JUNE 29, 1899] 
NATURE 
209 
next slide you will recognise as the picture of a peacock’s feather. 
There is much interest in these pictures of feathers, as they dis- 
tinguish the brilliant interference colours from those produced by 
certain pigments; the beautiful blue in the eye of the peacock’s 
feather is without action on the photographic plate. Butterflies’ 
wings, at leastsome of them, will draw, as you see, their own 
pictures. Linseed oil, which is a constituent of all printing ink, 
makes it an active body, and it can, like the zinc and other 
active bodies, act through considerable distances. In the picture 
before you the ink was at a distance of one inch from the plate, 
and the next slide shows what a remarkably clear and dark 
picture ordinary printing can produce. As the composition of 
printing ink varies so does its activity, and here are pieces of 
three different newspapers which have acted under the same 
conditions on the same plate, and you see how different the 
pictures are in intensity. Printed pictures, of course, act in the 
same way ; here is a likeness of Sir H. Tate taken from ‘‘ The 
Year’s Art.” The pictures and printing in Puch always print 
well, so does the yellow ticket for the Friday evening lectures at 
the Royal Institution; also the rude trade-mark on Wills’s 
tobacco, and it is of interest because the red pigment produces 
a very clear picture, but the blue printing is without action on 
the plate. 
An interesting and important peculiarity of all these actions 
is that it is able to pass through certain media; for instance, 
through a thin sheet of gelatin. Here are two plates of zinc ; 
both have been scratched by sand-paper ; one is laid directly 
on the photographic plate, and the other one has a sheet of 
gelatin, its colour is of ne note, laid between it and the sensitive 
plate ; the picture in this case is, of course, not so sharp as 
when no gelatin is present, but it is a good and clear likeness of 
the scratches. 
Celluloid is also a body which allows the action to pass 
through it, as is seen in this picture of a piece of perforated 
zinc, a picture which was produced at ordinary temperatures. 
Gold-beaters’ skin, albumen, collodion, gutta-percha, are also 
bodies which are transparent to the action of the zinc and the 
other active bodies. On the other hand, many bodies do not 
allow the transmission of the action through them ; for instance, 
paraffin does not, and among common substances writing ink 
does not, as is easily shown by placing ordinary paper with 
writing on it between the active body and the photographic 
plate. The active body may conveniently be either a plate of 
zinc or a card painted with copal varnish and allowed to dry, 
ora dish of drying oil. The picture of an ordinarily directed 
envelope shows this opacity of ink well. It is a property long 
retained by the ink, as this picture of the direction of a letter, 
written in 1801, shows ; also this letter of Dr. Priestley’s, dated 
1795, and here is also some very faded writing of 1810, which 
still gives a very good and clear picture. Even if the writing 
be on parchment, the action passes through the parchment, but 
not through the ink, and hence a picture is formed. 
With bodies which are porous, such as most papers, for 
instance, the action passes gradually through the interstices, and 
impresses the plate with a picture of the general structure of 
the intervening substance. For instance, the following pictures 
show the structure and the water-mark of certain old and 
modern writing papers. Some modern writing papers are, 
however, quite opaque ; but usually paper allows the action to 
take place through it, and combining this fact with the fact of 
the strong activity of the printing ink, the apparently confused 
appearance produced on obtaining a picture from paper with 
printing on both sides is accounted for, as the printing on the 
side away from the photographic plate, as well as that next to 
it, prints through the paper, and is, of course, reversed. 
