B. Silliman on determining the Photometric power, ete. 879 
Art. XLI.—On the Determination of the Photometric Power of 
a rich gas by dilution with a poor gas of known value: the 
“method of mixtures ;” by B. SILLIMAN. 
In a paper on ‘Farmer’s Theorem,’ * I have given several 
examples of the method of determining the intensity or photo- 
metric power of a rich gas by diluting it with several times its 
own volume of a poorer gas of known intensity, and then caleu- 
lating its value from the increment of intensity. Having dem- 
onstrated in the paper before mentioned the worthlessness of all 
determinations of the intensity of gases of high illuminating 
power made by burning them in volumes less than five cubic 
feet, and then calculating their intensity by the rule of three up 
to that volume, I have shown how much more exact results 
were obtained when the results were calculated upon the theo- 
rem of Mr. Farmer; this greater exactness being predicated 
largely upon the confirmation drawn from parallel observations 
upon the same gases when measured by the method of mixtures. 
The results thus obtained having, however, been questioned by 
Mr. Stimpson,+ on the ground that the method itself had not 
been experimentally demonstrated, I have undertaken lately, in 
connection with Mr. Farmer, to make some experiments calcu- 
lated to test its accuracy. ; 
The results which go to support the accuracy of the method 
were obtained with the use of a new photometric apparatus, 
constructed for the Manhattan Gas Co., under my direction, by 
Sugg of London, and which was designed to embrace all the 
best approved features which recent experience has indicated 
in photometry. A discussion of these details would be out of 
— in this connection. Before detailing our results, it will 
€ proper to present the method of determination of intensity 
for gas of high illuminating power as practiced by Mr. Farmer 
at the Manhattan Gas Works in New York, and which I have 
called the method of mixtures. 
_ To find the candle power of a gas aiee for > an 
intensity goa than 20 candles, mix the rich gas of unknown 
power with a poorer of known power in such proportions 
that the Sareney of a mixture all not te preter” haar 20 
candles power, when consumed at the agreed rate of not over 
five cubic feet per hour. Then to compute the candle power 
(intensity) of the rich gas,— ; 
* This Journal, II, xlix, 17; also Proceedings of American Association for Ad- 
vancement of Science, Salem meeting, 1869, p. 149. 
+ See page 272, this volume. — 
