1908.| Allied Bodies on a Photographic Plate in the Dark. 381 
erystalline form. By repeating this process it may be purified, and will then 
have a melting-point of 156°C. It is with an acid so prepared that the 
following experiments have been made. If a small glass vessel be nearly 
filled with the crystalline acid and a photo plate be laid on the top, not 
touching the acid, at ordinary temperatures, after two hours no action will 
have occurred, but after 18 hours the plate will give a strong dark picture. 
If, however, the acid be kept at a temperature of 40°, then a fairly good 
picture can be obtained in two hours, and with longer exposure a very dark 
one. 
Thus it acts in the same way as resin, and has about the same amount 
of activity. Exposed to sunlight or to the are light, its activity is much 
increased. Exposed to the are light for an hour, it gives a good and dark 
picture, and even on an exposure of half that time a picture only slightly 
lighter is obtained; in fact, in little more than half an hour it is charged 
to its maximum amount. Light acts upon it as it does on resin, 
The acid dissolves readily in alcohol, and if the solution be allowed to 
evaporate on a glass plate, it gives a film suitable for experimenting with. 
Paper saturated with the solution becomes very active. The acid also 
dissolves in ether, benzene, chloroform, etc., and behaves in the same way as 
with an alcoholic solution. 
If the acid be heated to 100° it slowly loses its activity ; after eight hours’ 
heating the picture it gives is only slightly fainter than before heating, but 
after 56 hours’ heating it has become much fainter, and after being heated 
for 152 hours it has lost entirely its power of acting on a photo plate. 
If the acid be fused it becomes quite inactive, but its activity is restored 
if it be powdered, or if its surface be rubbed with sand-paper—in fact, if the 
smooth surface be broken up. If exposed to sunlight or to the are light its 
activity is much increased, and different coloured rays affect it as they do 
resin. Exposed under blue or white glass to six hours’ sunshine it gives a 
dark picture, but under a red glass only a faint one. All the metallic salts 
of this acid are entirely without action on a photo plate: neutralise a solution 
of the acid with potash or soda and its activity has gone, and there is the 
same loss of activity with the copper and the lead salts, whether in solution 
or in the solid state. Decompose the metallic salts and the liberated acid is 
as active as before. 
To purify the acid the lead salt, which is very insoluble even in alcohol 
and other organic liquids, was boiled several times with pure alcohol, and 
afterwards treated with sulphuretted hydrogen, the acid well washed and 
dried, and recrystallised from alcohol, and it was found to be quite as active 
as before this treatment. Another specimen of the acid was treated with 
