placed at/* instantly the liquid will be depressed in the leg of the thermome- 

 ter, and the presence of the source of heat greater than that of the surrounding 

 nit ilium will be thus indicated. That this source of heat is derived from the 

 vessel of hot water in the focus f may be easily proved. Let this vessel be 

 removed, and immediately the liquid in the thermometer will rise to its ordina- 

 ry level ; but it may be said that the effect is produced on the thermometer by 

 the heat transmitted direct from/ 7 toy. This, however, may be .proved not to 

 be the case ; for let the hot water be placed as before at/ 7 , and let the mirror 

 M be removed, the effect produced on the thermometer will immediately erase. 



The rapidity with which the heat thus radiated from/' and reflected by ihe 

 mirrors to/is propagated, may be shown by interposing between/*and f a screen, 

 composed of any substance not pervious to calorific rays. When the screen is 

 thus interposed, the liquid in the thermometer will recover its ordinary level ; 

 but the moment the screen is again withdrawn, the liquid instantly fails in the 

 focal leg ; and this takes place by whatever distance the two mirrors may be 

 separated. 



Of the two hypotheses already mentioned, which have been proposed for the 

 explanation of the phenomena observed in the prismatic spectruin, that which 

 supposes light to consist of three distinct principles seems to be attended with 

 a variety of circumstances which throw improbability upon it. The three 

 principles thus distinguished enjoy the same leading properties. They all obey, 

 with the most minute precision, the ordinary laws of optics, and, in fact, pos- 

 sess every property of light except the most prominent and obvious one of 

 affecting the sight. The other hypothesis, on the contrary, has the advantage 

 of great simplicity ; in it light is considered as compounded of a number of 

 rays unequally refrangible, and possessing, consequently, different influences 

 on other bodies, and on vision. The calorific and chemical properties which 

 disappear alternately at the extremities of the spectrum, are cqnsidered as de- 

 pending on, or connected with, the difference of refrangibility, and as becom- 

 ing insensible under different variations in that property; it is very conceivable 

 that the calorific power of rays may vary in some inverse proportion with re- 

 spect to their refrungibility, while the energy of the chemical power may change 

 in a contrary direction. In a word, since all the rays refracted by the prism 

 agree in by for the greater number of their properties, and disagree only in 

 some peculiar effects ; and since even this disagreement may be considered 

 more as apparent than real, and may arise from the want of sufficient sensibili- 

 ty in the tests by which these effects may be practically ascertained, it seems 

 more philosophical to regard all the rays as of one species, than to adopt an 

 hypothesis which classes things alike in all their leading qualities, under differ- 

 ent denominations. It is not, however, necessary to assume either supposition, 

 nor to adopt it as the basis of reasoning. Experiment is the sure and only 

 guide in physics ; and whether heat be obscure and imperceptible light, or a 

 distinct physical agent, we shall regard it as a principle attended with certain 

 sensible effects, capable of being ascertained by experiment or observation, and 

 from such effects arid such only, can legitimate inferences be drawn. 



The heat which passes from a body by radiation has a tendency to cause its 

 temperature to fall ; and, the rate of this process of cooling, is propomoiiate * 

 to the difference between the temperature of the body and that of the surround- 

 ing medium, when this difference is not of very great amount. It follows, the,:, > 

 that a hot body at first, when its temperature greatly exceeds that of the sur- 

 rounding air, cools rapidly ; but as its temperature falls, and approaches ncaier ) 

 to equality with the temperature of the medium in which it is placed, the rate i 

 at which it cools gradually diminishes. This law of bodies cooling was !ir>* . 

 observed by Newton, and reduced to an exact mathematical expression, i>y < 



