On Indigo. 435 
In the Ist vol. of the Annals of Philosophy 
(1813) I pointed out a safe and easy method 
of estimating the quantity of oxymuriatic 
acid, in solutions of oxymuriate of lime, 
not by solutions of indigo, which must be 
variable from the quality of the indigo, but 
by solutions of protosulphate of iron, which 
can always be obtained of the saine strength. 
I say safe and easy method, notwithstanding 
we are gravely told by one professor of che- 
mistry, that he had tried the method and was 
nearly killed by it; and another states that 
he had attempted to follow my method, but 
did not succeed. Any person who is tolerably 
skilled in chemical manipulations, and has 
the two liquids, sulphate of iron of a known 
strength, and oxymuriate of lime of a known 
specific gravity, before him, may determine 
the strength of the oxymuriate in the space 
of five minutes. In this time I found the 
strength of the oxymuriate of lime used on 
the present occasion. Having by me a so- 
lution of protosulphate of iron containing 8 
per cent. oxide, I touk 50 grain measures of 
it and poured them into a wine glass, then 
100 of the oxymuriate, stirring the mixture, 
after which no smell was perceived; 100 more 
were poured in, still no smell; then dropped in 
ten grains at atime by adropping tube, stirring 
