336 Prof. Tyndall on Luminous and Obscure Radiation. 



Radiation from Electric Light. — Experiment No. I. 

 Through transparent CS a . Through opake solution. 

 72°-0 ^70°'0 



Experiment No. II. 



76°-5 75°0 



Experiment No. III. 



77 0< 5 



76-5 



Calculating from these measurements the proportion of lumi- 

 nous to obscure heat, the result may be thus expressed : — 



30. Dividing the radiation from the electric light emitted by car- 

 bon points, and excited by a Grove s battery of forty cells, into ten 

 equal parts, one of those parts is luminous and nine obscure. 



31. The results may be thus presented in a tabular form : — 



Table III. — Radiation through dissolved Iodine. 



Source. Absorption. Transmission. 



Dark spiral 100 



Lampblack at 212° Fahr. . 100 



Red-hot spiral .... 100 



Hydrogen-flame .... 100 



Oil-flame 3 97 



Gas-flame 4 96 



White-hot spiral .... 4-6 95'4 



Electric light 10 90 



Repeated experiments may slightly alter these results, but they 

 are extremely near the truth. 



32. Having thus in the solution of iodine found a means of 

 almost perfectly detaching the obscure from the luminous heat- 

 rays of any source, we are able to operate at will upon the former. 

 Here are some illustrations : — The rock-salt lens was so placed in 

 the camera that the coal points themselves and their image 

 beyond the lens were equally distant from the latter. A battery 

 of forty cells being employed, the track of the cone of rays emer- 

 gent from the lamp was plainly seen in the air, and their point 

 of convergence therefore easily fixed. The cell containing the 

 opake solution was now placed in front of the lamp. The lumi- 

 nous cone was thereby entirely cut off, but the intolerable tem- 

 perature of the focus, when the hand was placed there, showed 

 that the calorific rays were still transmitted. Thin plates of tin 

 and zinc were placed successively in the dark focus and speedily 

 fused; matches were ignited, gun-cotton exploded, and brown 

 paper set on fire. Employing the iodine solution and a battery 

 of sixty of Grove's cells, all these results were readily obtained 



