sid, 
SECRET INKS. 
Secret Inks. 
. By “M.” 
Secrecy is probably as old as man himself, and from time immemorial 
men and women have adopted subterfuges to enable them to communicate 
with each other in a way unintelligible to others. The Greek and Roman 
military commanders had devised various methods of sending secret 
messages, and though crude, these probably served their purpose. 
A favorite plan was to shave the head of a slave, write a message 
on his scalp, and despatch him to his destination after the hair had 
grown again. Another method often used was to send a message on a 
strip of parchment which could, in those days, only be deciphered when 
the strip was rolled round a rod of particular shape in the possession of 
the recipient. 
As knowledge accumulated, so the methods of secret communications 
pen correspondingly complex, and to-day the whole range of science 
has been brought to bear on this exceedingly interesting work. To 
devise a safe method of secret communication is now a task not nie A ebaNy 
of the best mathematical, chemical and physical brains. 
In time of war it is especially important to have safe (7.¢., refined) 
ways of secret communication. The intelligence work of an army, or 
rather of a nation, is a good index to its fighting power, for modern 
war machines can act and move so rapidly that it is of paramount im- 
portance to acquire accurate information of the enemy’s resources and 
dispositions. It has been said, in effect, that science has altered war 
from a game of cards to a game of chess. 
Broadly, there are two ways of sending a secret message. One is 
to write it in a selected code, and the other is to write it in a secret ink. 
The first calls for mathematical skill, the second rather for chemical 
and physical ability. Sometimes the two methods are combined, but this _ 
article will deal only with the latter. . 
What is a secret or sympathetic ink? It is a fluid in which a per- 
fectly invisible message may be written, but which, after suitable treat- 
ment with a “developer,” becomes coloured and hence visible. Most 
schoolboys know that if a message is written on ordinary paper with 
onion juice, lemon juice or diluted milk, it will be practically invisible 
when dry, but will show up dark brown if Heated witha ordinary iron. 
The fact that some cobalt salts are almost colourless when cold but 
highly tinted when hot has long been known, and must have struck many 
as affording a ready means of secret communication. That morbid 
genius, Edgar Allan Poe, wrote an interesting story, “ The Gols Bug,” 
the plot of which hinges on this fact. . 
In making an efficient secret message a gold nib must be used, for 
most invisible inks have an action on steel, and the presence of salts 
of iron would quite overcome the delicacy of some of the reactions, for 
some inks are used at a dilution of .001 per‘cent. It is essential that 
C.5690.—4 225 
