Cheapest Form of Light. 279 



devised ; and that, finally, there seems to be no reason why 

 we are forbidden to hope that we may yet discover a method 

 (since such a one certainly exists and is in use on the small 

 scale) of obtaining an enormously greater result than we now 

 do from our present ordinary means for producing light. 



Appendix. 



Determination, in Calories, of the Heat in the Luminous (Abdo- 

 minal) Radiation of Pyrophorus noctilucus. 



The determination is reached by two steps : — (1) The cali- 

 bration of the galvanometer, so as to give the value of its 

 division in calories ; and (2) the inference from the observed 

 deflexion in divisions of the total of calories radiated. 



(1) The bolometer, whose face occupied 0*19 square centim. 

 (a), gave a deflexion of 342 divisions (6), at a distance of 

 25 centim. (r) from a 5 centim. circular aperture filled by a 

 blackened Leslie cube. Seen from the centre of this aperture, 



the bolometer occupied then - — ^ =0*0000484 of the hemi- 

 sphere, and would have received this fraction of the total 

 radiation, except that, being placed exactly opposite the radia- 

 ting surface, more than the mean radiation fell on it in a 

 proportion which calculation shows to be about J. The 

 fraction of the total radiation which it actually received, then, 

 was 0-0000645 (c). 



Accordingly the total radiation would have caused a 



deflexion - =5,300,000 divisions. 

 c ' 



The surface of the cube was at a temperature of 99° C, 



and was limited by the diaphragm to an area of 19 "6 square 



centim. (d). The total radiation from one centimetre, then, 



would have caused a deflexion of —: =270,400 div. The 



cd 



temperature of the bolometer, which was that of the apart- 

 ment, was 20° C. According to Dulong and Petit's law, the 

 radiation from such a surface at 99° C. to one at 20° C. would 

 be 1-11 cal. per minute (e), which does not greatly differ 

 from our own independent determinations; and for 10 sec. 

 =0*167 min. (/) (the time of the galvanometer-swing) it 



equals 0'185 cal. (<?/). Hence * =* ^^ = 1462000 div. 



caef 0*185 

 is the potentiality of work in 1 calorie, to be expressed in the 

 swing of the galvanometer-needle, and 1 div. =0*000000684 

 cal. 



