698 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



slip of deal, £ inch thick, and 1 inch broad, in a horizontal direction, so as to be ot 

 an equal height, in the middle of its thickness, with the centre of the mirror. 

 Against the side, facing the mirror, were fixed the 2 thermometers N° 2 and N° 3, 

 with their balls within half an inch of each other, and the scales turned the op- 

 posite way. A little of the wood was cut out of the slip, to make room for the 

 balls to be freely exposed. That of N° 2 was in the axis of the mirror ; and the 

 ball of N° 3 was screened from the reflected rays, by a small piece of pasteboard tied 

 to the scale. The small ivory scales of the thermometers, with the slip of wood at 

 their back, which however was feather-edged towards the stove, intercepted some 

 heat; but it will be seen presently that there was enough N <> N< > 3 



to spare. When the Stove was of a good heat, I brought Min. In the Focus. Screened. 

 the apparatus to a place ready prepared for it. Here we ° & ^ 



find that, in 1 minute, the invisible culinary heat raised 



the thermometer N° 2, 39 degrees ; while N° 3, from change of temperature, rose 

 only 1 , though its exposure to the stove was in every respect equal to that of N° 3, 

 except so far as relates to the rays returned by the mirror; and therefore the radiant 

 nature of these invisible rays, their power of heating bodies, and their being 

 subject to the laws of reflection, are equally established by this experiment. 



Exper. 10. Reflection of the Invisible Rays of Heat of a Poker, cooled from being 

 red-hot till it could no longer be seen in a dark Place. — The great abundance of heat 

 in the last experiment, would not allow of its being carried on without injury to the 

 thermometer, the scale of which is not extensive ; I therefore placed a poker, when 



of a proper black heat, at 12 inches from the steel 



Min N 2 



mirror, fig. 1, and received the effect of its condensed Mirror covered ' 61' 



rays on the thermometer N° 2, placed in the focus. Open 1 68 



* , , . , • .i • Covered 2 6l 



Then, alternately covering and uncovering the mirror, 0pen 3 ^ 



1 minute at a time, the effect was thus. Here, in 6 m , Covered 4 59 



we have a repeated result of alternate elevations and de- covered '. '. ". *. *. 6 58 



pressions of the thermometer, all of which confirm the 



reflexibility, the radiant nature, and the heating power, of the invisible rays that 

 came from the poker. 



From these experiments it is now sufficiently evident, that in every supposed 

 case of solar and terrestrial heat, we have traced out rays that are subject to the re- 

 gular laws of reflection, and are invested with a power of heating bodies ; and this 

 independent of light. For though, in 4 cases out of 6, we had illuminating as 

 well as heating rays, it is to be noticed that our proof goes only to the power of 

 occasioning heat, which has been strictly ascertained by the thermometer. If it 

 should be said, that the power of illuminating objects, of these same rays, is as 

 strictly proved by the same experiments, I must remark, that from the cases of in- 

 visible rays brought forward in the last 4 experiments, it is evident that the con- 

 clusion, that rays must have illuminating power, because they have a power of oc- 

 casioning heat, is erroneous; and, as this must be admitted, we have a right to ask 



