The radiation of heat was exhibited in a still more de cisive manner by Pictet, who seems to have undertaken his experiments on the subject at the suggestion of Saussure. This distinguished naturalist was led to form some specu lations concerning caloric, from certain atmospherical phe nomena, which he noticed during his travels among the Alps, where he conceived that the communication of heat could not be accomplished by the contact of the heated body. He refers to an experiment of Mariotte's, which was published in the Memoirs of the Academy of Sci ences for the year 1682, in which he states, that " the warmth of a fire reflected by a burning mirror is sensible at its focus." Lambert also informs us, in his Pyrometrie, that he placed a burning body in the focus of a concave mirror, and that he was able by means of it to inflame ano ther body, placed in the focus of an opposite mirror, at the distance of above 20 feet. In this experiment, Lambert distinctly marks the difference between what be calls " luminous heat" and " obscure heat," and attributes the effect to the latter principle, i. e. to heat, properly so called, in opposition to light.
Saussure repeated the experiment in conjunction with Pictet ; and, in order to prevent the interference of the ac tion of light, they employed a ball of iron, heated to a de gree short of what would render it luminous in the dark. They used tin mirrors, that were placed more than 12 feet from each other : and, when the iron ball was put into one focus, they suspended a thermometer in the other, and observed the instrument to be very perceptibly affected, more than another thermometer, equally near the ball, but out of the focus. The former was raised from 4° to 141. of Reaumur ; the latter from 4° to 61° only: (Voyages Bans les illfies,§ 926.) The result of this experiment is easily explicable, upon the supposition that heat, whatever be its nature, radiates in straight lines ; that it impinges against solids that are opposed to its course ; and that, ac cording to circumstances, it either raises the temperature of this body, by being united to it, or is reflected from its surface. The heated ball, in this case, emitted rays of heat in every direction : those that were contiguous to the mir ror fell upon it ; but, owing to its polished surface, were reflected in straight lines to the other mirror, and were again reflected from this, according to the laws of mecha nical impulse, into the focus in which the thermometer was suspended. This thermometer received the effect both of the rays that were sent of by the iron ball, on the side con tiguous to it, and of those which were on the contrary side of the ball, next to the mirror ; whereas the thermo meter not in the focus, only received heat from the sick of the ball opposed to it. Pictet's apparatus is shewn in Plate
CCLX XXVIII. Fig. 9. See Description of Plates.
M. Pictet still further prosecuted these experiments, and varied them in different ways, so as to repel the objec tions that might be urged against the conclusions which he derived from them. In order to separate the light from the heat, and to chew the distinct operations of each, he plac ed a lighted candle in one of the foci, and noticed its effects upon a thermometer placed in the other focus. He then interposed a plate of glass between the candle and the thermometer ; and he found, as he had expected, that al though the light passed as before, a considerable portion of the heat was intercepted in its passage from one mirror to the other ; in this way confirming the results that had been formerly obtained by Scheele. But, in order to re move more effectually all suspicion that the effect in this case depended upon the rays of light, he placed a small flask of boiling water in the focus, from which we may be confident that no light could be emitted : and this he found to radiate heal, and to raise the thermometer very percep tibly. The experiments of Pictet may therefore be con sidered as completely establishing the point, that heat is sent off in right lines from bodies, where light cannot be supposed to be present. He also rendered the radiating power of caloric still more obvious, by shelving, that when rays of heat impinge against a body, if it have a polished surface, they are reflected from it ; but, if the surface be such as not to admit of reflection, they enter into it, and raise its temperature. In pursuance of this idea, he found, that when the bulb of the thermometer which he employ ed was blackened, it rose more rapidly, or absorbed more heat, than when its surface was clean and bright, thus prov ing, that the heat from boiling water was, in this respect, similar to the heat in the sun's rays, or that emitted during combustion : (Essay on Fire,§ 51, et seq.) We have al ready referred to the experiments of Dr Herschel, in which he analysed the sun's rays, and separated the part which produces heat, from that which excites the sensa tion of colour. These experiments proved, in a very de cisive manner, the radiating power of the calorific part of the solar beam ; and he afterwards made experiments of a similar kind, upon different species of heat, extricated from bodies on the surface of the earth, such as burning fuel and red hot iron. Phil. Trans. 1800, p. 316.