40 THE WONDERFUL CENTURY. CHAP. v. 



of radiation termed the X, or Rontgen Rays, from Pro- 

 fessor Rontgen of Wiirzburg, who was the first to in- 

 vestigate their properties and make practical applications 

 of them. These rays are produced by a special form of 

 electrical current sent through a vacuum tube, in or 

 around which is some fluorescent substance, which under 

 the action of the current become intensely luminous. But 

 this luminosity has totally different properties from ordi- 

 nary light, inasmuch as the substances which are opaque 

 or transparent to it are not the same as those to which we 

 usually apply the terms, but often the very contrary. 

 Paper, for instance, is so transparent that the rays will 

 pass through a book of a thousand pages, or through 

 two packs of cards, both of which would be absolutely 

 opaque to the most brilliant ordinary light. Alumin- 

 ium, tin, and glass of the same thickness are all trans- 

 parent, but they keep out a portion of the rays; whereas 

 platinum and lead are quite opaque. To these rays 

 aluminium is two hundred times as transparent as plati- 

 num. Wood, carbon, leather, and slate are much more 

 transparent to the X-Rays than is glass; some kinds of 

 glass being almost opaque, though quite transparent to 

 ordinary light. Flesh and skin are transparent in 

 moderate thicknesses, while bone is opaque. Hence, if 

 the rays are passed through the hand the bones cast a 

 shadow, though an invisible one; and as, most fortu- 

 nately, the rays act upon photographic plates almost like 

 ordinary light, hands or other parts of the body can be 

 photographed by their shadows, and will show the bones 

 by a much darker tint. Hence their use in surgery, to 

 detect the exact position of bullets or other objects em- 

 bedded in the flesh or bone. A needle which pene- 



