1908.] Allsed Bodies on a Photographic Plate in the Dark. 385 
details. Fig. 4 (Plate 10) is the picture of a vertical section of a Nottingham 
coal enlarged three times. The vertical section of a coal gives usually a more 
interesting picture than the horizontal section. Figs. 5 and 6 (Plate 11) 
show vertical and horizontal sections of a Seaham coal. Through the kindness 
of Dr. Teall and Mr. Strahan, of the Museum of Practical Geology, I have 
had the opportunity of examining coals from different localities. Taking first 
the specimens of English coals, they all seem to be active, that is, have the 
power of acting on the photographic film in the dark. The best way of trying 
them is, first to saw off a piece from the rough block, and then rub it down 
first on coarse sand-paper and then on fine, till the surface is flat and true, 
then on laying this flat surface on a photo plate at about 50° C, for in most 
cases about 18 hours, but in some cases it may be well to continue the contact 
for as long as 48 hours, a good picture is obtained. If the coal contains much 
water, it must be dried, either by heating it for a short time at a temperature 
of about 40° C. or by drying it over sulphuric acid. 
In place of using a slab of coal, it is sometimes convenient to use it in 
the form of powder, and this is done, as in previous cases, either by simply 
placing the powder on the photo plate or by filling a small glass vessel with 
it and placing the photo plate on the top, either in contact with the powder 
or at a small distance above it. As long as the coal is used in form of a 
slab and is fairly dry, its action is very uniform, different pieces of the same 
coal giving pictures of the same density ; but when the coal is in powder, a 
small amount of moisture modifies the density of the picture to a very 
considerable extent. | 
The effect of slightly heating a coal is shown by the following experi- 
ment :—Four samples of a Seaham coal in powder were treated as follows : 
One sample was at once put up with a plate, and gave a fairly good picture ; 
another was heated for 24 hours at 100° C., and gave a much darker picture ; 
a third one was heated at 150° for the same length of time, and its picture 
was much lighter, only slightly darker than the first one; and the fourth 
sample was heated for 24 hours at 200°, and gave no picture. 
In another case the heating was continued for only three hours at 200° 
and it gave a faint picture. If the drying be effected by placing the powder 
over sulphuric acid, phosphorus pentoxide, or solid caustic potash, it seems in 
many cases to increase the activity of the coal to a very considerable extent, 
so much so that some coals which under ordinary conditions give only a faint 
picture can be made to give a dark one. But on the other hand there are 
coals which are not altered by this process of drying. One specimen of coal, 
a Seaham coal, powdered, was exposed 11 times in a glass vessel over 
sulphuric acid, each time for 24 hours, without any diminution of its activity ; 
