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XXXIII. On the Mechanical Equivalent of Light. 

 By Professor Julius Thomsen of Copenhagen*. 



THAT the luminous ray exerts a mechanical action cannot 

 be doubted ; but as to its magnitude, there are at present 

 no determinations. I have proposed to myself to determine this, 

 at all events approximately. Since light can change into heat 

 when it is absorbed by a black and non-luminous surface, we 

 may, from the heating-effect of luminous rays freed from thermal 

 rays, draw conclusions as to the mechanical activity of light. The 

 production of heat by luminous rays was detected by Melloni's 

 apparatus ; but as this apparatus has only been used for relative 

 measurements, it was necessary to reduce the results to absolute 

 measurement. 



Hence the indications of the therm o-multiplier were compared 

 with the absolute radiation of a known source of heat. As 

 source of heat, a glass bulb was used filled with warm water, 

 and placed at various distances from the thermo-pile. The bulb 

 contained (including the reduced value in water of the glass) 

 1351 grms. water; at a temperature of 50° C. the cooling of the 

 bulb was 0°*185 a minute, and the bulb experienced therefore a 

 loss of 1351 gr. x0 o, 185 = 250c (c — caloric or thermal unit) in 

 a minute. 



But this loss arises partly from radiation, and partly from 

 cooling by contact with the air. If by Dulong's formula that 

 part be calculated which is due to radiation, it is found to be 

 102c for one minute, the temperature of the air being 17°. 



This source of heat was placed at a distance of 0*8 metre from 

 the thermo-pile ; and the multiplier indicated then a constant de- 

 flection of 17°'8. From this it may be inferred that a source of 

 light or of heat which, placed at a distance of 0*8 metre from the 

 thermo-pile, produces a deflection of 17°*8, radiates in a minute 

 102 thermal units. But as the indications of the multiplier are, 

 within certain limits, proportional to the radiation, under these 

 circumstances a deflection of 1° corresponds to a radiation of 

 5 '76c in a minute. 



By placing the bulb successively at different distances from 

 the pile, and observing the position of the needle when the tem- 

 perature was only 50°, similar factors were determined for a few 

 other distances, in which the different sources of light should 

 subsequently be placed. 



I then placed a candle at a distance of 0*8 metre from the 

 pile, and obtained a constant deflection of the needle of 36°'5f. 



* From PoggcndorfPs Annalen, June 1865. 



t In this, as in other cases in which the deflection was large, the expe- 

 riment was made so that the needle before the experiment was deflected 

 othe opposite side, and that the actual deflection from 0° did not exceed 30°. 



