1819.] Prof. Leslie on Heat and Climate. 7 



intensity of the incident light. If the initial effect, for instance, 

 of the sun's rays on the blackened bulb of a thermometer be 

 diminished one-tenth by the interposition of a bit of plate-glass, 

 that reduced quantity will also lose a tenth by the addition of 

 another bit of the same glass ; and thus, by continuing the appli- 

 cation of the transparent plates, the changes produced upon the 

 thermometer will form a descending geometrical progression. 

 The same consequences will follow if the light, previous to its 

 falling upon the bulb of the thermometer, be repeatedly reflected 

 from mirrors. I find likewise that the intensity of the rays 

 emitted from the sun at different altitudes as indicated by the 

 thermometer, corresponds precisely with the quantities formerly 

 assigned by the ingenious M. Bouguer from very different prin- 

 ciples. If heat then accompanies invariably the absorption of 

 light, and is proportional to the quantity absorbed, what more is 

 requisite to establish their identity. 



If the blackened bulb of a thermometer be inclosed by a slow 

 conducting substance with a front of glass or mica, and held per- 

 pendicularly to a sun-beam, the mercury will rise at first with a 

 regular and uniform motion. The celerity of ascent, however, 

 soon begins to deviate from equability, and will by successive 

 diminutions at last disappear. But it is easy to account for these 

 anomalies. After the bulb of the thermometer is sensibly 

 affected by the solar rays, the contiguous air also becomes 

 warm by communication, and flows from that bulb towards 

 the case or including substance ; a circulation is thereby pro- 

 duced, and the rapidity with which the heat is conducted off 

 is proportional to the increase of temperature in the bulb itself. 

 While the accumulation of heat, therefore, is perfectly equable, 

 its dissipation continually augments, till these opposite causes 

 come to balance each other, and then the mercury will remain 

 stationary at its extreme height. The initial change on the 

 thermometer is in every case the only certain and accurate 

 measure of the communication of heat.* 



The rays of the sun collected in the focus of a powerful 

 burning lens or mirror are able to fuse, and even to volatilize, the 

 metals, and the most refractory and opaque stones ; yet they 

 produce no remarkable effect on glass and pellucid crystals, and 

 still less on water. These experiments evince that the heat is 

 not excited by the impulse of the light, but is communicated 

 merely by absorption. In the case of the opaque bodies, the 

 influence of the incident beam being confined almost to the sur- 

 face, the heat accumulates there with vast rapidity, which is not 

 sensibly diminished by the subsequent transfusion through the 

 internal mass or by dissipation in the air. A part only of the light 

 which falls on pellucid substances is absorbed in its passage, and 



* I did not then suspect that I should afterwards invent so delicate an instru- 

 ment a* the Photometer.— A. 



