374 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEinCAL EESEAECHES. 
(1) That the heat evolved in the insolation-vessel from the photo-chemical combustion 
of the chlorine and hydrogen exerts no perceptible influence upon the combining povrer 
of these gases, or on the indications of om- instrument ; and 
(2) That in the first few seconds after a change from one amount of light to another, 
irregularities in the indications of the instrument will be obsem'ed, but that these irregu- 
larities do not extend over a period longer than a few seconds, after which time a constant 
temperature in the insolation-vessel is attained. 
Having thus calculated from the heat of combustion and from the cooling-velocity, the 
constant temperature which the gaseous mixture attains by exposure to various amounts 
of light, it is easy to solve the two other questions stated at the commencement of the 
inquiry. Prom the preceding considerations we have determined the value of — ^V„), 
or the excess of temperatui’e of the gas above the surrounding medium. Hence the 
decrease of volume A, which the gas sufiers in coohng from "VYj to W„, is known. If this 
calculated decrease of volume A is equal to that actually observed, Aj, the whole of the 
phenomenon is explained by the contraction from diminution of temperature. H, 
however, the calculated decrease A is not so large as the observed decrease Aj, a certain 
diminution of volume Aj — A must be caused by absorption of the hydrochloric acid, or 
by a chemical action taking place after removal of hght. From the following Table, in 
which the observed and calculated contractions are expressed as per-centage of the total 
volume, it is seen that the flrst supposition is the true one, as the values of Aj — A are 
extremely small, and all negative. 
A,. 
A. 
Aj — A. 
Exp. 1 
0-033 
0-042 
— 0-009 
Exp. 2 
0-034 
0-042 
-0-008 
Exp. 3 
0-108 
0-119 
— 0-011 
Hence we may conclude, — 
(1) That the amount of hydrochloric acid formed during the insolation is absorbed 
almost as quickly as it is produced, and therefore the insolated gas can contain onlv a 
trace of this substance ; and 
(2) That the chemical action ceases instantly on exclusion of the light, pro^inw the 
absence of any perceptible amount of after-action. 
The light emitted from a Scott’s annular burner, although it gives, as we have sIioato, 
satisfactory results, and may serve for many photometric observations, is, however, not 
sufficiently constant to serve for more accurate measurements. We therefore endea- 
voured to find means to obtain a flame of coal-gas of constant dimensions and luminous 
intensity. 
Flames of coal-gas issuing under the usual pressure of several fines of water, and 
burning free in the air, vary and flicker far too much to allow them to be made use of 
