Non-luminous Radiation emitted by a Gas Flame. 65 



incident light traversed Cell IV ; hence it would appear, if we assume 

 that neither the air nor the aqueous vapour absorbed a measurable 

 amount of the radiation, that the total radiation of the gas burner 



contained 1*646 x ^,? or 1*94 per cent, of luminous radiation, a 

 84*68 r ' 



result that agrees with that obtained by Julius Thomsen. 



As has already been stated, the method employed was based on the 

 assumption that the amount of absorption due to the air or to the 

 aqueous vapour it contained was negligible, or at least that it could 

 be allowed for. The experiments were made in an underground room 

 in which the temperature and the hygrometric condition of the air 

 varied but slightly. The readings of a wet and dry bulb thermometer 

 never differed during the course of the experiments more than about 

 1° F., the temperature of the air varying from about 59° F. to 67° F. 

 Hence it was always nearly saturated, and the mass of water-vapour 

 per unit volume of air was nearly the same. ' 



Professor Tyndall states (* Contributions to Molecular Physics,' 

 p. 133) that 4 feet of saturated air (the temperature of the air and 

 the nature of the source of radiation, which from the diagram was 

 apparently a gas flame, are not mentioned) absorbed 5J per cent, of 

 the total radiation. 



If we assume that for very small angles (and in the course of these 

 experiments the angles never exceeded 4°) the deflections of the gal- 

 vanometer were strictly proportional to the amount of radiant energy 

 incident upon the face of the thermopile, and that the radiation from 

 the lamp suffered no absorption before it reached the thermopile, 

 then the deflections of the galvanometer would vary inversely with 

 the square of the distance of the lamp. Table II shows that the mean 

 deflection when the lamp was 150 cm. from the thermopile was 414 

 scale divisions. The deflections corresponding to other positions of 



the lamp were calculated by the expression x ^ ^ = d where 



x 2 



x is the distance of the lamp, and d the scale reading; the results are 

 set forth in Table IX, column 2. 



If, however, a portion of the radiation from the lamp was absorbed 

 by the air, or the aqueous vapour it contained, then the decrease in 

 the deflection as the lamp was moved further and further off would be 

 partially due to the increased amount of absorption produced by the 

 longer column of air and aqueous vapour. 



Taking Professor Tyndall's value for the absorption (5 5 per cent, 

 in 4 feet), the percentage amount that would be absorbed in passing 

 through 10 cm., 50 cm., 60 cm., 70 cm., and 80 cm. of saturated air 

 was calculated, and thence, by the expression given above, the value 

 for the deflection ; x 2 being taken as (Stance oflamp^xlOO 



100 — absorption 



VOL. XLVII. -p 



