F. J. Rogers — Magnesium as a Source of Light. 313 



a sperm candle to 3 "7 calories in the case of gas. His values 

 are too high, for he used distilled water to absorb the radiant 

 heat instead of an alum solution, and made no correction for 

 the heat transmitted. Dr. Nichols* puts the thermal equiva- 

 lent of the light from an incandescent lamp at 3 - 6 calories. 

 .Magnesium-light is much richer than gaslight in those rays 

 (green) which are most effective in producing vision. Never- 

 theless it is highly improbable that its thermal equivalent is 

 much less than two-thirds that of the incandescent lamp or in 

 other words, that it is much less than 24 calories. But if the 

 total efficiency of magnesium light is considerably less than -10 

 then its thermal equivalent must be considerably less than 24 

 calories. 



It may be interesting to compare magnesium with gas as a 

 producer of luminous energy. The radiant efficiency of gas- 

 light is about *015 ; but of the total energy of combustion in 

 the case of gas only 45 or *20 is radiant energy. Therefore 

 the total efficiency of gaslight is "0022 or '003. This means 

 that potential energy in the form of uncombined magnesium 

 and oxygen is from 30 to 40 times more effective as a producer 

 of luminous energy than the same quantity of potential energy 

 in the form of illuminating gas and oxygen. Even this state- 

 ment does not give the full measure of the superiority of mag- 

 nesium light. If the thermal equivalents of gaslight and 

 magnesium light are 3'6 and 24 respectively, the same quan- 

 tity of luminous energy in the form of the latter will produce 

 50 per cent more candle power than if it were luminous 

 energy in the form of gaslight. 



The results of this investigation may be summed up as 

 follows : 



1. The spectrum of burning magnesium, as has already been 

 pointed out by Pickering, approaches much more nearly that 

 of sunlight than does the spectrum of any other artificial 

 illuminant. 



2. The temperature of the magnesium flame, about 1340° C, 

 lies between that of the Bunsen burner and that of the air 

 blast, lamp, although the character of its spectrum is such as 

 would correspond to a temperature of nearly 5000° C. were its 

 light due to ordinary incandescence. 



3. The "radiant efficiency" is 13|- per cent ; a value higher 

 than that for any other artificial illuminant (excepting perhaps 

 the light of the electric discharge in vacuo for which Dr. 

 Staub of Zurich has found an efficiency of about 34 per cent. 



4. The radiant energy emitted by burning magnesium is 

 about 4630 calories per gram of the metal burned or 75 per 



* American Inst, of E. E., vol. vi, p. 173. 



