X-RAYS: A SKETCH OF THEIR HISTORY. 
material, become extremely serious. The time of exposure must also be 
increased to an extent which makes the taking of such radiographs both 
tedious and expensive. 
FIG. 8—Radiograph of Steel Casting, showing large flaw. . (Davey General 
: Electric Review, 1915.) 
”, 
FIG. 9.—Radiograph of a block of pure copper (left) side by side with a block 
of ‘‘Boronised ” copper (right). (Ibid.} 
Very convenient methods of chemical analysis can be based either upon 
the existence of characteristic absorption, or upon that of characteristic 
radiation. It is only necessary to photograph the spectrum of the X-rays 
_ produced by. the substance as radiator in the one or as absorber inthe other 
case. The position of a “ bright line” in the spectrum in the one case or 
of a definitely placed: absorption-band in the other indicates unequivocally 
the presence of atoms of a particular species, and that irrespective of the 
presence of other atoms of different species. “ 
A beautiful method of chemical analysis applicable to all crystalline 
substances has been developed by A. W. Hull, of the: General Electric 
Research Laboratory. A narrow beam of approximately monochromatic 
537 
