Power 



SCIENTIFIC SIDE-LIGHTS 



544 



warmer than the white surfaces, and this 

 greater warmth is communicated to the air- 

 molecules, and causes them to rebound with 

 greater rapidity from the dark surfaces and 

 back again from the glass of the vessel, and 

 the reaction, being all in one direction, 

 causes the arms to revolve. The near ap- 

 proach to a vacuum is necessary, both to di- 

 minish resistance, and by greatly reducing 

 the number of molecules in the vessel, to 

 allow the very small differential action to 

 produce a sensible effect. Sir William 

 Crookes has found that there is a degree 

 of rarefaction where the action is at a maxi- 

 mum, and that when a nearer approach 

 to a perfect vacuum is attained the motion 

 rapidly diminishes. A proof is thus given 

 of the correctness of the explanation, and 

 the instrument may, therefore, be considered 

 to afford us an experimental illustration of 

 the molecular theory of gases. WALLACE 

 The Wonderful Century, ch. 8, p. 59. (D. 

 M. & Co., 1899.) 



2683 



Heat-waves Con- 



trasted with Rays of Light. Besides those 

 which produce light, the sun sends forth in- 

 cessantly a multitude of waves which pro- 

 duce no light. The largest waves which the 

 sun sends forth are of this non-luminous 

 character, tho they possess the highest heat- 

 ing power. 



A common sunbeam contains waves of all 

 kinds, but it is possible to sift or filter the 

 beam so as to intercept all its light, and to 

 allow its obscure heat to pass unimpeded. 

 For substances have been discovered which, 

 while intensely opaque to the light-waves, 

 are almost perfectly transparent to the 

 others. On the other hand, it is possible, 

 by the choice of proper substances, to inter- 

 cept in a great degree the pure heat-waves, 

 and to allow the pure light-waves free trans- 

 mission. . . . Supposing, then, that we 

 withdraw, in the first instance, the large 

 heat-waves and allow the light-waves alone 

 to pass. These may be concentrated by 

 suitable lenses and sent into water without 

 sensibly warming it. Let the light-waves 

 now be withdrawn and the larger heat-waves 

 concentrated in the same manner; they may 

 be caused to boil the water almost instanta- 

 neously. . . . The light- waves, even when 

 concentrated to the uttermost, are unable 

 to melt the most delicate hoar-frost. TYN- 

 DALL Forms of Water, p. 12. (A., 1899.) 



2684. 



The Sun's Attract- 



ive Energy. The sun, besides sustaining us 

 by his light-giving and heat-supplying pow- 

 ers, keeps us always near to him by that 

 mighty force of attraction which his vast 

 bulk enables him to exert. When we look 

 at the sun as he rises ( even as " the glory 

 of God coming from the way of the east ") 

 how seldom is the thought present in our 

 minds that in that ruddy orb there exists 

 the most tremendous power, swaying not 

 only this vast globe on which we live, but 



orbs yet vaster than she is and traveling 

 on far wider courses. PROCTOR Expanse of 

 Heaven, ch. 2, p. 13. (L. G. & Co., 1897.) 



2685. 



Transporting Ac- 



tion of the Wind Granite Cliffs Blown 

 Away. The transporting action of the wind, 

 or " deflation," as it is termed, goes on with- 

 out ceasing day and night and during all 

 seasons; and the result is seen in the deep- 

 ly eroded rocks, enormous masses of which, 

 it can be shown, have been thus gradually 

 removed. The evidence of denudation is con- 

 spicuous, but its products have for the most 

 part been carried away. In some places, as 

 Professor Walther remarks of the Libyan 

 Desert, are great walls of granite rising to 

 heights of 6,000 feet, but showing no slopes 

 of de"bris below, as would infallibly be pres- 

 ent under temperate conditions of climate. 

 In other places, again, are deeply excavated 

 wadies containing no beds of gravel, grit, 

 and sand, such as would not fail to show 

 themselves had the depressions in question 

 been formed by water-action alone. Every- 

 where deep, cavelike hollows have been worn 

 out in the rocks, and yet these hold no sedi- 

 ment or detritus, but are swept bare. The 

 wind tends, in short, to transport all loose 

 material from the scene of its origin to the 

 borders of the desert. GEIKIE Earth Sculp- 

 ture, ch. 2, p. 24. (G. P. P., 1898.) 



2686. 



Unseen Rays of 



Spectrum Rich in Heat. The great pioneer 

 in this domain of science [the study of radi- 

 ation] was Sir William Herschel. Causing 

 a beam of solar light to pass through a 

 prism, he resolved it into its colored con- 

 stituents; he formed what is technically 

 called the solar spectrum. Exposing ther- 

 mometers to the successive colors, he de- 

 termined their heating power, and found it 

 to augment from the violet or most re- 

 fracted end to the red or least refracted 

 end of the spectrum. But he did not stop 

 here. Pushing his thermometers into the 

 dark space beyond the red, he found that, 

 tho the light had disappeared, the radiant 

 heat falling on the instruments was more 

 intense than that at any visible part of the 

 spectrum. In fact, Sir William Herschel 

 showed, and his results have been verified 

 by various philosophers since his time, that, 

 besides its luminous rays, the sun pours 

 forth a multitude of other rays, more pow- 

 erfully calorific than the luminous ones, but 

 entirely unsuited to the purposes of vision. 

 TYNDALL Fragments of Science, ch. 2, p. 

 32. (A., 1897.) 



2687. Unseen Rays of 



Spectrum Yield Chemical Energy, as Well as 

 Heat. At the less refrangible end of the 

 solar spectrum, then, the range of the sun's 

 radiation is not limited by that of the eye. 

 The same statement applies to the more re- 

 frangible end. Hitter discovered the exten- 

 sion of the spectrum into the invisible region 

 beyond the violet, and in recent times this 



