298 The Hon. B. J. Sfrutt [April 24, 



happens is called the critical temperature. Now we know that the 

 liquid mercury is an excellent conductor of electricity. Mercury 

 vapour, however, as obtained by boiling mercury at the ordinary 

 pressure of the air, does not conduct at all, or at least it only possesses 

 the very feeble conducting power conferred on it by the Becquerel 

 rays from the vessel walls, as I explained to you before. And yet, at 

 the critical temperature, it must conduct as much as the liquid at that 

 temperature, for indeed the tw^o, liquid and vapour, are indistinguish- 

 able. 



It is evident, therefore, that at these high temperatures and 

 pressures, some very profound change must occur in the electrical 

 properties of mercury. Either the liquid must be very much less 

 conducting, or else the vapour immeasurably more so, than it would 

 be under ordinary conditions. It might seem an easy matter to put 

 this question to the test of experiment. But the practical difficulties 

 are very great. It is necessary to confine the mercury in a closed 

 vessel. This vessel must be capable of standing an enormous pres- 

 sure ; it must be able to stand a very high temperature without 

 melting, and it must be made of electrically insulating material. 

 These qualities cannot be found in sufficient degree in any known 

 material. But they are most closely approached by vitreous silica, 

 obtained by fusing rock-crystal in the oxy-hydrogen blow-pipe, and 

 working it into tubes by the ingenious methods which were not long 

 ago explained in this Institution by Mr. W. A. Shenstone. 



I have here a tube of quartz, with thick walls. Some mercury 

 has been hermetically sealed up in it. I place it over the flame of 

 the blowpipe, and you see that the mercury, instead of boiling at a 

 moderate temperature, as it would in an open vessel, is heated to 

 full redness. In experimenting j)rivately, I have been able to raise 

 the temperature to a yellow heat. At that point the mercury vapour 

 begins to show a steely blue absorption tint. Soon after this appears, 

 the strongest tubes burst with the pressure of the vapour. 



I will now show you a diagram of the tube used for measuring 

 the electrical conductivity of mercury, and its vapour at a red heat 

 (Fig. 3). The quartz tube took the form of an inverted Y, a b b. It 

 was constricted t(j a very small diameter at the parts d d for a length 

 of about 1 cm. on either side of the joint. The lower part of the 

 limbs b b were of much larger diameter. The tube was filled with 

 mercury up to the level c, the current being led in and out by iron 

 wires e e, which projected some distance up, inside the arms b b. 



The iron wires terminated in brass cups //, carrying appropriate 

 binding screws. These cups were filled with sealing-wax, which 

 cemented them to the quartz tube. This sealing-wax had been 

 sucked up the limbs while hot for a considerable distance, nearly up 

 to the points g g, so as to fill the space between the iron wires e e 

 and the lower parts of the quartz tube. The tops of the iron wires 

 projected out through the sealing-wax, making contact with the 

 mercury. The electrical resistance between the electrodes // lay 



