158 THE ATOMIC WEIGHTS. 



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, alumi- 

 num bromide was titrated with a standard solution of silver. 

 Third, metallic aluminum was attacked by caustic soda, 

 and the hydrogen evolved was measured. Fourth, hydro- 

 gen was set free by aluminum, and weighed as water. 

 Every weight was carefully verified, the verification being 

 based upon the direct comparison, by J. E. Hilgard, of a 

 kilogramme weight witli the standard kilogramme at Wash- 

 ington. The specific gravity of each piece was determined, 

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

 During each weighmg both barometer and thermometer 

 were observed, so that every result represents a real weight 

 in vacuo. 



The ammonium akim used in the first series of experi- 

 ments w^s specially prepared, and was absolutely free from 

 ascertainable impurities. The salt was found, however, to 

 lose traces of water at ordinary temperatures ; a circum- 

 stance which tended towards a slight elevation of the appar- 

 ent atomic \v'eight of aluminum as calculated from the 

 weighings. Two sets of experiments were made with the 

 alum ; one upon a sample air-dried for two hours at 21°-25°, 

 the other upon material 'dried for twenty-four hours at 

 19°-26°. These sets, marked A and B respectively, differ 

 slightly ; B being the less trustworthy of the two, judged 

 from a chemical standpoint. ^Mathematically it is the better 

 of the two. Calcination was effected with a great variety of 

 precautions, concerning which the original memoir must be 

 consulted. To Mallet's weighings I append the percentages 

 of AUOg deduced from them : 



Scries A. 



8.2144 grm. of the alum gave .925S grm. Al.^O^. 11.270 per cent. 



14.0378 " 1.5825 " 11.273 " 



5.6201 " .6337 " 11.275 " 



11.2227 " 1.2657 " 11.278 " 



10.8435 " I. 2216 " 11.266 " 



Mean, 11.2724, ± .0014 



