B. Silliman—Relation between the intensity of Light, etc. 17 
Art. II.—On the relation between the Intensity of Light pro- 
duced from the combustion of Illuminating Gas and the 
volume of Gas consumed; by B. SILLIMAN.* 
(Reud at the Salem meeting of the Am. Assoc. for Adv. of Science, Aug, 1869.) 
In photometric observations made to determine the illuminat- 
ing power or intensity of street gas, it is the practice of obser- 
vers to compute their observations upon the assumed standard 
of five cubic feet of gas, consumed for one hour, and in the 
constantly occurring case, of a variation from this standard, 
whether in the volume of the gas consumed or in the weight 
of spermaceti burned, the observed data are computed by the 
“rule of three,” up or down, to the stated terms. The stand- 
ard spermaceti candle is assumed to consume 120 grains of 
sperm in one hour, a rate which is. rarely found exactly in 
actual expericnce. 
or example, a given gas, too rich to burn in a standard 
argand burner at the rate of five cubic feet per hour without 
smoking, is consumed at the rate of 32 cubic feet to the hour, 
with an observed effect of 20 candles power. This result, pre- 
viously corrected by the same rule for the sperm consumed, is 
then brought to the standard of five cubic feet by the ratio 
3°35 : 20=5 : 28°57 
The candle power of the gas is therefure stated as 28:57 
candles, and this result has been universally accepted as a true 
expression of the intensity of the gas in question, or the rela- 
tive value of the two consumptions. | 
n common with other observers, I have long suspected that 
this mode of computation was seriously in error, as an expression 
of the true intensity of illuminating flames, and that there 
were other conditions besides the volume of gas or weight of 
sperm consumed which must influence, and greatly modify th 
results. As most of these conditions are considered somewhat 
at length in a paper on “ Flame Temperatures,’ prepared 
The results of many trials, made with the purpose of deter- 
mining the value of these photometric ratios, indicate clearly 
” thee Wha tre i in intensity in illuminating 
- flames is, within certain limits, expressed by the following 
* The main points of this r were made the subject of a verbal communica- 
tion to the an, Academy or Ane and Sciences at their session, June 17th, 1869. 
Aw. Jour. Sct.—Srconp Sertes, Vou. XLIX, No. 145.—Jan., 1870. 
2 
