PEOFESSOE EOSCOE’S EESEAECHES ON VANADIUM. 
7 
exact number which the following experiments yield, when calculated on Bekzelius’s 
hypothesis, is 6 7 ’3. 
In the following determinations of atomic weights the quantity of powdered pentoxide 
employed was always more than 5 grms. * It was placed either in a bent tube of hard 
glass, which when heated in a magnesia bath was found not to lose more than (M)001 
grm. after repeated ignition and cooling, or in a platinum boat placed in a glass tube. 
Before reduction the powdered vanadium pentoxide was gently heated in a current of 
dry air until the weighings, after two successive operations, were found to be constant. 
After the reduction, the tube was allowed to cool completely in a current of hydrogen, 
and this gas was then displaced by a stream of dry air. If the reduced oxide be even 
slightly warm when it is exposed to the air it absorbs oxygen, suddenly glows, and is 
superficially converted into a blue oxide ; if, however, the oxide be cold, it may be ex- 
posed to dry air for some time without change of weight, an experiment in which nitro- 
gen was substituted for air giving results identical with those in which air was used. 
In the experiments in which the oxide was contained in the glass tube the latter was 
carefully stoppered, and allowed to remain for half an hour in the balance-case before 
weighing ; when the platinum boat was used it was carefully withdrawn, with the re- 
duced oxide, from the combustion-tube and placed in a stoppered test-tube for weighing. 
These precautions are absolutely necessary in order to ensure accuracy, owing to the 
hygroscopic nature of the trioxide. 
The hydrogen employed was most carefully purified and dried ; all joints and stoppers 
in the apparatus were well secured by copper wire and paraffin. The gas passed through 
solutions of silver nitrate, sodium pyrogallate, caustic soda, and sulphuric acid, and a 
tube filled with metallic copper kept red-hot during the experiment was placed before 
the final drying-tube to ensure absence of oxygen. In order to ascertain whether 
atmospheric oxygen diffused into the apparatus, a weighed tube containing phosphorus 
pentoxide was attached to the further end of the combustion-tube during an experi- 
ment, and allowed to remain for four hours whilst hydrogen was passing through the 
heated tube; at the end of the operation the drying-tube had gained only 0 , 0002 grm., 
proving the absence of diffused oxygen. For the purpose of drying the hydrogen, boiled 
sulphuric acid alone can be used. In many of the preliminary experiments phosphorus 
pentoxide was used in the last drying-tube, and in all these cases the reducing action of 
the hydrogen was not complete, the numbers thus obtained for the atomic weight being 
all too high. After much labour this was found to be entirely caused by small traces 
of the light particles of the phosphorus pentoxide, which were invariably carried over, in 
spite of stoppers of cotton-wool, with the air and hydrogen into the vanadic acid, 
* In order to ensure accuracy, it is absolutely necessary to employ a larger quantity of material than was 
taken by Beezelius. The largest amount which he used was 2-2585 grms. vanadic acid, and the smallest 
0-6499 grm. An error of one milligramme on the first weight will produce a variation of +0-2 in the atomic- 
weight, whilst a similar error on the smaller quantity would throw out the result by +0-7. If 5 grms. of sub- 
stance are operated upon the variation arising from a milligramme error is +0-086. 
