42 PRESIDENT’S ADDRESS. 
bones as shadows by means of suitable fluorescent screens, 
and to take photographs of them or of solid foreign sub- 
stances lodged in the body, such as bullets, or coins, which 
have been accidentally swallowed. Such photographs are 
now familiar to most of us; further, they have proved of 
use in surgery, since by their means the positions of foreign 
metallic bodies have been located, and the nature of certain 
injuries to the bones have been rendered more or less visible. 
These X rays are by no means the only invisible rays 
known; there are several other kinds, viz., the ultra violet, 
the infra red, the Hertzian waves, the kathode, the Lenard, 
the Weidemann, the Becquerel, and other rays. All of 
these, except perhaps the infra red and Hertzian, produce 
photographic effects, and the kathode, the Lenard, Weide- 
mann, Becquerel, and X rays pass through sheets of metallic 
aluminium. 
The X rays do not bring about the combination of hydro- 
gen and chlorine, but the ultra violet and Becquerel rays do. 
It appears to be quite clear that the origin of the X rays 
is at the surface of the object upon which the kathode 
rays impinge, 7.e., the metallic reflector or the walls of the 
bulb. 
One of the most recent and valuable accounts of the 
physical properties of the Réntgen rays is that given by 
Sir G. G. Stokes, Bart., F.R.S., before the Manchester 
Literary and Philosophical Society (the Wilde lecture, 2nd 
July, 1897), wherein he suggests that they may consist of 
innumerable solitary transverse waves. Others, however, 
contend that they are waves of but very short length. 
Another use of the X rays to chemists has been shown 
by Messrs. Heycock and Neville. Gold forms a solid alloy 
with sodium ; if slices, about 12 mm., or about 4} inch, thick 
of these opaque substances be examined by the X rays, the 
individual crystals of gold alloy are clearly seen, as ,the 
metallic sodium allows the X rays to pass through; hence 
photographs can be obtained of the crystals of the gold and 
sodium ailoy in black upon a light ground. 
