in Fluids. 377 



no Heat will be generated in either of them by the sun's 

 light. 



If now a small particle of any opaque solid body be 

 suspended in the midst of the water in the tumbler, 

 those rays of light, which, impinging against it, are ab- 

 sorbed by it, must necessarily generate Heat in the very 

 moment when they are stopped. This is an incontro- 

 vertible fact, which nobody will dispute. 



In order to render this imaginary experiment more 

 interesting, we will suppose the solid body put into the 

 water to be a small particle of yellow amber ; and that 

 its specific gravity is so exactly equal to that of the wa- 

 ter that it has no tendency to move in it, either upwards 

 or downwards, and consequently will remain in the situ- 

 ation where it is placed, without being suspended ; and 

 we will suppose, farther, that this solid particle of amber 

 is nearly globular, and y^Vo" f an i ncn i n diameter, 

 which is just equal to the diameter of a single thread of 

 silk, as spun by the worm, and is probably one of the 

 smallest objects that is perceptible by the human eye, 

 unassisted by art. 



As it is evident that Heat must be generated or ex- 



D 



cited in this small particle of amber by the light it stops 

 or absorbs, the points which remain to be discussed 

 are, therefore, what its intensity is at the moment of its 

 existence ; and what are the effects which it ought to 

 produce in consequence of that intensity. 



The reasons have already been mentioned which ren- 

 der it probable that when Heat is generated by the rays 

 of light, its intensity, where it is generated, and before it 

 has been diminished in consequence of its dispersion, is 

 always the same ; and, taking it for granted that this is 

 the case in fact, we will endeavour to trace the operations 



