On the Sfflar and Terrejirial Rays which oeeafion Htat, 69 



without fuch afliftance, when certain circum fiances, fuch as a too argillaceous nature of 

 the foil, &c. are little favourable to its produdion ; fo that at a fliort depth tAuriate o£ 

 foda only is difcoverable. 



I wiftied to afcertain the truth of this explanation in a trough placed in one of the gprdens 

 belonging to the Inftitute. For this purpofe fome carbonate of lime and Cliceous earth, 

 both well wafhed, were mixed together, to which was added a certain proportion of 

 muriate of foda : a hole was made in this mixture, for the purpofe of pouring in water oc- 

 cafionally, and keeping up the neceflary degree of muifture. An incruftation of muriate of 

 foda is formed at the furface, which already ftrongly changes the colour of paper tinged 

 with fernambuc, like the alkalis ; but we cannot expedt to obtain an efflorefcence fo con- 

 fiderable as to be perceptible to the fight, till after a much greater length of time. 



II. The preceding obfervacions fliew that the only difference which diftinguifhes the- 

 complex affinities from thofe called eleElive, is, that in the former, fubftances are brought 

 Into a£tion which are nearly in an uniform degree of faturation ; and that in the latter, 

 there is a fubftance prefent which is not yet faturated (or fcveral fuch fubftances may be 

 prefent) j fo that in the former, a new degree of faturation is eftabliftied only in proportioa 

 to the combinations which are capable of being feparated ; whereas, in the latter, the a£lion 

 of the unfaturated fubftances becomes in equilibrio with that of the fubftances which were 

 already fo ; whence it happens that the force of cohefion and that of elafticity produce their 

 effeft more completely in the complex than in the eledlive affinities. 



VI. 



Experiments on the Solar and on the Terrejlrial Rays that occafion Heat ; with a comparative 

 View of the Laws to which Light and Heat, or rather the Rays which occafion them art 

 JubjeEl, in order to determine whether they are the fame or different. By Wld. HERSCHBLtif, 

 LL.D.F.R.S.* 



XN the former part of this paper, of which an abridgment was given at page 360 of our 

 laft volume, it was (hewn that heat derived immediately from the fun, or from ignited ter- 

 reftrial fubftances, is occafioned by rays emanating from them, and that fuch rays are fub- 

 jeft to the laws of reflection and of refraftion. The fimilarity between light and heat was 

 the moft remarkable feature of the experiments there related. Li the prefent memoir the au- 

 thor proceeds to point out fome ftriking and fubftantial differences in the rays of light and 

 heat which afford data for difculling whether there two effedls be produced by one and the. 

 feme power. 



* Abridged from the Philofophl«al Tranfa&ions, 1800, page 437. 



The 



