DR. HAROLD A. WILSON ON THE 
252 
pentoxide was sealed on to the apparatus near the tnhe containing the hot wire. The 
P 0 O 5 used was a specially good sample, which was found to he quite free from lower 
oxides and contained no lumps of meta-phosphoric acid. In some of the earlier 
experiments a small tube packed tightly with gold leaf was put in between the hot 
wire tnhe and the rest of the apparatus to absorb mercury vapour. About 50 gold 
leaves were packed into a tube 1 centim. in diameter and 3 centims. long, and it was 
found that the gold near the end of the tube next the pump and gauge became 
amalgamated, whereas the rest of the gold remained for months perfectly bright. I 
believe that such a tube entirely prevents the diffusion through it of mercury vapour, 
and tliat previous failures in using this method of absorption are due to too few gold 
leaves having been used and not sufficiently tightly packed. Of course, such a tube 
does not allow very free communication between its ends, so that some little time 
has to he allowed to elapse after changing the pressure in the apparatus hefoie 
making any measurements. It was found that the leak was very little affected b} 
the absorption of the mercury vapour, so that in the later experiments this golddeaf 
tube was abandoned. Some of the experiments in which the gold leaf was used will 
he described in the section on the leak in hydrogen. 
In the earlier experiments very inconsistent results were often obtained. This was 
found to he due to gas, proliably mostly hydrogen, given off by tlie wire when heated. 
A very small quantity of hydrogen has a very large effect on the leak, as v ill be 
shown in Section ( 4 ). When the platinum wire is welded to its electrodes it is 
heated in a blow-pipe flame, and probably absorbs some hydrogen, which is afterwards 
given out wlien the wire is heated in a vacuum. A new wire was generally found to 
give very large leaks, which varied in an irregular manner with the time. Long- 
continued heating in a good vacuum ultimately nearly gets rid of these irregularities, 
but long continued heating also causes the wire to disintegrate, and its suiface 
becomes roughened and covered with a netwoi'k of deep cracks. It was found that 
the irregular liehaviour of the wire could lie entirely got rid of without long continued 
heating by the simple process of boiling it in strong nitric acid, after welding it to its 
electrodes and sealing them into tlie tube. Tlie tube was about half filled with pine 
nitric acid and boiled over a spirit lamp for a few minutes, the acid then poured out, 
and the process repeated two or three times. The tube was then washed out vith 
distilled water. If the platinum loop had been bent by the boiling process, it vas 
straiglitened by a clean platinum wire hook Introduced through the side tube, and 
tlien the tube was sealed on to the pump and gauge, &c. The apparatus was then 
pumped out and filled up with air several times, and then on heating the vire the 
negative leak at once took up an almost steady value. The positive leak was very 
small, and did not fall off witli the time after the first few seconds. The process of 
filling the apparatus with air and pumping out several times with the vire kept hot 
all the time was repeated at frequent intervals to ensure that the apparatus should 
only contain pure dry air. 
