1880.] Revision of the Atomic Weight of Aluminium. 



331 



No. 

 Mean 

 P. e. 



Series I. 



A.. 



5 



27-040 

 ±•0073 



B. 



5 



27-096 

 + •0054 



Series II. 



A. B. C. 



No. ...... 3 5 3 



Mean 27'034 27'023 ...... 27-018 



P. e +-0049 +-0052 +'0069 



Series III. 



A. B. 



No. .- 6 3 



Mean 27'005 26*990 



P. e +-0033 +-0046 



In view of the gradual loss of water which crystallised ammonium 

 alum has been shown to undergo on exposure to the air, the author 

 considers that of these various sets of experiments, Series I, B, is 

 entitled to least confidence. He is inclined to attach most weight to 

 Series III, A, since the method used was very simple in principle, 

 the determination of one of the two quantities concerned was rendered 

 very exact by the great volume occupied by the hydrogen, the com- 

 parison was made directly with the standard element in our system of 

 atomic; weights, and not through the intervention of any other sub- 

 stance whose atomic weight must be assumed, and the agreement 

 of the results among themselves is particularly good. 



The general mean from all the thirty experiments, if all be included, 

 is Al=27"032, with a probable error for this mean of + '0045. If 

 Series I, B, be excluded, the mean of all the remaining twenty-five 

 experiments is Al = 27'019, with a probable error of +'0030. 



The general result adds, the author hopes, aluminium to the, un- 

 fortunately still limited, list of those elementary substances whose 

 atomic weights have been determined within the limits of precision 

 attainable with our present means of experiment. 



This result also adds one to the cases already on record of the 

 numbers representing carefully determined atomic weights approaching 

 closely to integers, and leads the author to say a word on the recon- 

 sideration of " Prout's Law." Taking the following 18 elements as 

 the only ones of which the atomic weights may be fairly considered ac 

 determined, with reference to hydrogen, with the greatest attainable 

 precision, or a near approach thereto, namely oxygen, nitrogen, chlorine, 



VOL. XXX. 2 A 



