488 SCIENCE PROGRESS 



per centimetre amounts to about 188 pairs, when reduced to 

 atmospheric pressure. 



In taking the photograph shown in fig. 10 the X-rays were 

 made to traverse the air before instead of after the expansion. 

 The ions liberated along the track of each cathode-ray were 

 thus free to move under the action of the vertical electric force 

 maintained in the cloud chamber, the positive travelling down- 

 wards, the negative upwards. Each trail was thus divided 

 into two portions, one consisting of negative, the other of 

 positive ions, before being converted into visible cloudlets 

 by expansion of the moist air; the ions of each trail have also 

 had time to be considerably scattered by diffusion. 



The representations of X-ray clouds shown thus far have 

 all been from photographs taken with the camera pointed 

 horizontally and so placed that a magnified image was obtained. 

 The remaining photographs were obtained with the camera 

 pointed vertically downwards, the conditions being such that 

 the whole visible contents of a horizontal stratum of the cloud 

 chamber, about 2 cm. in thickness, were photographed just as 

 in the case of the alpha-ray pictures. Very intense illumination 

 is required to make the cathode-ray tracks visible in a picture 

 taken in this way and it is only recently that I have succeeded 

 in photographing them. 



A thin sheet of copper was fixed in the centre of the cloud 

 chamber in the path of a narrow beam of X-rays, which was 

 made to traverse the supersaturated air of the cloud chamber 

 immediately after its expansion. 



The absorption of X-rays by the copper is evident at a 

 glance (fig. 11) from the difference of the density of the clouds 

 condensed on the incident and transmitted beams. 



In passing through the copper the X-rays produce immense 

 numbers of cathode-rays which form dense clouds immediately 

 in front of and behind the copper plate. The clouds are not 

 quite in contact with the copper, the clear space next the plate 

 being due to the air becoming warmed by contact with the 

 copper before the passage of the rays, so that the ions fail to find 

 the supersaturation necessary for their growth into water drops. 



From the researches of Barkla and others we know that 

 when exposed to X-rays the copper plate will emit secondary 

 rays — the homogeneous or characteristic or fluorescent rays of 

 copper. These will in turn cause the air to emit secondary 



