PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 377 
In order to have still more striking proof of the accuracy of the indications of our in- 
strument, we have compared the results obtained from the standard flame of coal-gas in 
June with those of the same-sized flame in September, when all the liquids contained in 
the apparatus had been renewed. The standard flame, of the same size as previously 
employed, was placed at a distance of 0“T365 from the insolation-vessel, and the instru- 
ment was furnished with a new observation-tub^, whose degrees were 3-40 times larger 
than that employed in Series X. An action of 10T7 degrees per minute was obtained 
as a mean of 96 well-agreeing observations: this 10T7, reduced to the former scale, 
represented 34-58 degrees, which gives for the distance 0“-216 the action 13-80, which 
differs from the June mean 13-91 by only 0-11. 
The exact agreement of these numbers at such distant periods of time is the more 
remarkable, as it could not have been expected that the coal-gas generated by a common 
manufacturing process should possess so constant a composition that it could be used 
during the space of several months as a fixed unit for the measurement of photo- 
chemical action. In order to form an idea in how far the intensity of the chemical rays 
emanating from a flame is dependent upon the composition of the gases undergoing com- 
bustion, it was necessary to determine the variation in the composition of the gas em- 
ployed in the experiments. 
Although we have not analysed the gas employed in each experiment, we are still able 
to give the composition of the gas employed by us from a series of analyses made for 
another purpose by Dr. Laxdolt at various times during the course of our investigations. 
Four samples of Heidelberg coal-gas, collected by Laxdolt on the spot and at the time 
at which our experiments were conducted, gave, on analysis, the following results : — 
Hydrogen 
I. 
39-30 
II. 
41-04 
III. 
44-00 
IV. 
41-37 
Light carburetted hydrogen. 
40-56 
40-71 
38-40 
38-30 
Carbonic oxide 
4-95 
7-64 
5-73 
5-56 
Elayl 
4-04 
5-10 
4-13 
5-00 
Ditetr)'! 
3-15 
2-18 
3-14 
4-34 
Nitrogen 
8-00 
2-75 
4-23 
5-43 
Oxygen 
0-00 
0-00 
0-00 
0-00 
Carbonic acid 
0-00 
0-58 
0-37 
0-00 
100-00 
100-00 
100-00 
100-00 
Hence it is seen that the composition of the Heidelberg coal-gas varies but little during 
a considerable period, but still sufficiently to render the exact agreement of the photo- 
chemical measurements very remarkable. 
In order to determine which of the ingredients of the flame principally influences the 
amount of the chemically active rays, we have made the following experiments with a 
gas-lamp, which has been introduced by one of us into the laboratory to supersede the 
ordinary wire-gauze flame, and is particularly well suited to produce a flame of various 
MDCCCLVII. 3 D 
