186 Mr. J. Spiller on the Composition of 
These results, agreeing so closely with those obtained by cal- 
culation, prove that such a compound of the oxide of iron and 
phosphoric acid was produced, otherwise the calculated amount 
of iron would have been very different from that obtained by 
experiment. 
They also show how very accurate this method is, and how 
constant are the results obtained by its adoption ; and the ease 
and expedition with which the estimation of phosphoric acid 
is effected, renders it a very useful means for the determination, 
not only of that acid itself, but likewise for that of many of its 
compounds, which can easily be calculated from the amount of 
phosphoric acid present. 
I have already found that it is particularly useful.in estimating 
the quantity of soluble and insoluble phosphates in superphos- 
phate, a manure the analysis of which has hitherto been attended 
with considerable trouble. And from my experiments I have no 
doubt that it will be found to be extremely useful in the esti- 
mation of the phosphates in different manures, the ashes of 
plants, and many other cases of common occurrence where an 
expeditious determination of the quantity of those substances is 
required. 
Laboratory of the Royal Dublin Society, 
February 3, 1860. 
XXIV. On the Composition of the Photographic Image. 
By Joun Spruyer, F.C.S., of the War Department*. 
iy ies composition and chemical nature of the photographic 
image, as produced by the action of light upon the chlo- 
ride of silver, is even at the present moment, notwithstanding 
the numerous experiments recorded on the subject, one upon 
which authorities are divided. While there is abundant evidence 
to show thatthe darkening consequent on exposure to the sun’s 
rays is a process of reduction accompanied with the evolution of 
chlorine, there are yet two opinions entertained as to the extent 
to which this reducing action ordinarily proceeds. In accordance 
with one hypothesis, the white or protochloride of silver (Ag Cl) 
is assumed to suffer the full decomposition into its elements, 
two cases I have tried, viz. the magnesia and soda salts) to be easily 
effected by heating them for a few minutes with a little hydrochloric acid. 
But in these as in other cases, the solution of the phosphate must be 
suffered to cool to the ordinary temperature before the estimation of the 
phosphoric acid is attempted, as heat alters the conditions, and appears to 
give rise to a different compound of the oxide of iron and phosphoric acid. 
* Communicated by the Author. 
