330 



Mr. J. W. Mallet. 



[Apr, 22, 



weight, which occupied a large part of his leisure time during the 

 last three years. In this investigation he spared no pains to attain 

 as precise a result as possible, and aimed especially at the discovery, 

 and as far as possible removal, of sources of error connected with 

 the methods employed. The following general principles were kept 

 in view : — 



1st. That each process should be as simple as possible, and should 

 involve as little as possible of known liability to error. 



2nd. That different and independent processes should be resorted to 

 as the means of checking each other's results, even though it may 

 fairly be assumed that one is more advantageous than another. 



3rd. That each process should be carried out with quantities of 

 material differing considerably from each other in successive ex- 

 periments. 



4th. That only such other atomic weights should be involved as 

 maybe counted among those already known with the nearest approach 

 to accuracy. 



The most scrupulous care was taken in the purification and exami- 

 nation of all the reagents used, and, as far as possible, vessels of 

 platinum or of hard porcelain were substituted for those of glass. 



In the author's first series of experiments, the alumina was de- 

 termined which resulted from the ignition of a known weight of 

 ammonium alum. 



In the second series, a solution was made of a known weight of 

 bromide of aluminium, and was very exactly precipitated by a solution 

 of nitrate of silver, and the weight of bromide of aluminium com- 

 pared with that of the silver required for forming the solution of 

 nitrate. 



In the third series, bromide of aluminium was used for the prepara- 

 tion of pure aluminium, by reduction by sodium ; a carefully weighed 

 portion of the pure aluminium was dissolved in a solution of caustic 

 soda, and the hydrogen given off was estimated by volume ; or else 

 burnt, and the weight of resulting water determined. 



In the following epitome of the results, A, B, C denote groups of 

 experiments under each series, the experiments of each group differing 

 only in the quantity of material operated on, while the different 

 groups differ in the particular preparation used, or in some feature of 

 the method. Thus in Series I, A was made with alum dried by 

 exposure to air for two hours ; B with alum dried by exposure for 

 twenty-four hours. In Series II, A, B, C were made with successive 

 portions of a distillate of bromide of aluminium. In Series III, A was 

 made by estimating the hydrogen by volume, B by estimating by 

 weight the water resulting from its combustion. Only the number of 

 experiments in each group, the mean result from the group, and the 

 probable error of the mean, are here given. 



