1014 
PROFESSOR J. W. MALLET ON A REVISION 
and thermometer being observed at the time of each weighing, all weights hereafter 
mentioned in this paper represent real values in vacuo . 
Three separate series of experiments were made, by methods to be presently stated. 
A fourth series was attempted, involving the conversion of metallic aluminum into 
oxide and determination of the amount of oxygen taken up, but this process was found 
to be attended with much difficulty from various causes, amongst others from the 
liability to loss by spirting if the metal were treated with acid in open or small 
vessels, from the necessity of transfer to such vessels for final ignition if larger ones 
were at first used, and from the appreciable solubility of the hydrate of aluminum if 
this were precipitated in order to avoid evaporation of the original solution. The few 
results obtained in this way agreed generally with those of the other methods, but 
varied among themselves within unsatisfactorily wide limits, and were manifestly not 
deserving of equal confidence. Hence the work was not pushed further in this 
direction. 
First series of experiments . 
Purification of ammonium alum .—Ammonium alum of commerce was dissolved 
in water, a very little nitric acid added, and the liquid boiled in large glass beakers by 
passing in a current of steam. When the solution had become cold a little ammonium 
fer rocyanide was added—a very small quantity sufficed to throw down the traces of 
iron present—and a, little animal charcoal, previously well boiled with strong hydro¬ 
chloric and nitric acid and thoroughly washed, was stirred in to aid in the subsidence 
of the minute amount of very finely divided Prussian blue which had been formed. 
The clear liquid was after about ten days drawn off, and evaporated until the larger 
part of the alum crystallised out on cooling. The crystals, of which there was obtained 
more than a kilogramme, were re-dissolved in hot water, and re-crystallised several 
times (throwing away all the mother liquors), the last time in a porcelain vessel, and 
with agitation, so as to obtain a granular crystalline powder, which was washed with 
cold distilled water. 
Re-dissolving in water the so fax purified alum, now much reduced in quantity, 
it became necessary to secure the exclusion of the metals of the fixed alkalies, lest 
their alums should exist in isomorphous admixture with the pure ammonium alum 
required. To this end aluminum hydrate was thrown down from the solution by 
addition of ammonia, using not quite enough of the reagent for complete precipitation. 
The precipitate was well washed with abundance of water, this tedious process being 
much expedited by the use of a siphon-filter delivering the liquid drawn off into a 
large flask connected with a powerful aspirator. The bulk of the precipitate was 
twice re-dissolved in pure hydrochloric acid (each time avoiding complete solution), 
thrown down again by ammonia, and again washed. 
This hydrate was now dissolved in just the necessary amount of dilute and very 
carefully purified sulphuric acid, and just the proper amount added of ammonium sul- 
