Because it is impossible to collect all the rays, and also because a portion of the caloric, imparting heat to the rays, is absorbed by the air, and by the various other bodies with which the rays come in contact.
215. Can heat be reflected in any great degree of intensity?
Yes; to such a degree that inflammable matters may be ignited by it. If a cannon ball be made red hot, and then be placed in an iron stand between two bright reflectors, inflammable materials, placed in a proper position to catch the reflected rays, will ignite from the heat.
There is a curious and an exceptional fact with reference to reflected heat, for which we confess that we are unable to give "The Reason Why." It is found that snow, which lies near the trunks of trees or the base of upright stones, melts before that which is at a distance from them, though the sun may shine equally upon both. If a blackened card is placed upon ice or snow under the sun's rays, the frozen body underneath it will be thawed before that which surrounds it. But if we reflect the sun's rays from a metal surface, the result is directly contrary—the exposed snow is the first to melt, leaving the card standing as upon a pyramid. Snow melts under heat which is reflected from the trees or stones while it withstands the effect of the direct solar rays. In passing through a cemetery this winter (1857), when the snow lay deep, we were struck with the circumstance that the snow in front of the head-stones facing the sun was completely dissolved, and, in nearly every instance, the space on which the snow had melted assumed a coffin-like shape. This forced itself so much upon our attention that we remained some time to endeavour to analyse the phenomena; and it was not until we remembered the curious effect of reflected heat that we could account for it. It is obvious that the rays falling from the upper part of the head-stone on to the foot of the grave would be less powerful than those that radiated from the centre of the stone to the centre of the grave. Hence it was that the heat dissolved at the foot of the grave only a narrow piece of snow, which widened towards the centre, and narrowed again as it approached the foot of the head-stone, where the lines of radiation would naturally decrease. Such a phenomena would prove sufficient to raise superstition in untutored minds.
"The light of the righteous rejoiceth, but the lamp of the wicked shall be put out."—Proverbs xiii.
216. Are good reflectors of heat also good absorbers?
No; for reflectors at once send back the heat which they receive, while absorbers retain it. It is obvious, therefore, that reflectors cannot be good absorbers.