ATOMIC WEIGHTS 263 



In the second experiment the AICI3 contained traces of iron. Eejecting 

 this experiment, the remaining six give a mean of 41.344, ±.007. These 

 data give a value for Al approximating to 27.4, and were for many years 

 regarded as satisfactory. It now seems probable that the chloride con- 

 tained traces of an oxy-compound, which would tend to raise the atomic 

 weight. 



In 1879 Terrell ^ published a new determination of the atomic weight 

 under consideration, based upon a direct comparison of the metal with 

 hydrogen. Metallic aluminum, contained in a tube of hard glass, was 

 heated strongly in a current of dry hydrochloric acid. Hydrogen was 

 set free, and was collected over a strong solution of caustic potash. 

 0.410 grm. of aluminum thus was found equivalent to 508.2 cc, or 

 .045671 grm. of hydrogen. Hence Al = 27.142. 



About a year after Terrell's determination appeared, the lower value 

 for aluminum was thoroughly confirmed by J. W. Mallet." After giving 

 a full resume of the work done by others, exclusive of Isnard, the author 

 describes his own experiments, Avhich may be summarized as follows : 



Four methods of determination were employed, each one simple and 

 direct, and at the same time independent of the others. First, pure 

 ammonia alum was calcined, and the residue of aluminum oxide was 

 estimated. Second, aluminum bromide was titrated with a standard 

 solution of silver. Third, metallic aluminum was attacked by caustic 

 soda, and the hydrogen evolved was measured. Fourth, hydrogen was 

 set free by aluminum, and weighed as water. Every weight was care- 

 fully verified, the verification being based upon the direct comparison, 

 by J. E. Hilgard, of a kilogramm.e weight with the standard kilogramme 

 at Washington. The specific gravity of each piece was determined, and 

 also of all materials and vessels used in the weighings. During each 

 weighing both barometer and tliermometer were observed, so that every 

 result represents a real weight in vacuo. 



The ammonium alum used in the first series of experiments was 

 specially prepared, and was absolutely free from ascertainable impuri- 

 ties. The salt was found, however, to lose traces of water at ordinary 



1 Bull. Soc. Chim., 31, 153. 



2 Phil. Trans., 1S80, p. 1003. 



