TUNGSTEN COMPOUNDS. 97 



In order to avoid the danger to which all the former 

 determinations are liable, consequent upon the partial re- 

 duction to green oxide, which cannot again be oxidized, 

 and the production of which appears to be caused by the 

 presence of traces of alkali, the tungstic acid used was pre- 

 pared by decomposing pure oxychloride with water. The 

 acid thus obtained has a bright canary-yellow colour, which 

 becomes darker on heating. After igniting a portion of 

 this oxide in a glass tube, in a current of air, it lost its 

 bright yellow colour and assumed a greenish tint, owing to 

 reduction by alkaline impurities which it had taken up 

 from the glass. Hence the acid used for the atomic- 

 weight determinations was ignited on porcelain, and the 

 determination of the loss of weight on reduction and the 

 subsequent gain of weight on oxidation was several times 

 repeated. 



The oxide was for this purpose placed in a large por- 

 celain boat (platinum is attacked by tungsten), and heated 

 in a porcelain tube alternately in pure hydrogen and in a 

 current of air. After each reduction the boat was found 

 to be partially coated inside with a thin black film having 

 a metallic appearance, which oxidized completely when 

 heated in the air. A second boat was placed beyond that 

 containing the substance, for the purpose of ascertaining 

 whether any metal was volatilized ; but the boat was not 

 found to have become the least discoloured. 



The following are the experimental data : — 



grms. 

 i. Weighing-tube and boat (after beating for three hours in air) ... 63-0573 



2. ,, -|- tungsten trioxide (after heating for three 1 



u • • \ f 70-9413 



hours in air) J 



3. „ -f- tungsten (after reduction in hydrogen) 69*301 1 



4. „ -f tungsten trioxide (after heating for three 1 



hours in air) J 



5. ,. 4- tungsten (after reduction in hydrogen) 69-3054 



6. „ -f tungsten (after reheating in hydrogen) 69-3061 



7. „ -)- tungsten trioxide (after oxidation in air) 70-9365 



SER. III. VOL. V. H 



