RELATION OF HEAT AND LIGHT. 193 



exceeds the heat of a powerful furnace. A piece of gold placed in the focus 

 of such a glass, has not only been melted, but has actually been converted into 

 vapor, by Lavoisier. This fact was proved by a piece of silver placed at some 

 height above the gold, having been gilded by the condensation of the vapor of 

 the gold on its surface. 



Artificial lights are generally accompanied by heat in various degrees, and, 

 generally, the more intensely brilliant the light, the more powerful will be the 

 calorific effects. It would appear, however, from some remarkable differences 

 which are observed in the transmission of artificial light through transparent 

 bodies, that the invisible calorific rays exist in such light in a much greater 

 proportion than in solar light. If a screen of plate-glass be placed before a 

 coal fire, although scarcely any light will be intercepted, nearly all the heat 

 will be immediately stopped. This has been generally adduced as a proof 

 that light and heat are distinct principles, since the glass, in this case, is said 

 to separate them. The effect, however, admits of explanation with equal fa- 

 cility, on the supposition that heat is a quality of light, and that the luminous 

 property may have so weak a force in some rays, as to be incapable of affect- 

 ing the sight. The light from the fire, in the case just mentioned, is generally 

 of a red color, like that of the rays at the lowest point of the luminous spec- 

 trum ; it is probable, therefore, that it may contain also the more calorific in- 

 visible rays which are, in that neighborhood, in the spectrum. If this be ad- 

 mitted, the light emitted by a fire will consist of a much larger proportion of 

 the invisible calorific rays than is found in sunlight. The proportion, there- 

 fore, which the visible rays transmitted by the glass bears to the invisible rays 

 which may not be transmitted, will be much less than in sunlight, and conse- 

 quently the rays transmitted by the glass will possess comparatively a much 

 less heating power. 



One of the most remarkable exceptions to the general fact, that the presence 

 of light necessarily infers the presence of heat, is the fact, that moonlight, in 

 whatever degree it can be concentrated by the most powerful burning-glasses, 

 has never yet been found to affect the most sensible thermometer. De-la-Hire 

 collected the rays of the full moon, when on the meridian, by a burning-glass 

 of about three feet in diameter, in the focus of which he placed a delicate air 

 thermometer. The density of the lunar rays was in this case increased in the 

 proportion of about 300 to 1, and yet not the slightest effect was produced. 

 This anomaly is, however, easily accounted for. Admitting that the moon ab- 

 sorbs no part of the invisible calorific rays of the solar light, it will follow that 

 the heating power of moonlight cannot be in a greater proportion to that of 

 sunlight than the relative brilliancy of the two lights. Now, to determine the 

 comparative splendor of moonlight and sunlight, let the moon, when seen in the 

 firmament during the day, be compared with a white cloud near it ; its bright- 

 ness, and that of the cloud, will appear very nearly the same. Assuming that 

 they are exactly the same, it will follow that in the day, when the whole fir- 

 mament is covered with white fleecy clouds, the brilliancy of the light would 

 be the same as if the whole firmament were covered with an illuminated sur- 

 face similar to that of the moon. The light, therefore, of a cloudy day of this 

 kind, will be as much more brilliant than the light of the moon, as the magni- 

 tude of the whole firmament is greater than that portion of it occupied by the 

 full moon. This proportion is nearly that of 300,000 to 1 ; and hence the 

 light of a cloudy day is 300,000 times brighter than moonlight : consequently, 

 the intensity of the moon's rays is certainly not greater than 3-^oFo P art °^ t ^ le 

 intensity of sunlight. In the experiment of De-la-Hire, just explained, where 

 the moon's rays were concentrated in the proportion of 300 to 1, the effect of 

 the concentrated light in the focus of a burning-glass would not amount to 



VOL,. II.— 13 



