55^ PoRRETT on the nature of the Salts, &c. 
they precipitate the last atom of that oxide from its solutions 
in the state of an insoluble salt, whose composition is known. 
In order that it should have this effect however, the oxide of 
copper should exist in the solution as protoxide, (in which 
state it is seldom obtained in analyses) or which answers the 
same purpose, a deoxidating agent, such as sulphurous acid, 
or its salts, or a salt of protoxide of tin, or of iron, must be 
added to it. 
Other useful applications of these salts and of their acid will 
also naturally occur to the ingenious analyst who wishes to 
avail himself of its peculiar actions on metallic bodies. It would 
be tedious to point these out, and their recital would extend a 
communication that is already of greater length than I could 
have wished, and which I here close. 
Tower, June 6, 1814. 
