154 
Transactions of the Royal Society of South Africa. 
and hence independent of the temperature, for values of the temperatures 
less than about 15° C, while it decreased with increase of temperature for 
values greater than 15° C, as shown by the dotted curve in Fig. 21. 
Summary. 
1. Water vapour is unstable in the presence of aluminium and mercury 
vapour in a vacuum tube through which the electric discharge has been 
passed. Eeduction of the vapour, probably by mercury-aluminium-couple 
appears to take place. 
Though the experiments made by M. Henry were essentially different 
to those of the writer, the tubes used by M. Henry were fitted with 
aluminium electrodes and a mercury manometer. It seems difficult, there- 
fore, to reconcile the results obtained by the writer with those of M. Henry, 
since it would appear that any vapour admitted would suffer reduction ; 
hydrogen would be liberated, and the pressure of gas in the tube would 
continually increase so long as the vapour remained unabsorbed in 
the tube. 
2. No such reduction takes place on allowing water vapour to stand 
in a tube fitted with platinum electrodes only, and containing no 
aluminium. If the discharge is passed through the vapour, there is a rise 
in pressure, indicating the formation of some gas or gases — a result possibly 
due to partial dissociation of the vapour by the discharge. When the^ 
electrodes have suffered some disintegration, so that there is a deposit of 
platinum-black on the walls of the tube, the pressure of gas formed does 
not rise to a permanent value ; it passes through a maximum after a few 
minutes, and then falls off with time at a diminishing rate. 
3. The passage of the discharge through water vapour under pressures 
of the order of 1 mm. in an electrodeless tube brings about the appearance 
of a brownish-black coloration on the w\alls of the tube — a result probably 
due to chemical action of cathode rays on the glass. The coloration acts 
somewhat similarly to the platinum-black in the previous case. The dis- 
charge again causes a rapid formation of gas. There is a rapid rise in 
pressure to a maximum value, after which the pressure falls off at a 
diminishing rate. The value of this maximum pressure of gas increases 
as the pressure under which water vapour is admitted increases, till this 
quantity attains to a definite critical value. In vapour under this pressure 
the coloration is no longer stable. The passage of the discharge through 
vapour under such a pressure causes a rapid disappearance of the colour, 
and the higher the pressure under which vapour is admitted the more 
rapid is the disappearance. 
Mere exposure to moisture causes no appreciable diminution in the 
intensity of the coloration. 
