NATURAL PHILOSOPHY SWANN" 243 



every hundred thousand million of the molecules originally present 

 remains. This improvement in the technique of producing high 

 vacua, rendered necessary for investigation in pure science, has ren- 

 dered possible the electric lamps which we use to-day. It has ren- 

 dered possible the modern X-ray tube — an instrument not only 

 infinitely more reliable than the weak and capricious tubes of 20 

 years ago, but controllable in intensity to amounts twenty times as 

 great as those formerly attainable. It is only through the aid of 

 modern vacuum technique that the modern broadcasting station has 

 been rendered possible, that the radio amplifying tube has become a 

 reality, and that we can have wireless transmission of signals, speech, 

 and photographs. 



If, 20 years ago we had wished to give an example of a type of 

 research which was least likely to have an utilitarian value, we could 

 hardly have chosen a more fitting example than the inve,stigations 

 which Prof. O. W. Richardson had been carrying on in England, and 

 later at Princeton, on the emission of electrons from heated wires; 

 yet it is to these investigations, combined with the power to produce 

 high vacua, that we owe the modern radio tube, the X-ray tube, and 

 a, variety of appliances used in the general fields of radio transmission 

 and X-ray technique. 



It has long been known that light when falling upon the surface of 

 certain substances possesses the power to eject electrons from them, 

 and the study of this phenomenon has been one of primary importance 

 in relation to our knowledge of atomic processes. But it, too, has 

 had its practical application, for it is this phenomenon which has 

 rendered possible the wireless transmission of picture^, and a variety 

 of other things hardly less important even though less spectacular. 



Many years ago, Rowland, of Johns Hopkins University, showed 

 us how to rule very fine closely spaced parallel lines on speculum 

 metal, and use them to analyze light into its component colors. That 

 which the grating does to the light can be predicted if we know the 

 spacing of the lines; and, conversely, if we had knowm beforehand 

 the nature of the light, we could have gained information regarding 

 the spacing of the lines. Since X rays are of the same general 

 nature as light, but of much shorter wave length, it became a matter 

 of interest to inquire how far such methods could be applied to them. 

 It ,soon appeared, however, that for the successful pursuit of this 

 problem, it would be necessary to rule lines whose distance apart was 

 of the order of one hundred-millionth of a centimeter. We can not, 

 of course, make such ruling, but nature ha,s provided us with some- 

 thing very like them in the regularly spaced atoms which constitute 

 a crystal of rock salt, for example. About 15 years ago, Laue found 

 that he could make a crystal act in relation to X rays in vcrj' much 

 the same way as Rowland's gratings acted toward light. The matter 



