of certain inflammable gaseous compounds. 27 
charged : upon making the contact, the effect of the light 
upon the mixed gases was very remarkable; fumes of mu- 
riatic vapour were instantly produced, the water rose in the 
tube in consequence of the production of muriatic acid, and in 
about five minutes the absorption was entire ; but the most 
curious circumstance was, that in two instances an explosion 
of the gases took place the moment they felt the impulse of 
the electric light. 
12. As I have in no case been able to produce an analogous 
effect by any other terrestrial light, however intense, I cannot 
but consider the phenomenon as dependent upon some pecu- 
liar property belonging to the rays of solar and electric 
light. 
The lunar rays produce no effect upon mixed chlorine and 
hydrogen, nor upon chloride of silver; neither was the 
whiteness of the latter in the slightest degree impaired by the 
most powerful luminous focus that I could obtain from an 
olefiant gas flame. 
13. In some experiments connected with the subjects of 
this communication, I have availed myself of a photometric 
thermometer, acting upon the principle of that described by 
Mr. Leslie, but infinitely more sensible ; it is constructed 
nearly in the same way as the differential thermometer, but 
instead of containing air, the balls are filled with the vapour 
of ether, and the stem contains a column of that liquid ; it 
thus forms a very delicate differential thermometer. To con- 
vert it into a photometer, the upper bulb is covered with a 
thin coating of Indian ink, and the lower one with silver or 
gold leaf ; the whole instrument is then placed in a pellucid 
glass tube : when taken out of its case the influence of light 
