VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 773 



certain conclusions, that will entirely alter the form of our inquiry. Thus, from 

 the J 8th experiment it appears, that 21 degrees of solar heat were given in 1 

 minute to a thermometer, by rays which had no power of illuminating objects, 

 and which could not be rendered visible, notwithstanding they were brought toge- 

 ther in the focus of a burning lens. The same has also been proved of terrestrial 

 heat, in the 9th experiment ; where, in one minute, 3Q° of it were given to a 

 thermometer, by rays totally invisible, even when condensed by a concave mirror. 

 Hence it Is established, by incontrovertible facts, that there are rays of heat, both 

 solar and terrestrial, not endowed with a power of rendering objects visible. 



It has also been proyed, by the whole tenour of our prismatic experiments, that 

 this invisible heat is continued, from the beginning of the least refrangible rays 

 towards the most refrangible ones, in a series of uninterrupted gradation, from a 

 gentle beginning to a certain maximum ; and that it afterwards declines as uni- 

 formly to a vanishing state. These phenomena have been ascertained by an instru- 

 ment which, figuratively speaking, we may call blind, and which therefore could 

 give us no information about light; yet, by its faithful report, the thermometer, 

 which is the instrument alluded to, can leave no doubt about the existence of the 

 different degrees of heat in the prismatic spectrum. This consideration, as has 

 been observed, must alter the form of our proposed inquiry; for the question being 

 thus at least partly decided, since it is ascertained that we have rays of heat which 

 give no light, it can only become a subject of inquiry, whether some of these heat- 

 making rays may not have a power of rendering objects visible, superadded to their 

 now already established power of heating bodies. This being the case, it is evident 

 that the onus probandi ought to lie with those who are willing to establish such an 

 hypothesis ; for it does not appear that nature is in the habit of using one and the 

 same mechanism with any 2 of our senses; witness the vibration of air that makes 

 sound; the effluvia that occasion smells ; the particles that produce taste; the re- 

 sistance or repulsive powers that affect the touch : all these are evidently suited to 

 their respective organs of sense. Are we then here, on the contrary, to suppose 

 that the same mechanism should be the cause of such different sensations, as the 

 delicate perceptions of vision, and the very grossest of all affections, which are 

 common to the coarsest parts of our bodies, when exposed to heat ? 



But, let us see what light may now be obtained from the several articles that 

 have been discussed in this paper. It has been shown, that the effect of heat and 

 of illumination may be represented by the two united spectra, which we have 

 given. See fig. 12, pi. 13. Now when these are compared, it appears that those 

 who would have the rays of heat also to do the office of light, must be obliged to 

 maintain the following arbitrary and revolting positions ; namely, that a set of rays 

 conveying heat, should all at once, in a certain part of the spectrum, begin to 

 give a small degree of light ; that this newly acquired power of illumination should 

 increase, while the power of heating is on the decline ; that when the illuminating 

 principle is come to a maximum, it should, in its turn, also decline very rapidly, 



