356 On the Absence of Iron in Hydrochloric Acid. 
For the removal of the nitric acid from oil of vitriol, either of the 
two following methods may be adopted :— 
1: Three volames of the acid are to be mixed with one of water and 
sulphurous acid transmitted through the liquid until it smells strongly, 
it is then to be boiled till all odour of sulphurous acid has disap- 
peared. 
2. Instead of diluting the acid with water, the same bulk of a 
saturated solution of sulphurous acid may be used, which has the 
advantage that a supply of the solution may be kept for use when 
required, 
In preparing an acid free from lead, as well as nitric acid, it will 
be found necessary to dilute the oil of vitriol, with half its bulk of 
water. As sulphate of lead is slightly soluble in oil of vitriol of spe- 
cific gravity 1-715 ; mixed with half its bulk of water, its density is 
about 1-650. The oil of vitriol used in these experiments was from 
one of the first manufactories.in the country, and had a_ specific 
gravity of 1°838. 
2. On the Absence of Iron in Hydrochloric Acid, prepared by 
Professor Gregory’s process. 
Professor Gregory recommended the use of patent salt, as free 
from iron, to yield a pure acid. But, although an acid free from iron 
may thus be obtained, I was struck with the fact that the sulphate 
of soda remaining in the flask had always a yellowish colour, On 
testing it, I found iron present in the residue in every case. It 
was therefore plain that even the patent. salt was not free from iron, 
and that the absence of iron in the hydrochloric acid, made from 
such materials, depended on some cause which prevented the per- 
chloride of iron from passing over. This cause, Professor Gregory 
suggests, may be the low temperature at which the operation is 
carried on, or the probable effect of an excess of sulphuric acid in 
preventing the formation of the perchloride of iron. At all events, 
I found that, even when iron filings, or peroxide of iron were added 
to the materials in considerable quantity, no iron could be detected 
in the hydrochloric acid, This was the case, even when the oil of 
vitriol contained so much nitric acid as to yield a very dark-coloured 
product, coloured by free chlorine and nitrous acid. 
This observation is practically valuable, since it enables us to ob- 
tain, by Professor Gregory’s process, perfectly pure and colourless 
hydrochloric acid from the commonest sea-salt, although it contains 
a good deal of iron, and thus still further to reduce the cost of a re- 
agent so indispensable as pure hydrochloric acid. 
Professor Gregory formerly detected traces of iron in the hydro- 
chloric acid made with the common kitchen salt, which induced him 
to use patent salt. This iron may have heen carried over as perchlo- 
ride, in consequence of the distillation having been pushed too far, 
