THE SCOPE OF RADIOLOGY. 357 



disease-bearitio- l)acteria, yet few of us care to enquire how 

 Nature protects us in this case. The so'lar heat dries the pools, 

 unfortunately heat also encourag^es the growth of bacteria. At 

 the same time, the blue rays come to our aid ; they kill off the 

 bacteria in a short time, and also penetrate to a greater depth 

 than the red (heat) rays. 



In experiments with actinic rays of mercury lamps it was 

 found that the dangerous bacteria of cholera, typhoid, and 

 dysentery were killed in less t/lian 20 seconds, whereas the harm- 

 less yeast bacteria lived after 300 seconds. 



From what has been said, it will readily be seen that ex- 

 posed pools are less dangerous than those hidden in sheltered 

 places. The latter receive heat from the earth by conduction, 

 and are at the same time screened from the actinic rays. 



This bactericidal action of the actinic rays is applied in 

 modern waterworks to sterilize the water. 



Actinic rays produce 'hydrogen peroxide — a good oxidizer — 

 when they pass through water, but it is not the effect of this 

 action which is princii^ally responsible for this sterilization. 



Thus far we have considered radiations whose wave-lengths 

 lie between the values 0.0008 millimetre and o.oooi millimetre. 

 Let us now consider A'-rays, which lie far beyond this region 

 with o.ooooooooi milimetre. A''-rays are characterized by the 

 following general jjroperties : — 



(a) Penetrating effect ; 



( b ) Photographic and chemical effect ; 



(c) Fluorescent effect ; 



(d) Ionizing effect ; 



(c) Physiological effect; and 

 (/) Optical properties. 



These pro])erties will be better understood when we consider 

 briefly the origin of A^-rays. 



Under certain conditions A-rays are produced as the result 

 of the bombardment of solids by corpuscles moving at a very 

 high speed. The energy which these corpuscles impart to the 

 solid sets the electrons vibrating in a definilte manner, con- 

 sequently the character of the radiation will be determined by 

 the constitution of the target and the energy of the bombarding 

 corpuscle, which may cause some or all of the C(^rpuscles of the 

 solid to give off" their characteristic radiations. 



(a) Penetrating Effect. 



Substances which are opaque to ordinary light are more or 

 less transparent to these rays. Targets of high atomic weight 

 generally produce rays which are more penetrating, ceteris pan- 

 bus, than targets of lower atomic weight, and dense substances 

 are generally less transparent than substances of low density. 

 This readily explains that it is easy to obtain a photograph of 

 the bones and the opacjue parts of a body. 



