84 Mr, Emmet t on the [Feb. 



will be refl3cted parallel to each other as B D, C E. If they 

 fall upon another similar mirror D E, whose axis coincides with 

 that of B C, they meet and are concentrated in its focus F : if,, 

 therefore, a thermometer be placed in the focus F, it will indl*, 

 cate a temperature superior to that of the surrounding medium^ 

 which process must continue until A is cooled to the temperature 

 of the surrounding medium, when the radiation ceases. If A 

 have, the temperature of the surrounding medium, and F be a 

 cold mass of matter, as a frigorific mixture, A will experience a 

 diminution of temperature, which phenomenon has often been 

 absurdly termed tke radiation of cold, and is produced as fol- 

 lows : F, having a temperature below that of the surrounding 

 medium, will absorb caloric from it, until the temperatures become 

 the same. During this operation, F attracts towards its centre, 

 which is the focus of the mirror D E, the caloric from the sur- 

 rounding medium. Now it is easily proved from the principles 

 of mechanics, that the particles of caloric being mutually repel- 

 lent, the only rays that can impinge upon the mirror D E, so as 

 to flow in succession to the focus, are such as make the angle 

 of incidence equal to the angle of reflection, which property,, 

 from the nature of the parabola, belongs only to the parallel 

 rays B D, C E, &c. whence, by reason of the attraction of F for. 

 caioric, it flows from the parts B C, &c. in parallel lines to D E, 

 Sec. and for the same reason, that the equilibrium of the medium, 

 may be preserved, must flow from A in divergent rays, imping- 

 ing upon B C ; wherefore the temperature of A will be reduced. 

 If, .therefore, A be a deUcate thermometer, it will indicate a 

 temperature below that of the surrounding medium. These two 

 phenomena of the radiation of heat are only modifications of one 



feneral principle: in the first, the caloric is put into motion, 

 ecause the body is exposed in all directions to a medium, hav- 

 ing a lower temperature than its own; in the second, to a medium 

 w^ich is cooler in only one of its parts. 



,- If the mirrors be of such a nature as to absorb a considerable 

 quantity of the incident caloric, the reflection of heat is in part 

 destroyed ; for then the mirrors become heated, and instead of 

 reflecting the heat in parallel lines, or to their foci, the caloric 

 radiates from each point upon their surfaces, as from a centre ; 

 consequently little or no effect will be produced in their foci.. 

 The same attains, if a thin transparent lamina be placed between. 

 them, which has a powerful attraction for, and readily absorbs, 

 the incident caloric, which explains some phenomena which arise 

 when plates of glass, &c. are placed between the mirrors. 



Crystallization. 



It will be proper first to premise, that the particles which 

 constitute a regular crystal must, at some previous time, have 

 been separated from each other, either by the repulsive force of 



