PEOrESSOE BUNSEN AND DE. H, E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 401 
action. The phenomenon is in reality a simple consequence of the laws of photo-chemical 
induction, and proves that these relations, which we have examined only in the case of 
chlorine and hydrogen, occur in a slightly modified form in other photo-chemical pro- 
cesses. If the chlorine and hydrogen mixture were a solid substance which could be 
fixed upon paper without diminishing its sensibility to light, this paper must exhibit 
exactly the same singular property which Becquekel first observed with the dagueri’eo- 
tj-pe plate, or other photo-chemically sensitive surface. In order to prove this asser- 
tion, we will imagine that such a sohd chlorine and hydrogen surface was as sensitive as 
the gas in Series of Experiments IV. h of this Part. We will further imagine that 
half of this sensitive surface was darkened, and the other half insolated for five minutes 
with the luminous intensity I, as in Experiment I. The action on both halves of the 
plate after the five minutes’ insolation is therefore, as the numbers of the experiment 
show, still 0. The plate has undergone therefore no visible alteration, nor could a 
difference be effected by any reagent. If the whole surface be now exposed to a uniform 
insolation, the half previously exposed wil| act quite differently from the half which 
remained in the dark. The former will undergo an alteration in the first five minutes, 
which is represented by the amounts of hydrochloric acid formed, namely, 
2-I4-I4-64-29-2-1-3I-4+30-I = I07-4, 
whilst the non-insolated portion of the surface exposed during the same space of time 
to the same amount of light does not show, according to Experiment I. Series IV., any 
alteration in its chemical properties. If the formation of hydrochloric acid were accom- 
panied by a deepening of colour, the half of the chlorine and hydrogen plate, which 
was not preVously exposed, would still appear white, whilst the other half would be of 
a dark colour, thus acting exactly like the sensitive surface of a daguerreotype plate. 
Let us next consider the second case, in which an image commenced by lights of 
various degrees of intensity is developed by a subsequent uniform exposure in depths of 
colouring proportional to the various original intensities. We again imagine a sohd 
layer of chlorine and hydrogen which is as sensitive as the chlorine and hydrogen mixture 
employed in Series of Experiments IV., and suppose that at the point of the 
surface the various intensities of the light I, I‘78, 2'45, 4T7 were acting. According 
then to Experiments I e, 2 c, and 3 c, the amount of chemical action measured by the 
hydrochloric acid formed in the minute is atp, 0, atyjg 2T, at ^3 5-2, and at ^4 65-6. If 
these numbers are dirided by their respective amounts of light, we obtain the quantity 
of action which is effected by the unit amount of light at the point ^j, p. 2 , p^, p^ ; these 
are 0, ITS, 2T2 and I5'7. These numbers express the combining power, or what is 
the same thing, the sensibility of the plate at the points p.^, p^, p^. After an exposure 
of two minutes the sensibility of the plate at the same points would be 0 , 6'07, 26-8 and 
29-3, and after three minutes’ exposure 0, 18-6, 32-7 and 54-7. It is thus seen that the 
sensibility of the plate is altered by the first exposure to light, and in such a manner 
that the greater luminous intensity corresponds to the greatest sensibility. If the varying 
MDCCCLVII. 3 G 
