Chemistry was also in great request for secret writing, and various substances were found to afford a fluid which would leave no mark behind the pen, until some chemical agent were applied. For example, if a letter be written with a pen dipped in the juice of lemon, the words will be invisible until the paper is held before the fire. This is caused by the action of the heat. Again, if a solution of nitrate of iron be the fluid used, the writing cannot be seen until it is dipped into a solution of galls, or even into tea, which will act upon the iron, and become ink. It was found that if a plain sheet of paper were sent, and intercepted, the very fact of its being plain rendered it suspicious, and every means were used to render visible any writing that might be on it. A letter was therefore written with ordinary ink, on indifferent subjects, and between the lines the required information was added in some sympathetic ink. But writing with these or other sympathetic inks is unsafe, because the agents employed to render them visible are too generally known. Hence, the chiffre indéchiffrable, as it is called, has come very much into use, because it is easily applied, difficult to be deciphered, and the key may be preserved in the memory and easily changed. It consists of a table in which the letters of the alphabet, or any other signs agreed upon, are arranged as follow:
z a b c d e f g h i k l m n o p q r s t u v w x y z
a b c d e f g h i k l m n o p q r s t u v w x y z a
b c d e f g h i k l m n o p q r s t u v w x y z a b
c d e f g h i k l m n o p q r s t u v w x y z a b c
d e f g h i k l m n o p q r s t u v w x y z a b c d
e f g h i k l m n o p q r s t u v w x y z a b c d e
f g h i k l m n o p q r s t u v w x y z a b c d e f
g h i k l m n o p q r s t u v w x y z a b c d c f g
h i k l m n o p q r s t u v w x y z a b c d e f g h
i k l m n o p q r s t u v w x y z a b c d e f g h i
k l m n o p q r s t u v w x y z a b c d e f g h i k
l m n o p q r s t u v w x y z a b c d e f g h i k l
m n o p q r s t u v w x y z a b c d e f g h i k l m
n o p q r s t u v w x y z a b c d e f g h i k l m n
o p q r s t u v w x y z a b c d e f g h i k l m n o
p q r s t u v w x y z a b c d e f g h i k l m n o p
q r s t u v w x y z a b c d e f g h i k l m n o p q
r s t u v w x y z a b c d e f g h i k l m n o p q r
s t u v w x y z a b c d e f g h i k l m n o p q r s
t u v w x y z a b c d e f g h i k l m n o p q r s t
u v w x y z a b c d e f g h i k l m n o p q r s t u
v w x y z a b c d e f g h i k l m n o p q r s t u v
w x y z a b c d e f g h i k l m n o p q r s t u v w
x y z a b c d e f g h i k l m n o p q r s t u v w x
y z a b c d e f g h i k l m n o p q r s t u v w x y
z a b c d e f g h i k l m n o p q r s t u v w x y z
Any word is now taken for a key. The word Paris, for example. This is a short word, and for the sake of secresy it would be well to choose for the key some one or more words less striking. Suppose we wish to write in this cypher with this key the phrase, "We lost a battle," we must write Paris over the phrase, repeating it as often as is necessary, thus:
Pa risP a risPar
We lost a battle.
We now take cypher for w, the letter which we find in the square opposite w in the left margin column, and under p on the top, which is m. Instead of e we take the letter opposite e, and under a, which is f; for l, the letter opposite c, and under z, and so on.
Proceeding thus, we should obtain the following series of letters:
mf cxli b tkmimw
The person who receives the epistle writes the key over the letters
P a r i s P a r i s P a r
m f c x l i b t k m i m w
He now goes down in the perpendicular line, at the top of which is p, until he meets m, opposite to which, in the left marginal column, he finds w. Next, going in the line of a down to f he finds, on the left, e. In the same way r gives l, i gives o, and so on. Or you may reverse the process; begin with p, in the left marginal column, and look along horizontally till you find m, over which, in the top line, you will find w. It is easily seen that the same letter is not always designated by the same cypher; thus e and a occur twice in the phrase selected, and they are designated respectively by the cyphers f and w, b and k. Thus the possibility of finding out the secret writing is almost impossible.