SS ——— | 
SCIENCE AND INDUSTRY. 
the paper should be unglazed, tough and colourless. If glazed paper is 
used, the solution removes the casein or other filler, and thus becomes 
visible. = 
In order to circumvent attempts by prisoners of war to send secret 
messages, the British military authorities compelled internees to write 
their letters on special highly-glazed letter heads supplied to them, and 
any effort to tamper with these could be readily detected with the naked 
eye. The Germans took no such precautions with their prisoners and 
internees, and our people were often able to get secret messages through 
to their homes. Many common substances, such as perfumes, can at a 
pinch be used for secret writing, and most writing fluids serve as de- 
velopers for certain inks, so in British camps means were taken to 
prevent prisoners obtaining such materials. In place of ordinary writing 
inks those with a carbon basis were supplied. 
Usually a secret message is first written on a sheet of paper, and 
then, to avoid suspicion, an innocent ordinary message 1s written over 
and at right angles to it. The secret ink slightly buckles the paper, and 
to replace the fibres the sheet is held over a jet of steam. 
With the exception of inks developed by physical means, such as 
heat, what one may term a “chromo-reaction” is the basis of secret 
writing, and, of course, it is not necessary for the developing solution to 
be colourless. The point is that the developer and the ink react to pro- 
duce a coloured substance which retains the same form as the invisible 
writing solution. 
Only careful experiment will enable one to find an efficient ink, and 
often reactions on paper are markedly different from those that occur 
in test tubes. Needless to say, the developed message must adhere to the 
paper, and quite a number of inks that promised well have had to be 
rejected because they failed in this way. 
As a simple secret ink, one may cite starch solution. This was often 
used by prisoners in German camps, because it was so easily obtainable. 
When painted with a solution of iodine the well-knwon dark-blue colour 
is developed. Another pair of solutions which, on account of their 
simplicity, has often been used is copper sulphate and potassium ferro- 
cyanide. Either solution may be used as the ink, but as both are 
coloured, the fluid must be so dilute that it shows no trace on white 
paper. A marked development results, but as both solutions can be very * 
pally detected under systematic test, no one but an amateur would use 
them. 
As the study of secret inks developed the search for the ideal medium 
resolved itself into finding an ink such that it could be used in extremely 
dilute solution and could only be made visible by one specific developer. 
Some‘ exhaustive work has been carried out, and a few excellent inks 
have been discovered. The chemistry of the reactions involved is highly 
complex, and lack of space and other reasons precludes considering them 
here. It will be sufficient to say that most of the inks and developers 
are organic. 
One of the most interesting aspects of secret writing is connected 
with espionage. For a spy to acquire information is one thing, but to 
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