W. Gibbs—Analytical Notices. 111 
better result is obtained by precipitating at a boiling heat, when 
the mercurous chromate almost immediately becomes highly 
crystalline, its color changing to a bright scarlet. It may then 
be washed with the greatest ease, and ignited in the usual 
manner. It is absolutely necessary in applying this method 
that the mercurous nitrate used should be perfectly free from 
nitrous acid. Want of attention to this point led me formerly 
into an error, which I desire to correct in this place. ave 
stated in a former paper* that hot solutions must not be 
employed on account of the reduction of chromic acid by mer- 
curous nitrate. This reduction is not due to the temperature, 
but to the presence of a small quantity of nitrous acid in the 
mercurous nitrate employed. It is easy to avoid this source of 
error by dissolving’ the mercury in nitric acid, in an open 
vessel, and crystallizing the nitrate two or three times, using 
for solution dilute nitric acid which has been perfectly freed 
from nitrous acid by a current of air or carbonic dioxide. 
0 test the method thoroughly, the following analyses were 
made with pure potassic dichromate : : 
L Salt precipitated at a boiling heat by mercurous nitrate and 
washed with hot water alone 
1. 0°6003 gr. gave 0°3030 gr. €r, 0, = 50°47% Cr, 0,. 
2.04744 or. > 08007 ge. Et oe TE 
The formula K,Cr,0, requires 51-73% if we take Cr = 52-2. 
IL Salt precipitated cold by mercurous nitrate and washed 
with cold water only. 
3. 0°2641 gr. gave 0°1344 gr. €r, 0, = 50°89% Cr, 9,. 
4. 5008 pro 99-9607 gr 58> oe OP 1S 
III. Salt precipitated cold, then boiled and washed with boiling 
water only. 
5. 0°4957 gr. gave 0°2503 gr. €r,0, = 50°49% Cr, 0,. 
o-3208 gre 3% ee 
6. 06393 gr. 
IV. Salt precipitated cold, then boiled and washed with hot 
water containing mercurous nitrate. 
In these last analyses, the error of the mean is only 0-044. 
We arrive, however, more quickly at our object when we pre- 
Cipitate at once at the boiling point, and then wash with a hot 
dilute solution of the nitrate. cs 
several works on Analytical Chemistry it is recommended 
to precipitate chromic acid from its solutions by plumbic 
acetate, and to weigh the resultin, chromate of lead. In 
ials 
pea 
ever to prevent the precipitated plumbie chromate from passin 
more oe a dirough the tilter ea to render the filtrate barbide 
* This Journal [II], vol. xxxix, p. 59. 
