238 W. Gibbs and \F. A. Genth 
and if the heat be applied only after the first action of the acid 
is over. The mixture is to be gently heated over a spirit lamp, 
until the excess of the acid, sulphate of ammonia, and other vol- 
* atile matters have been expelled. During the whole time of 
heating, the cover of the crucible must be so placed as to pre- 
vent the possibility of loss by spaitering, and at the same time 
to permit the escape of volatile matters. hen, however, the 
quantity of acid has not been too great, the whole process = 
on very quietly to the end, when the mass becomes dry. 
heat is finally to be raised, for an instant, to low redness, the 
cover of the crucible being quickly lifted off and then replaced. 
The crucible is then to be allowed to cool and weighed, when 
the quantity of cobalt may easily be calculated from the weight 
of the and pure sulphate. After the weighing, the mass in 
the crucible must be carefully examined. It should have a fine 
rose color, and be perfectly soluble in warm water, leaving no 
residue. In case this is observed, which happens only 
when the heat has been too high, a drop of sulphuric acid and a 
few drops of oxalic acid may be added, and the whole evaporated 
to dryness, and again ignited. When, however, there is much 
—_ of cobalt present, it is better to reject the analysis at once. 
With a little care and practice the operation succeeds almost 1n- 
variably, and the result, as we shall hereafter show, leaves noth- 
ing to be desired in point of accuracy. When chlorine is pres 
ent in the salt to be analyzed, a little free chlorine is sometimes 
found among the products of the action of the sulphuric acid, 
and the platinum crucible is slightly acted upon. In such cases 
we usually add a little oxalate of ammonia to the salt before 
dropping the acid upon it. The quantity of salt to be taken for 
analysis may vary from three to five decigrammes; when more 
is used, there is apt to be some loss from effervescence. In con- 
. 
sequence of the small quantities of substante employed, the 
weighings mu ccurate as possible. In calculating the 
weight of the cobalt from that of the sulphate, we have the ad- 
of determining one substan another with an 
equivalent more than twice as high. vate cae 00 
with a solution of caustic potash, washin 101 
oughly, and estimating the ignited precipitate as CoeOz, or a8 
metallic cobalt after reduction by hydrogen. Frémy justly ob- 
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