VOL. LXXVir.] PHILOSOPHICAL TRANSACTIONS. 2QI 



The apparatus with the pasteboard diaphragni was exposed still to the sun ; 

 that with the iron was removed, and suffered to cool till its thermometer shov/ed 

 107°; it was then exposed again to the sun till it had acquired the heat of 1 10^, 

 to which degree the apparatus with the pasteboard hardly reached. The windows 

 were now shut. The heat of the room had arisen to 80°. Both the apparatuses 

 were placed on a table; the doors were shut, so that there was no current of air. 

 Pasteboard apparatus, after 30 minutes. Iron apparatus. 



96 104 



After 75 minutes, or 1^ 45' from the beginning. 



83 89 



After 2^ or 3^ 45' from removal from the sun. 



78 room 75 80 



A similar result arose when there were no glasses to exclude the external air. 

 Likewise when the diaphragms were changed from one apparatus to the other. 



The first thing to be noted in this experiment is, that the rays of the sun 

 acted on the same black paint only; for it was so thick, that the rays could not 

 penetrate /to the iron or pasteboard below. The colour was the same, and there 

 was the same gloss; if any thing, that on the iron, in the experiment related, 

 was rather more glossy, that it might not be favoured, as in former experiments 

 the results h*ad been in favour of the iron apparatus acquiring the greatest heat. 

 Every thing therefore was the same, except that the iron and pasteboard were 

 of different weights, of different capacities of heat, and of different degrees of 

 readiness to acquire heat, and communicate it. 



It is evident, that a greater quantity of heat was actually produced in the ap. 

 paratus with the iron diaphragm : for though in the first 2 or 3 minutes the 

 pasteboard became hotter than the iron, yet as soon as the iron began to be sen- 

 sibly heated, it became hot faster than the pasteboard, and actually became hotter, 

 and even continued to do so, when the pasteboard no longer could produce more 

 heat than was dissipated from the surface of the apparatus into the air. When 

 they were set in an air equally cold the apparatus with the iron diaphragm was 

 longer in cooling, though they were both of the same degree of heat when set 

 by. This greater quantity of heat I ascribe to the iron's taking the heat from 

 the black paint faster than the pasteboard, as being a better conductor. Just as 

 if a plate of glass was placed on a plate of steel, and another, perfectly similar, 

 was placed on a plate of clay, and both were placed equally among equal vibrating 

 bodies. In this case it is clear, that much greater vibration would take place if 

 the same means of exciting it were applied to that plate of glass attached to the 

 plate of steel, than if they were applied to that attached to the clay. I do not 

 mean to say, that heat is vibration; but merely to illustrate my idea of heat 

 being only a quality, and not a substance. I am led to this not only by this ex- 



p p 2 



