70 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC. 



did not take the very important precaution of fusing the sulphate before 

 weighing ; a and moreover, the possibility that the reduced silver might con- 

 tain argentic sulphide or sulphate was by no means excluded. 



A brief review of subsequent work may not be out of place, although it 

 is of a different order of precision. Cooke reduced argentic sulphide in a 

 current of hydrogen, and concluded that the atomic weight of sulphur 

 must lie between the limits 32.14 and 31.98. In summing up his results 

 he stated that "this is equivalent to confirming the accepted value of 

 this constant, so far as any experiments on a scale less extensive than those 

 of Stas can be of value to this end." : 



The determination of the atomic weight of sulphur by Richards was 

 made incidentally in his work on the atomic weight of copper. The ratio 

 Na 2 CO 3 : Na 2 SO 4 gave as a result S = 32.075. If sodium is taken as 

 23.008 instead of 23.053, the result becomes S = 32.043. No very great 

 confidence was placed in these results at the time, as is shown by the fol- 

 lowing sentence : "The results are hardly capable of deciding the present 

 uncertainty in the atomic weight of sulphur." 3 It is probable that the 

 result is too low, as no proof could be obtained of the thorough desicca- 

 tion of the sodic carbonate. 



Very recently numerous atomic-weight determinations have appeared, 

 depending on a purely physical method, based on the assumption that 

 Avogadro's hypothesis and the simple gas law PV = RT is strictly true 

 for gases when infinitely expanded. Since it is impossible to determine the 

 ratios of the densities of gases with sufficient accuracy at very low pres- 

 sures, it is necessary to determine this ratio under normal conditions and 

 apply a different correction in each case, depending on the deviation of 

 the gas from the simple gas law. It is probable that the ratio of the den- 

 sities can be, and in most cases has been, determined with sufficient per- 

 centage accuracy. On the other hand, the correction is hypothetical 

 and much less certain ; and accordingly, the method has but little value 

 when the correction is large. 



According to Leduc 4 the ratio between the densities of sulphur dioxide 

 and oxygen is 2.04835. If no correction is applied for the imperfection of 

 the gases, this value leads to 33.55 for the atomic weight of sulphur. The 

 correction in this case is unusually large and therefore must be known 

 with a high percentage accuracy if the result is to have any value as 

 a determination of atomic weights. The following are the general meth- 

 ods of calculating the correction, but none seems to be of sufficient value 

 for the present case. 



iRichards, Proc. Amer. Phil. Soc., 42, 28 (1903). 

 2 Cooke. Proc. Amer. Acad., 13. 52 (1878). 

 3 Richards, Proc. Amer. Acad., 26, 269 (1891). 

 4 Leduc, C. R., 117, 219 (1894). 



