ATOMIC WEIGHTS 407 



Moissan, in five experiments, found F = 19.05 to 19.09. Assuming 

 that he put Ba = 137, and S = 32.074 as before, these two extremes 

 become — 



1.3311 

 1.3305 



Mean, 1.3308 



Hence F = 19.02. 



Giving- this equal weight with Louyet's mean, we get the subjoined 

 combination : 



Louyet 1.331, ± .0004 



Moissan 1.3308, ± .0004 



General mean 1.3309, ± .00028 



The experiments with lead fiuoride are due to Louyet, and a new 

 method of treatment was adopted. The salt was fused, powdered, dis- 

 solved in nitric acid, and precipitated by dilute sulphuric acid. The 

 evaporation of the fluid and the ignition of the sulphate was then effected 

 without transfer. Five grammes of fluoride were taken in each opera- 

 tion, yielding of sulphate: 



6.179 

 6.178 

 6.178 



Mean, 6.1783, ± .0002 



Hence F = 19.14. 



In Christensen's determinations ^ we find a method adopted which is 

 radically unlike anything in the work of his predecessors. He started 

 out with the salt (N"H4)2MnF-,. When this is added to a mixture, in 

 solution, of potassium iodide and hydrochloric acid, iodine is set free, 

 and may be titrated with sodium thiosulphate. One molecule of the 

 salt (as written above) liberates one atom of iodine. In four experi- 

 ments Christensen obtained the following data : 



3.1199 grm. Am.MnF, gave 2.12748 I. 68.191 per cent, 



3.9190 " 2.67020 " 68.135 



3.5005 " 2.38429 " 68.113 



1.2727 " .86779 " 68.185 



Mean. 68.156. ± .0128 



Hence F = 19.038. 



Journ. prakt. Chem. (2), 35, 541. Christensen assigns to the salt double the formula here given. 



