<5g4 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1800, 



are liable to be stopped, in certain proportions, when transmitted through diapha- 

 nous bodies. 6. They are liable to be scattered on rough surfaces. 7. They have 

 hitherto been supposed to have a power of heating bodies ; but this remains to be 

 examined. 



The similar propositions relating to heat, which are intended to be proved in this 

 paper, will stand as follows. 1. Heat, both solar and terrestrial, is a sensation oc- 

 casioned by rays emanating from candent substances, which have a power of heating 

 bodies. 2. These rays are subject to the laws of reflection. 3. They are also sub- 

 ject to the laws of refraction. 4. They are of different refrangibility. 5. They are 

 liable to be stopped, in certain proportions, when transmitted through diaphanous 

 bodies. 6. They are liable to be scattered on rough surfaces. 7. They may be 

 supposed, when in a certain state of energy, to have a power of illuminating objects ; 

 but this remains to be examined. 



I have to mention, that the number of experiments which will be required to 

 make good all these points, exceeds the usual length of my papers ; on which ac- 

 count, I shall divide the present one into 2 parts. Proceeding therefore now to an 

 investigation of the first 3 heads that have been proposed, I reserve the next 3, and 

 a discussion which will be brought on by the 7th article, for the 2d part. 



Exper. 1 . Reflection of the Heat of the Sun. — I exposed the thermometer, which 

 in a former paper has been denoted by N° 3, to the eye-end of a J 0-feet Newtonian 

 telescope, which carried a camera-eye-piece, but no eye-glass. When, by proper 

 adjustment, the focus came to the ball of the thermometer, it rose from 52 to 110°; 

 so that rays which came from the sun, underwent 3 regular reflections ; one, on a 

 concave mirror, and the other 2, on 2 plain ones. Now these rays, whether they 

 were those of light or not, for that our experiment cannot ascertain, had a power of 

 occasioning heat, which was manifested in raising the thermometer 58°. 



Exper. 2. Refection of the Heat of a Candle. — At the distance of 29 inches 

 from a candle, I planted a small steel-mirror, of 3-^ inches diameter, and about 

 24- inches focal length. (See pi. 12, fig, l). In the secondary focus of it, I placed 

 the ball of the thermometer which in my paper 

 has been marked N° 2 ; and very near it, but 

 out of the reach of reflection, the thermometer 

 N° 3. Having covered the mirror till both were 

 come to the temperature of their stations, I 

 began as annexed. Here, in 5 minutes, the ther- 

 mometer N° 2 received 34- degrees of heat from the 

 candle, by reflected rays. I now covered the mirror, 

 but left all the rest of the apparatus untouched. — 

 Here, in 6 minutes the thermometer lost the 3^- 

 degrees of heat again, which it had gained before. 

 I uncovered the mirror once more ; and, in 5 mi- 

 nutes, the 34- degrees of heat were regained. In 

 consequence of which, we are assured that certain 



