Objections to the method.
Marignac[6] offered the objection to this method for determining the atomic weight of zinc that the zinc oxide dissociated when heated in platinum over the blast-lamp. The same objection might be urged against this method for determining the atomic weight of cadmium, had it not been shown that the objection does not hold for zinc[7]. What took place was a reduction of the zinc oxide by the highly heated hydrogen which passed through the hot platinum.
It was shown that zinc oxide can be heated in a platinum vessel in a muffle furnace, to the melting point of steel, without undergoing any dissociation, or in any wise losing in weight. This source of error was avoided by using porcelain vessels, which were not brought into contact with the free flame.
The statement of Marignac that the oxide of zinc derived from the nitrate retains oxides of nitrogen even when heated to the temperature at which it begins to undergo dissociation, was shown by the same authors to be without foundation. The basis of this objection is doubtless to be found in the imperfect method of testing for such oxides.
It might be urged as an objection to this method that the difference in weight between the metal and oxide is not very great, therefore any error in weighing would be multiplied in the result. At first sight this objection may appear valid, but since the substances weighed were so well adapted to that purpose and the weighings could be made with such a high degree of accuracy no appreciable error could have resulted from this source.
A crucible with its contents was repeatedly weighed against its tare and weights to ascertain the difference between successive weighings under the conditions employed. A number of weighings agreed to .00002 gr. and in some instances to half this amount.