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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



square inches of surface, to be 444 millionths of 

 a grain. This he called " weighing a beam of 

 light." In his subsequent lecture at the Royal 

 Institution, he stated it as the result of his ex- 

 periments that the radiant energy of the sun 

 equals that of 1,000 candles at twelve inches' 

 distance, its mechanical power upon two square 

 inches of surface being thus equal to 444 thou- 

 sandths of a grain. This is equivalent to about 

 32 grains per square foot, to two hundred-weight 

 per acre, to 57 tons per square mile, or to nearly 

 3,000,000,000 tons on the exposed surface of the 

 globe — sufficient to " knock the earth out of its 

 orbit if it came upon it suddenly," and to " drive 

 the globe into space if it were not counteracted 

 by the force of gravitation." But, as Mr. Crookes 

 considerately added, " it must be remembered that 

 our earth is not a lamp-blacked body inclosed in 

 a glass case ; nor is its shape such as to give a maxi- 

 mum of surface with the minimum of weight." 



For comparative estimates, however, Mr. 

 Crookes devised another form of "torsion-bal- 

 ance," in which one half of the pith-bar was 

 blacked and the other left white, so that, when 

 the whole was subjected to a broad beam of light, 

 the bar would be made to swing by its -different 

 action on the white and on the black surfaces. 

 This bar being made to carry a mirror and a 

 small magnet, its slightest deflection from the 

 north and south zero could be detected by the 

 movement of a spot of light reflected from the 

 mirror along a graduated scale : while the me- 

 chanical force required to produce any degree of 

 deflection could be very exactly estimated, the 

 apparatus being made more or less sensitive as 

 desired by the use of a controlling magnet on the 

 outside. The results of a long series of experi- 

 ments served to show the photometric value of 

 this apparatus when lights of the same kind were 

 compared ; the amount of deflection produced by 

 the light of the same standard candle at different 

 distances being obviously conformable to the law 

 of inverse squares, while the effect of two such 

 candles placed side by side was found to be prac- 

 tically double that of a single candle, and that of 

 three candles practically triple. Again, when a 

 candle was placed on either side of the apparatus 

 and equidistant from it, so that each would neu- 

 tralize the effect of the other, the index-spot of 

 light remained at zero ; but, by shading one or 

 other of the candles, the index-spot was made to 

 fly off to the extremity of the scale. 



This arrangement afforded a ready means of 

 comparing the radiant energy of different sources 

 of light. Thus, if a " standard candle " was 



placed on one side at a distance of 48 inches, 

 and a gas-burner on the other was found to bring 

 the index-spot to zero when removed to a dis- 

 tance of 113 inches, their relative motor powers 

 would be as 48 2 to 1 13-— that is, the radiant en- 

 ergy of the gas-burner was equal to that of 5£ 

 candles. But it was as pure an assumption on 

 Mr. Crookes's part to affirm that the mechanical 

 action exerted by two flames of different kinds 

 would measure their relative illuminating powers 

 as it would have been to say that their heating 

 action would be proportional to their illuminating 

 action, which we know perfectly well not to be the 

 case — the gas-flame, as every one knows, having a 

 much greater heating-power than the candle-flame 

 in proportion to the light it gives. 



The same form of " torsion-balance " was em- 

 ployed by Mr. Crookes to determine the relative 

 effects of the interposition of screens of different 

 kinds. Thus, a deflection to 180° being pro- 

 duced by a candle at three feet distance, its 

 amount was reduced to 161° by the interposition 

 of a screen of yellow glass, to 128° by a screen 

 of red glass, and to 102° and 101° respectively 

 by screens of blue and green glass. A far more 

 potent effect, however, was produced by the in- 

 terposition of a glass trough containing water, 

 which brought down the deflection to 47°, while 

 a screen formed of a plate of alum reduced it to 

 2*7°. As the absorption of the luminous rays in 

 passing through such transparent media must 

 have been very slight, while they practically cut 

 off the rays of " dark heat," this marked reduc- 

 tion would seem mainly attributable to the ab- 

 straction of the latter ; but, strong in his convic- 

 tion of the immediate mechanical effect produced 

 by radiation, Mr. Crookes thus expressed himself 

 in regard to it : " There is no real difference be- 

 tween heat and light ; all we can take account of 

 is difference of wave-length, and a ray of a defi- 

 nite refrangibility cannot be split up into tw r o 

 rays, one being heat and one light. Take, for in- 

 stance, a ray of definite refrangibility in the red. 

 Falling on a thermometer it shows the action of 

 heat; on a thermopile it produces an electric 

 current; to the eye it appears as light and color; 

 on a photographic plate it causes chemical action ; 

 and on the suspended pith it causes motion." 

 Now, so far as light, heat, and chemical action, 

 are concerned, this mode of expressing their re- 

 lations is undoubtedly that which all physicists 

 now accept ; these agencies being regarded, not 

 as separate and distinct, but as different manifes- 

 tations of that one physical action which consti- 

 tutes radiation. This action, according to the 



