SECRET INKS. 
transmit it is quite another. In time of war belligerents, of course, estab- 
lish censorship of all postal and telegraphic facilities, and a spy finds 
himself compelled to adopt a subterfuge to evade it. As already ex- 
plained, this may be by, means of code or secret writing. There are 
many reasons why the latter method is more favoured. Most spies, or, 
as they are known in official circles, enemy agents, are not of high 
mental calibre, and they understand very little indeed of the chemistry 
of secret writing. The agent knows very little of the inner workings 
of the bureau which employs him, and for purposes of communication 
he is given an ink of which he knows neither the name nor the developer. 
For developing the secret message sent to him he is given a developer, but 
does not know the ink used. As the ink and developer with which he 
is supplied will not work together, he cannot reveal much information 
if he is captured. : 
In disguising materials for secret writing in order to bring them 
into the country, the agent, or rather his employer, displays considerable 
ingenuity. Many of the requisite chemicals are difficult to obtain, and in 
any case to purchase them through the ordinary channels might arouse 
suspicion. The agent therefore brings them in with his personal belong- 
ings. He has to run the gauntlet of a stringent examination at the hands 
of vigilant naval and Customs officers, and so it is well nigh impossible 
to conceal even the smallest bottle. One of the espionage bureaux sup- 
plies its agents with what is to ordinary appearances tooth-paste, but 
the tube really contains the ink and insoluble perfumed substances. On 
adding water to the “paste” the ink dissolves, the other matter sinks 
to the bottom, and the writing solution can then be decanted off for 
use. A favorite dodge was to carry the ink disseminated through a 
cake of ordinary soap. The virtue of this was that the presence of the 
ink did not prevent the lathering of the soap, and the agent could make 
an effort to prove the innocence of the material by washing with it. 
Later on refinements were devised when it was found that solutions 
of some of the inks did not affect textiles. Agents would soak their boot- 
laces, cloth buttons, handkerchiefs, or even their hair, in a solution of 
ink. When it was wanted for use, what was easier than to soak the 
article in water for a minute, thus dissolving out the required amount. 
As the late war progressed, the problem’ of combating espionage 
became increasingly important, and both sides built up as branches of 
the intelligence service highly-trained departments whose duties were 
principally to devise safe inks for the use of their own men and efficient 
methods of detecting and developing the inks used by the enemy. There 
is now little doubt that our own side usually won. The work was 
singularly exciting, and the hackneyed phrase, “ Romance of Science,” 
gained a new significance. 
_ The systematic testing of letters for the presence of secret messages 
is quite an intricate problem, for so many factors must be considered. 
The letters undergoing test could not be detained very long, and the 
fact that the material on which the operations were to be carried out 
was paper and might easily be injured somewhat restricted the scope of 
the tests. Most difficult of all, the possible inks were so diverse that a 
system of selective testing had to be thought out. Many people who 
received foreign correspondence in war time must have pondered over 
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