on the terrestrial Rays that occasion Heat. 507 



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 proved, by the whole tenour of our prismatic 

 experiments, that this invisible heat is continued, from the be- 

 ginning of the least refrangible rays towards the most refran- 

 gible ones, in a series of uninterrupted gradation, from a gentle 

 beginning to a certain maximum ; and that it afterwards declines, 

 as uniformly, to a vanishing state. These phenomena have been 

 ascertained by an instrument, which, figuratively speaking, we 

 may call blind, and which, therefore, could give us no infor- 

 mation 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 hypo- 

 thesis ; for it does not appear that nature is in the habit of using 

 one and the same mechanism with any two of our senses; 

 witness the vibrations of air that make sound ; the effluvia that 

 occasion smells ; the particles that produce taste ; the resistance 

 or repulsive powers that affect the touch: all these are evi- 

 dently suited to their respective organs of sense. Are we then 



