384 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
(3) Through this the indirect experimental proof is given that 
/ 
P 
n&t E = p" + n ^^~ E . T, 
dT 
i. e. , a law analogous to that of Gibbs-Helmholtz for ordinary cells holds good for 
galvanic cells created by light. The experiments (given in § 7) of the 8th till the 
13th of July, 20th of July, 1904, and of the 15th of February, 1905, Plate 19, 
Table V.,show, however, that there are enormous difficulties in this region of research 
to determine the value of 
n e €(y rp 
dT ‘ ’ 
experimentally, the value having proved to be too small for experimental determina¬ 
tion in view of other sources of error. 
(4) The heat of reaction p", or the difference of the heats of ionisation in light 
and in the dark can, for the same reason, be determined experimentally only 
approximately by putting p " — n e e 0 ]£E. The heat of reactions evidently changes as 
the E.M.F. docs, both with the intensity and composition of light, which is quite a 
characteristic feature for galvanic cells created by light. The heats of reaction are 
always exceedingly small, as the E.M.F.’s are. Thus the heat of reaction for one electro¬ 
chemical equivalent of Br ions in the system (Ag-BrAg plates in 0'1 norm. NaBr 
solution) giving, c.g., 106 x 10 -6 volt = 23040 x 106 x 10 -6 = 2-442 gram. cal. (see § 7, 
p. 365). The systems, Ag plates in N0 3 Ag solution, gave E.M.F.’s between 1 ’8 x 10 -6 volt 
and 54H x 10 _b volts. The value of p" for 1 electro-chemical equivalent of Ag ions is 
thus = 23040 x(P8 to 54"4)x 10~ 6 = 0-044 to P25 gram. cal. (see § 6, p. 355). 
(5) This research (§6, §7, II. and XV.) shows that the solution pressure of the 
electrodes in light is sui generis, since it changes, as well as the E.M.F., with the 
intensity of light and its composition. Experiments of § 7, II. and X., show also the 
effect of the capacity of the plates for absorption of light, the effects of previous 
illuminations, of physical changes in the dark, upon E.M.F. obtained, and 
consequently upon the solution pressure and heat of reaction of some systems. 
As to the variation of the solution pressure of an electrode on exposition to light, 
experiments show that it is very small: thus, in case of the system (Ag-BrAg plates 
in BrNa solution) giving 106 x 10” 6 volt, we get 
1 ? l 
lo 8 E7 = 
106 x 10 
-2 
Pd 0-860Tx 2'306 
VI 
at T = 290° (17° C.) : log — = 0-001846 and PI D0043Pd, 
Jl CV 
i.e., an increase of the solution pressure by 0"43 per cent. For Ag plates in N0 3 Ag 
solutions, giving (for different intensities) E.M.F.’s from T8x 10“ 6 volt to 54'4 x 10 -6 
volt, we get for 
T = 290° C. : log Pl/Pd = 0-000031 to 0-00095, PI - 1-00072 to l*0022Pd, 
