96 PROF. H. E. ROSCOE ON CERTAIN 



Calculated. Found. 



Tungsten, W =184 4-8'94 49 #l8 



Bromine, Br 2 =i6o 4 2 "55 4 2 '°5 



Oxygen, O a = 32 851 



376 100*00 



Iodide of Tungsten. 



On passing iodine-vapour, together with carbonic acid, 

 slowly over metallic tungsten heated to redness, a very 

 small quantity of soft scaly crystals, having a greenish me- 

 tallic lustre, is found to sublime near the heated part of 

 the tube. The same substance is formed (but also only in 

 very small quantities) when iodine-vapour is passed over 

 the heated brown oxide, or a mixture of a metal and an 

 oxide. The product was analyzed by passing air over the 

 heated iodide, when it is readily converted into tungstic 

 trioxide, iodine being volatilized. The sublimed iodine 

 was dissolved in sulphurous acid solution, and the iodide 

 of silver ultimately converted into silver chloride. 



Substance taken 0*9142 



Tungsten trioxide obtained °'4953 



Silver chloride 0*5850 



Calculated, WI 2 . Found. 



Hence tungsten, W= 1 84 42*01 4 2 '95 



„ iodine, I 2 =254 57*99 56*64 



438 100*00 99*59 



The iodide is infusible, and cannot be redistilled with- 

 out decomposition; it is not immediately acted upon by 

 water. 



Atomic Weight of Tungsten. 



1. By reduction of Tungsten Trioxide. — The difficulty 

 of obtaining perfectly pure tungstic acid, and the effect 

 impurity produces on the atomic-weight determinations, 

 has been pointed out by Dumas *. 



* Ann. Ch. Pharm. cv. p. 105. 



