56 
mr. j. c. Mclennan on electrical conductivity in gases 
In order to investigate the point an apparatus similar to that shown in fig. 3 
was adopted. Different thicknesses of aluminium foil were in turn used for the 
window, and the air in the chamber B was kept at a pressure low enough to 
absorb but little of any radiation coming from the window, and yet sufficiently 
high to afford considerable conductivity when ionized. 
With different thicknesses of the foil down to '04 millim., it was found that the 
electrode A did not gain any charge when the tube was excited. Further, if in 
these cases a charge, either positive or negative, was given independently to the 
electrode, this charge was maintained when the discharge passed in the tube, and 
no leak occurred. But when the window was made of foil '008 millim. in thickness, 
the effect obtained was such as that already described in the last paragraph. 
Under these conditions the electrode A, if carrying initially a positive or a negative 
charge, finally assumed a stationary state, in which it carried a definite negative 
charge whose value, as has already been pointed out, depended upon the pressure 
of the air in the chamber B. As, then, no leak from the electrode occurred when 
the aluminium was '04 millim. in thickness, it seems justifiable to conclude that if any 
Bontgen rays were present under these circumstances they were of an extremely 
weak character. If Bontgen rays of even very moderate intensity had entered the 
chamber, a leak would have taken place which could have been observed. In practice 
the aluminium foil used in my experiments was about '008 millim. in thickness, and 
with this foil intense ionization was observed. From the known character of Bontgen 
rays, it was quite impossible for this great ionization to be produced by rays which 
could be absorbed by a layer of aluminium '032 millim.—the difference in thickness 
of the two windows. 
Again, an ordinary focus tube illustrates very well the fact that the Bontgen 
rays produced issue in a large measure from the face of the anticathode, upon 
which the cathode rays fall, while the radiation appearing to come from the opposite 
face is always very weak. The theory now generally accepted is that the Bontgen 
rays are electromagnetic pulses sent through the ether when the moving electrified 
particles which constitute the cathode rays are suddenly stopped. If then the 
Bontgen radiation sent out in the direction of propagation of the cathode rays, when 
these carriers were stopped by foil '04 millim. in thickness, was at most but very 
feeble, it appears highly improbable that a strong radiation of this kind could be 
produced by those carriers that passed through the thinner foil without being 
stopped. 
The conductivity produced in a gas by cathode rays is, moreover, far in excess of 
that excited by even the strongest Bontgen rays. In order to make a direct com¬ 
parison, measurements were taken of the ionizations produced in the same chamber 
by both radiations, and the following illustration gives an indication of their 
respective efficiencies. By using the apparatus shown in fig. 1, it was found that, 
under the action of cathode rays with a saturating intensity of field, a capacity of 
