THE PROPERTIES OF X-RAYS 29 



requires an electric battery to work it. The 

 discharge strikes a small plate in the centre of the 

 bulb ; which plate is made of some highly refractory 

 metal because it might be raised to a very high 

 temperature by the discharge. In fact cooling 

 devices are often necessary. The X-rays radiate 

 in straight lines from the plate, and they pass 

 through all substances more or less. Materials 

 composed of atoms of small weight interfere 

 comparatively little with the kind of X-rays most 

 naturally produced and used, that is to say, they 

 are relatively transparent to the rays. Such 

 substances are wood, cloth, aluminium, skin and 

 muscle. Heavier atoms absorb the rays more 

 highly, so that they do not pass easily through 

 plates of lead or copper, for example; even the 

 bones which contain certain heavy atoms like 

 calcium interpose some obstruction to the passage 

 of the rays. Transparency to X-rays has nothing 

 to do with transparency to light rays ; some kinds 

 of glass which do not contain very heavy atoms 

 are fairly transparent, in fact the bulbs themselves 

 are made of such glass. But glass containing the 

 heavy atoms of lead makes a highly efficient 

 absorbing screen. Moreover if the rays are allowed 

 to fall on a photographic plate they so affect it 

 that when the plate is developed it shows marks 

 as if ordinary light had been acting on it. If then 

 the hand is laid flat on a photographic plate wrapped 

 up in a light-tight paper envelope, and an X-ray 



