F. J. Rogers — Magnesium as a Source of Light. 311 



the results of seven determinations by this method. A mean 

 of these gives 6010 lesser calories as the heat developed by the 

 combustion of one gram of magnesium in oxygen. 



Julius Thomsen* gives 6077 as the heat of combination of 

 magnesium and oxygen. He arrived at this result indirectly 

 by means of the heat developed in the formation of the hydrate. 



Y. 



Total efficiency of magnesium light. 



By total efficiency is meant the ratio of luminous energy to 

 the total energy expended in the production of the light. In 

 order to determine this ratio it is necessary to know the ratio 

 of radiant energy to total energy of combustion. The method 

 of obtaining the energy of combustion of a gram of magnesium 

 in lesser calories, has just been described, and next must be 

 indicated that of obtaining the radiant energy from a gram of 

 burning magnesium in the same units. The method of obtain- 

 ing this result was by calibrating the galvanometer and ther- 

 mopile for lesser calories of spherical radiation from a source 

 24 0m from the face of the thermopile. The known source of 

 heat was a spherical brass vessel coated with lamp black and 

 filled with hot water. Its' loss of heat was known by its fall 

 in temperature. The loss by convection and conduction was 

 determined by noticing the rate of cooling in air and in a 

 vacuum, under otherwise similar circumstances. By this means 

 it was found that a galvanometer "swing" of one scale division 

 corresponded to a spherical radiation of 2 '53 calories per 

 minute. Next the thermopile was exposed to the radiation 

 from burning magnesium, the ribbon being supplied at a uni- 

 form rate. From a mean of 24 observations it was found that 

 the burning of one gram of magnesium produced 4.630 calories 

 of radiant energy. As above shown *135 of this radiant energy 

 is light. The total energy of combustion of one gram of mag- 

 nesium is 6010 colories, therefore the total efficiency of mag- 



, . , T . . '135X4630 



nesium as a producer of luminous energy is = '1025. 



1 ftJ 6010 



It is assumed in this computation that the radiation from burn- 

 ing magnesium is of the same intensity in all directions. 

 Experiments were made for determining the validity of this 

 assumption, in which it was found that the candle power showed 

 but little diminution up to 70° from the horizontal plane. 



It is very remarkable that nearly -10 of the total heat of 

 combustion of any artificial illuminant is expended in produc- 

 ing light. The radiant efficiency of magnesium, as we have 



""Journal fur praktische Chemie, 1875. 



Am. Jour. Sci.— Third Series, Yol. XLIII, No. 256.— April, 1892. 



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