 

 dissipated in other ways, are called non-conductors. 



The metals in general are good conductors, but different metals 

 have different degrees of conductibility. The earths and woods 

 are bad conductors, and soft, porous, and spongy substances still 

 worse. 



11. Heat is propagated from bodies which contain it by 

 radiation in the same manner, and according to nearly the same 

 rules, as those which govern the radiation of light. Thus, 

 it proceeds in straight lines from the points whence it emanates, 

 diverging in every direction, these lines being called thermal 

 rays. 



12. Certain bodies are pervious to the rays of heat, just as glass 

 and other transparent media are pervious to the rays of light. 

 They are called diathermanous bodies. Thus, atmospheric air and 

 gaseous bodies in general are diathermanous. 



The rays of heat are reflected and refracted according to the 

 same laws as those of light. They are collected into foci by 

 spherical mirrors and lenses, they are polarised both by reflection 

 and refraction, and are subject to all the phenomena of double 

 refraction by certain crystals in a manner analogous to that 

 which takes place in relation to the rays of light. 



Bodies are diathermanous in different degrees. 



Imperfectly diathermanous bodies transmit some of the rays of 

 heat which impinge on them, and absorb others ; the portions 

 which they absorb raising their temperature, but those which they 

 transmit not affecting their temperature. 



13. The surfaces of bodies also reflect heat in different 

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