208 WELLS'S NATURAL PHILOSOPHT. 



kind of matter pervading all space, and entering into 

 combination in various proportions and quantities, with 

 all bodies, and producing by this combination all the va- 

 rious effects noticed. 



Explain theri- ^^'^- ^^^^ vibratorj thcorv, on the contrary, 

 bratory theory, gupposcs hcat to be merely the effect of a spe- 

 cies of motion, like a vibration or undulation, produced 

 either in the constituent particles of bodies, or in a subtle, 

 imponderable fluid which pervades them. 



When one end of a bar of iron is thrust into the fire and heated, the other 

 end soon becomes hot also. According to the mechanical theory, a subtilo 

 fluid coming out of the fire enters into tho iron, and passes from particle to 

 particle until it has spread through the whole. When the hand is applied to 

 the bar it passes into it also, and occasions the sensation of warmth. Ac- 

 cording to the vibratory theory, the heat of the fire communicates to the par- 

 ticles of the iron themselves, or to a subtile fluid pervading them, certain vi- 

 bratory motions, which motions are gradually transmitted in every direction, 

 and produce tho sensation of heat, in the same way that the undulations or 

 vibrations of air, produce the sensation of sound. 



„ , There seems to bo but little doubt at the present time among 



Ho-w are these . .r. , , , i • i -i xi -u 



two theories scientific men, that the theory which ascribes the pneuome- 



generalijr re- ^^ ^f jjg^^^ ^g 3^ series of vibrations, or undulations, either in 

 garded i ' ' 



matter, or a fluid pervading it, is substantially correct. At 



the same time it is not wholly satisfactory, and neither theory will perfectly 



explain all tho facts in relation to heat with which wo are acquainted. 



For the purpose of describing and explaining the phenomena and effects of 



heat, it is convenient, in many cases, to retain tho idea that heat is a substance. 



The fact that naturo nowhere presents us, neither has art 



What are pvi- succeeded in showing us, heat alono in a separate state, 



Gcnccsiu favor o i r i 



of the respect- is a Strong ground for believing that heat has no separate 



heat*? '^""'^^ material existence. Heat, moreover, can be produced without 



limit by friction, and intense heat is also produced by the ex- 

 plosion of gunpowder. On the contrary, as arguments in favor of the material 

 existence of heat, we have the foct, that heat can be communicated very 

 readily through a vacuum ; that it becomes instantly sensible on the condens- 

 ation of any material mass, as if it were squeezed out of it : as when, on re- 

 ducing the bulk of a piece of metal by hammering, we render it very hot (the 

 greatest amount of heat being emitted with the blows that most change its 

 bulk) ; and, finally, that the laws of the spreading of heat do not resemble 

 those of the spreading of sound, or of any other motion known to us. 



4G6. The relation between heat and light is a very intimate 



U^'the'"ri''*be"- one. Heat exists without light, but all the ordinary sources 



tween heat and of light are also sources of heat; and by whatever artificial 



means natural light is condensed, so as to increase its splen. 