I hope I have now given you a clear idea how a picture can 
be produced on a photographic plate in the dark, and the 
general character and appearance of such pictures. I now pass 
on to the important question of how they are produced. Moser 
suggested fifty years ago that there was ‘‘ dark light,” which 
gave rise to pictures on polished metallic plates, and lately it 
was suggested that pictures were produced by vapour given off 
by the metals themselves ; the explanation, however, which I 
have to offer you is, I think, simpler than either of these views, 
for I believe that the action on the photographic plate is due to 
the formation of a well-known chemical compound, hydrogen 
peroxide, which undergoing decomposition acts upon the plate 
and is the immediate cause of the pictures formed. The com- 
plicated changes which take place on the sensitive plate I have 
nothing to say about on the present occasion, but I desire to 
NO. 1548, VOL. 60] 
convince you that this body, hydrogen peroxide, is the direct 
cause of these pictures produced in the dark. Indirect proof 
has to be resorted to. Water cannot be entirely excluded, for 
an absolutely dry photographic plate would probably be perfectly 
inactive, and as long as water is present peroxide of hydrogen 
may be there also. But what are the conditions under which 
these pictures are formed? Only certain metals are capable of 
producing them. This list of active metals which I have 
mentioned to you was determined solely by experiment, and 
when completed it was not evident what common property 
bound them together. Now, however, the explanation has 
come, for these are the very metals which most readily cause, 
when exposed to air and moisture, the formation of this body, 
peroxide of hydrogen. Schdnbein showed as long ago as 1860 
that when zinc turnings were shaken up in a bottle with a little 
water hydrogen peroxide was formed, and the delicate tests which 
we now know for this body show that all the metals I named to 
you not only can in the presence of moisture produce it, but that 
their power of doing so follows the same order as their power of 
acting on a photographic plate. Again, what happened with 
regard to the organic bodies which act on the photographic 
plates ? I have already mentioned that in experimenting with the 
metals it was accidentally observed that copal varnish was an 
active substance producing a picture like that produced by zinc, 
and that the action was traced to the turpentine present ; again, 
a process very much like groping in the dark had to be carried 
on in order to determine which were active and which inactive 
organic bodies, and the result obtained was that the active 
substances essentially belonged to the class of bodies known 
to chemists as terpenes. Now a most characteristic property 
of this class of bodies is that in presence of moisture and air 
they cause the formation of hydrogen peroxide, sothat whether 
a metal or an organic body be used to produce a picture, it is 
in both cases a body capable, under the circumstances, of 
causing the formation of hydrogen peroxide. Passing now to 
experimental facts, which confirm this view of the action on 
sensitive plates, I may at once say that every result obtained 
by a metal or by an organic body can be exactly imitated by 
using the peroxide itself. It is a body now made in consider- 
ably quantity, and sold in solution in water. Even when in 
a very dilute condition it is extremely active. One part of the 
peroxide diluted with a million parts of water is capable of 
giving a picture. It can, of course, be used in the glass dishes 
like any other liquid, but it is often convenient not to have so 
much water present ; and then it is best to take white blotting- 
paper, wet it in the solution of the peroxide, and let it dry in 
the air. The paper remains active for about twenty-four hours ; 
or, what is still better, take ordinary plaster of Paris, wet it 
with the peroxide solution, and let it set ‘‘in a mould” so as 
to get aslab of it. This slab increases in activity for the first 
day or two after making, and retains its activity for a fortnight 
or more. Such a slab will give a good and dark picture in 
three or four seconds. 
To show how similar the pictures produced by the peroxide 
and those by zinc are, pictures of a Japanese paper stencil, 
which had been paraffined to make it quite opaque, have been 
made by both processes, and are shown with other instances 
in which turpentine was used in the following slides. It is 
also very easy to obtain good pictures with the peroxide alone 
of the structure of paper, &c.; see, for instance, this one of a 
five-pound note and these of lace. Again, the strict similarity 
between the action of the peroxide and that of the metals and 
organic bodies is further shown by the fact that its action passes 
through the same media as theirs does; and here are good 
pictures formed by the action of the peroxide after passing 
through a sheet of these substances. How this singular trans- 
mission can be explained, I have treated of elsewhere, and time 
does not allow of my discussing the matter to-night. 
There are many ways in which the bright, active zinc surface 
can be modified. Draw your finger across it, press your thumb 
upon it, and you stop its activity, as is shown by the picture it 
will give. Lay a printed paper on the zinc, and let the contact 
continue for three-quarters of an hour, at a temperature of 55°, 
then bring the zinc in contact with a sensitive plate, a picture of 
the printing is formed, but allow the contact between the zinc 
and printing to continue for eighteen hours at the same tem- 
perature, and the picture then given by the zinc is the reverse of 
the former one. Where the ink has been is now less active than 
the rest of the plate. Here are slides which show these positive 
and negative pictures. Another way of modifying the zinc surface 
