from Terrestrial Sources. tt 



nature, othenvise than by analogy ^\^th the similar power possessed 

 by terrestrial light. This we can trace to its source, and may thus 

 be enabled to form opinions with tolerable certainty as to \ta 

 nature. 



18. Philosophers have been much divided in their opinions re- 

 specting the nature of the relation thus subsisting between light 

 amd heat. One party have maintained the absolute identity of 

 those agents ; accounting for the different properties exhibited by 

 this heat, and by simple heat, only by supposing some modifica- 

 tions to take place in the state or form in which the " igneous 

 fluid" exists, and by which it becomes either light or heat, accord- 

 ing to circumstances ; each possessing many of the properties of- 

 the other. Another opinion has been that of the totally distinct 

 existence of the two ; although they accompany p^ch othe^ in th^ 

 closest and apparently most inseparable state of connexion. ^.v; 



The farmer of these opinions appears to me little more than §;■ 

 gratuitous assumption; an4 thp Utter is attended with in^urT 

 mountable difficulties, if we suppose the heat so accompanying 

 light to retain its separate existence and radiant properties. These, 

 i« fact, are entirely changed. 



13. It appears from'^the most decisive experiments, that the sui^'sf 

 rays have a power of producing heat in bodies in proportion ^. 

 the degree in which their surfaces (according to the commq^ pj^n . 

 pression) absorb light, from their darkness of colour, .y.^ ^^^^ 



The cause to which this effect is to be attribute^, whatever ^5ay 

 be its nature, is clearly shewn to be so closely associated with the 

 rays of light, that it seems impossible, u^der any ordinary circum- 

 stances at least, to effect a separation between them. It is trans- 

 mitted with light through transparent bodies; increases pr de-. 

 creases with the intensity of light ; and as nearly in exact proppFf ^ 

 tion as the nature of the appropriate experiments will allow us to 

 ascertain. Professor Leslie considers the proportion to be pre- 

 cise and undeviating: it is refracted with light to a focus ; an4 

 communicates no heat to transparent media through which it 

 passes ; it is reflected with the rays of light, and polarized with, 

 then^. |t is also, as is well known, subjected to a peculiar djstri'^ 



