72 
MR. P. V. BEVAN ON THE COMBINATION OF HYDROGEN 
Ill the same year, 1809, Dalton^ made several exjieriments with direct sunlight, and 
with diffuse daylight, as causes of the combination. He states, “ upon repeating the 
experiment with sundry variations it was confirmed that light is the cause of this 
rapid combustion of hydrogen and oxymuriatic gas ; that the more powerful is the 
light, the more rapid is the diminution of the mixture ; and that if the eudiometer 
be covered by an opake body, the mixture will scarcely be aftected with any 
diminution for a day, and will not completely disappear in two or three weeks. 
Moreover when the diminution is going on witli speed, if the hand, or any other 
opake body, is interposed to cut off the solar light, the diminution is instantly 
suspended.” Dalton also, as well as Ceuickshank, noticed that the diminution did 
not l^egin at once under the influence of sunlight. This was the first notice of the 
“period of induction” of Bunsen and Boscoe. Dalton, in his analysis of the 
oxymuriatic gas, mixes hydrogen and chlorine in an eudiometer over mercury and 
water, and exposes the mixture to the sunlight, “ when after remaining two or three 
minutes without any change, the wnter, and afteinvards the mercury ascend the tube 
with increasing and afterwards diminishing velocity, till they nearly reach the top.” 
Seebeck discovered the eftect of difference in the colour of the linht used, observino- 
that in a clear glass vessel explosion occurred with sunlight, in a dark blue vessel 
C(nnbination occurred cpnckly wnthout explosion, whilst in a dark red glass vessel the 
action was very slow. 
Deafer took up the investigation in 1840 and made a series of careful and detailed 
experiments resulting in the Invention of the “ tithonometer,” an instrument by 
which he endeavoured to refer the chemical action of light to a standard measure. 
His first jDaperf contains a description of this instrument. In 1844 he published a 
memoir on “tithonised” chlorine, in wdiich he gives some of his reasons for believing 
that the effect of light is to produce an allotropic form of chlorine. This view is 
further dealt with in a paj^er published in 1845. The allotropic modification is, 
according to Deafer, more active than chlorine which has not been illuminated, as it 
comlfines readily with hydrogen even in the dark.| This conclusion was contradicted 
by Bunsen and Boscoe, § who exposed the gases evolved by the electrolysis of 
hydrochloric acid separately to sunlight, and led them then into their “ insolation ” 
vessel. On exposing the mixture to light, they found no greater difference between 
the duration of the induction period of this pre-illuminated mixture and that of gases 
which had not been previously illuminated than is to he accounted for by unavoidable 
errors of experiment.|| This contradiction is explained by the fact that, in Deafer’s 
experiment, the gases were mixed in the vessel where they were submitted to the 
* Dalton, ‘A New System of Chemical Philosoph\",’ L, 300. 
t ‘Phil. Mag.,’ vol. 23, 1843. 
f. ‘Phil. Mag.,’ vol. 27, 1849, p. 339. 
§ ‘Phil. Trails.,’ 1857, p. 382. 
I ‘Phil. Trans.,’ 1857, p. 398. 
