77* PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



and vanish at the same time with the power of heating. How can effects that are 

 so opposite be ascribed to the same cause ? First of all, heat without light ; next to 

 this, decreasing heat, but increasing light ; then again, decreasing heat and de- 

 creasing light. What modification can we suppose to be superadded to the heat- 

 making power, that will produce such inconsistent results ? 



We must not omit to mention another difference between light and heat, which 

 may be gathered from the same article of the refrangibility of heat-making rays. 

 It is, that though light and heat are both refrangible, the ratio of the sines of 

 incidence and refraction of the mean rays is not the same in both. Heat is evi- 

 dently less refrangible than light ; whether we take a mean refrangible ray of each, 

 or, which I believe to be the better way of proceeding, whether we take the maxi- 

 mum of heat and light separately. This appears, not only from the view we have 

 taken of the two spectra already mentioned, but more evidently from the 23d 

 experiment, by which we find, that heat cannot be collected by a lens, to the same 

 focus where light is gathered together. Our 5th article, in which an account has 

 been given of the proportions of heat and light stopped by glasses and other sub- 

 stances, will afford us now an ample field for pointing out a striking difference 

 between these two principles. From the 24th to the 30th experiments, we have 

 the quantities intercepted by colourless substances as follows. 



Table 1. 

 Rays of heat. Of light. Rays of heat. Of light. 



Bluish-white glass stops 250 and 86 Iceland crystal 244 and 150 



White flint glass 91 34 Talc 139 90 



Greenish crown glass 259 203 Calcinable talc 184 288 



Coach glass 214 168 



Now, by casting an eye on the above table, it will be seen immediately, that no 



kind of regularity takes place among the proportions of rays of one sort and of 



another, which are stopped in their passage. Heat and light seem to be entirely 



unconnected. The bluish- white and flint glasses, for instance, stop nearly 3 times 



as much heat as light, whereas the greenish crown glass stops only about 4- more 



of the former than of the latter ; but, as coloured glasses take in a much greater 



range, I will now also give a tabular result of the experiments that have been given 



relating to them. 



Table 2. 

 Rays of heat. Of light. Rays of heat. Of light. 



Very dark red glass stops 800 and 999^ Pale-blue 812 and 684 



Dark-red 606 999 T *? Dark-blue 362 801 



Orange 604 779 Indigo 633 999rs 



Yellow 333 819 Pale-indigo 532 978 



Pale-green 633 535 Purple 583 933 



Dark-green 849 949 Violet 489 955 



Bluish-green 768 769 



From this table, I shall also point out a few of the most remarkable results. A 

 yellow glass, for instance, stops only 333 rays of heat, but stops 819 of light : on 

 the contrary, a pale blue stops 812 rays of heat, and but 684 of light. Again, 



