54^ Mr, PoRRETT on the nature of the Salts 
it a solution containing sulphate of copper and green sulphate 
of iron, in the proportions of two of the former salt to three of 
the latter, until the crimson colour disappears, when a copious 
white precipitate will be formed, which is an insoluble salt of 
copper composed of the protoxide of that metal combined with 
the red tinging acid. This salt must then be boiled with a 
solution of potash which takes from it its acid, leaving the 
oxide of copper as a bright yellow powder. The acid thus 
transferred to the potash may be obtained in a free state by mix- 
ing this combination with sulphuric acid, and distilling it. The 
red tinging acid comes over, as a liquid strongest towards the 
end of the process, it is not however yet quite pure, as it will be 
found to contain a little sulphur and sulphuric acid, the best 
mode of separating both of which, is to add carbonate of barytes 
until the acid is saturated with it, and afterwards to throw down 
the barytes by the careful addition of sulphuric acid. The red 
tinging acid will thus be obtained in a state of perfect purity. 
In this state it is generally colourless, but sometimes acquires 
a pinkish hue, which I believe to be owing to a commencement 
of oxidation. In the most concentrated form in which I have 
obtained it, it had the specific gravity of 1.022, and a smell as 
pungent, and a little resembling that of strong acetic acid ; at 
the boiling temperature it had a slight solvent action on sul- 
phur, when the latter had been presented to it in a very divided 
state, but most of the sulphur thus dissolved separated again 
on cooling, the rest which it still retained after cooling, and 
which is foreign to its composition, may be detected by nitrate 
of silver, or by nitrate of protoxide of mercury, which form 
with the red tinging acid in the pure state, precipitates which 
are white and continue so, but with the acid holding sulphur in 
