120 SOCIAL HABITATIONS. 
stance has been submitted to boiling heat, but not 
dissolved. Tea, for example, is, when properly made, 
a decoction of the leaf, though when made with hot, but 
not boiling water, it has no right to the name. The 
solution of copperas is only pale green, and that of the 
gall is nearly colourless, although when mixed, they 
become deeply black. The old practical joke of forcing 
a dupe to stain his hands and face black, depended on 
the knowledge of these properties. 
Before the victim went to wash his hands, some of 
the decoction of galls was poured into the water, while 
the towel with which he was supplied had been damped 
with the copperas solution and then dried. The con- 
sequence of this combination was, that although the 
hands and face might be washed perfectly clean, yet as 
soon as they were dried with the prepared towel the 
union of the two substances produced ink, and both 
hands and face were deeply stained. 
Now when a gall is cut with a knife, the slightly acid 
juice acts upon the steel, and so a kind of ink is pro- 
duced, which is pale, but still a veritable ink. There 
is a well-known method of secret writing which depends 
on this property of iron and tannin, the principle con- 
tained in the galls. 
A quill pen is dipped in the solution of copperas, and 
the required message is written, usually between the lines 
or among the words of a letter on unimportant subjects, 
so as to avoid the suspicions which would be aroused by 
a sheet of blank paper. The almost colourless solution 
leaves no mark, and the letter passes without comment, 
until it reaches the person who is in the secret. He 
pours some decoction of galls into a wide and flat vessel, 
and warily dips the letter into it, so as to wet it; or 
he saturates a cloth with the decoction, and lays the 
