256 
MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
Silver chloride, precipitated in the presence of a slight excess of sodium chloride, 
and subsequently washed, is a white powder which slowly darkens to a slate colour 
when exposed to daylight. . When the precipitation takes place in the presence of an 
excess of silver nitrate, the chloride darkens more rapidly on exposure to light, and 
the depth of colour depends in great measure on the amount of free silver nitrate 
left with the silver chloride, and not subsequently washed out. 
Silver iodide varies in colour according to the mode of preparation. If the potassium 
iodide is in excess, the silver iodide falls as a very pale yellow, almost white, pre- 
cipitate ; whilst, if precipitated in the presence of an excess of silver nitrate, it is 
of a decided straw-yellow colour. In the former case, it is not visibly affected by 
exposure to light, even to the sun. But the straw-yellow iodide formed in the 
presence of excess of silver salt is very sensitive to light. This sensitiveness is 
not rendered evident by much change of colour, since exposure to full day or even 
sunlight only causes it to change to a pale brown colour. If the sensitive silver 
iodide is exposed for one or two seconds to daylight, and then brought into a room 
faintly illuminated with orange light, no change of colour is visible ; but on pouring 
over it a mixture of silver nitrate and a reducing agent, immediate blackening takes 
place, showing that a very strong, although invisible, change has been produced in 
the silver iodide by the momentary exposure to light. 
In its behaviour to light, silver bromide may be considered intermediate between 
silver chloride and iodide. Its visible darkening, when exposed to daylight, is much 
less than that of silver chloride ; whilst it also shares with the silver iodide, although 
in a less degree, the power of assuming a latent change capable of subsequent develop- 
ment. In all cases the presence of free silver nitrate increases the sensitiveness 
to light, and as this salt is washed away the sensitiveness diminishes. The rays of 
light which produce this photographic action are chiefly the indigo and violet rays, the 
red, orange, and yellow having no action. The most sensitive salts of silver can there- 
fore be safely prepared in a room illuminated by a candle shining behind orange glass. 
I experimented with all varieties of the three silver salts. They were dried, then 
ground up with pure water to a paste, and carefully painted on disks of mica, as 
described at par. 222. In the case of the silver bromide and chloride (prepared in 
the dark, and not exposed to light before the standard candle of the apparatus shone 
on them), no material difference of action could be observed between the sensitive and 
non-sensitive varieties. The figures in the table are therefore the mean of all the 
results, which were fairly concordant. With the silver iodide, however, the case is 
different : the almost white, insensitive variety is most affected ; next comes the 
sensitive variety, from which, however, much of the free silver nitrate has been 
washed, so as to considerably reduce the sensitiveness. Lastly comes the silver 
iodide most sensitive to light ; this is less affected mechanically by radiation than 
any other powder hitherto tried. The incident radiation from the candle, instead 
of causing molecular pressure on the surface, and driving back the silver iodide, 
