242 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 192 8 



things to which our interest was confined, we should limit greatly the 

 possibilities which that city opened to us. If this is true of a rela- 

 tively simple structure like a city, how much more is it true of that 

 beautiful framework of science whose parts are so clearly interwoven 

 that it is almost impossible to touch one of them without producing 

 response in all the others. While, therefore, the man pf science must 

 pursue knowledge for its own sake, it is a remarkable fact that prac- 

 tically all of those achievements in the physics of the past 20 years 

 which might be classed as utilitarian, have arisen directly from, 

 or in relation to investigations pursued with no utilitarian motive 

 directly in view. X rays revealed themselves first in the light of 

 their importance in surgery. The study of their properties shed a 

 new light upon the structure of the atom and this light was reflected 

 back with enhanced intensity to clarify the properties of the X rays 

 themselves. The immediate application to photographic surgery was 

 obvious, but that field which is concerned with the effects of the rays 

 upon the body tissue, upon the cure of cancer and the like, was not 

 so evident. Bound up as it is with the properties of the rays in rela- 

 tion to their passage through matter, with their absorption in the 

 tissues, and the extent of the molecular disruption which they pro- 

 duce, it must draw for its development upon the more fine-grained 

 aspects of the study of X ra^^s which the physicist has made in the 

 field of his own interests. 



The study of radio-activity has taught us that in the spontaneous 

 disintegration of the atoms which accompanies this process, power- 

 ful radiations are emitted. First we have the alpha particle, a 

 positively charged atom of helium, with a velocity of 12,000 miles 

 per second. Then we have electrons traveling with a velocity 10 

 times as large, and finally we have a very hard type of X ray known 

 as gamma rays. These rays possess the power to disrupt molecules 

 through which they pass and it is this power which gives them, in 

 common with X rays, such great value in medicine. The surgeon's 

 knife can dissect the tissues and remove the larger malformations of 

 growth, but the X rays, the rays from radium, and those of ultra- 

 violet light can dissect the malformation on things ten thousand 

 times smaller than the smallest things which our microscope can 

 reveal. 



The detailed investigations of phenomena pertaining to the pas- 

 sage of electricity through gases — phenomena whose study led to the 

 discovery of the electron, necessitated an improvement in our methods 

 of producing high vacua. The pumps of to-day can accomplish in 

 15 seconds what would have taken a couple of hours 25 years ago, and 

 the vacua attainable are ten thousand or more times better than they 

 were in those days. We can now reduce the pressure in our appa- 

 ratus by means of modern pumps to such an extent that only one in 



