ATOMIC WEIGHTS 305 



Hence, 



From ratio 7 P=: 30.908, ± .0012 



" 3 30.920, rt .0243 



" 4 30.970, ± .0384 



" 1 31.018. ±: .0077 



" 2 31.027, ±: .0117 



" 6 31.051, ± .0039 



" 5 31.054, ± .0148 



General mean, P = 30.925, ± .0011 



Eatio 7 is here evidently overweighted to au enormous extent. It 

 outweighs all the others collectively, which is a highly improbable con- 

 dition. If we reject it altogether, the remaining six ratios give a general 

 mean of P =31.041, ±.0032, which appears to be more probable than the 

 much lower value given above. The arithmetic average of the seven 

 values is P = 30.993. The true value is probably near 31, as is com- 

 monly assumed. 



VANADIUM. 



Eoscoe's determination of the atomic weight of vanadium was the first 

 to have any scientific value. The results obtained by Berzelius ^ and by 

 Czudnowicz ^ were unquestionably too high, the error being probably 

 due to the presence of phosphoric acid in the vanadic acid employed. 

 Tliis particular impurity, as Roscoe has shown, prevents the complete 

 reduction of V2O5 to Vo^s by means of hydrogen. All vanadium ores 

 contain small quantities of phosphorus, which can only be detected with 

 ammonium molybdate— a reaction unknown in Berzelius* time. Fur- 

 thermore, the complete purification of vanadic acid from all traces of 

 phosphoric acid is a matter of great difficulty, and probably never was 

 accomplished until Eoscoe undertook his researches. 



In his determination of the atomic weight, Eoscoe ' studied two com- 

 pounds of vanadium, namely, the pentoxide, VoOg, and the oxychloride, 

 TOCI3. The pentoxide, absolutely pure, was reduced to V2O3 by heating 

 in hydrogen, with the following results : 



7.7397 grm. V.O5 gave 6.3827 grm. V0O3. 17.533 per cent. loss. 



6.5819 " 5.4296 " 17.507 



5.1895 " 4.2819 " 17.489 



5.0450 " 4.1614 " 17.515 



5.4296 grm. V.O,,, reoxidized, gave 6.5814 grm. V.O.,. 17.501 per cent, difference. 



Mean, 17.509, ± .005 



Hence V = 51.381, ±.0320. 



iPoggend. Annal., 22, 14. 1831. 



2 Poggend. Annal., 120, 17. 1863. 



3 Journ. Chem. Soc, 6, pp. 330 and 344. 1868. 



