484 MM. C. Lea—Identity of the Photosalts of Silver 
light may be made to reverse the action of hypophosphite. So 
exact a correspondence in these remarkable properties can 
searcely be fortuitous. 
i 
A silver haloid formed in the absence of light and subjected 
to the action of sodium hypophosphite gives rise to the gradual 
formation of subsalt, which combines with the normal salt 
in the manner described in the previous part of this paper. 
This action of the hypophosphite closely corresponds with that 
of light. In its initial stages it is invisible, but can be brought 
out, in both cases by development. 
If we form a film of chloride, bromide or iodide of silver and 
with a glass rod dipped in solution of hypophosphite, make 
marks upon it, these marks can with the utmost ease be devel- 
oped in precisely the same way as an image produced by expo- 
sure to light. 
A very simple mode of operating consists in imbibing photo- 
graphic paper with a solution of an allaline haloid, drying, ap- 
plying a silver solution and then thoroughly washing, all of 
course with careful exclusion of active light. If the silver solu- 
tion is acidulated with nitric acid, a drop to the ounce, the 
result is brighter, but this is not important In any case the 
washing must be thorough. 
Marks made on this paper can be depeloped with the oxalate 
developer with the utmost facility. If a strong solution of 
hypophosphite is applied cold, it may be washed off at the end’ 
of a minute, but a stronger impression is obtained by allowing 
it to wait a half an hour before developing. Or the action may 
be accelerated and increased in strength by laying the freshly 
marked paper on a hot surface, or better, by steaming it, before 
applying the developer. A convenient mode of steaming is to 
_lay two pieces of glass on a small water bath kept boiling, with 
a space between them. Over this space the paper is rested for 
two or three minutes. Paper prepared with a solution of KCl, 
KBr, or KI, dried and floated on acidulated solution of silver 
nitrate and well washed, if marked with strong solution of 
hypophosphite and steamed for two or three minutes, will de- 
velop the marks as black as ink on a white ground. The use 
of heat simply gives a blacker development, but a very vigor- 
ous image may be got without. 
(A similar result may be obtained by substituting for the 
hypophosphite a dilute solution of potash and an oxidable 
organic substance. With milk sugar the action is very ener- 
getic and heat is quite superfluous). 
Both these are the initial steps of reactions which when pro- 
longed result in the visible formation of the colored photosalts. 
