;i5<'^ the scope of radiology. 



(/') Photographic and Chemical Effect. 



The chemical action of these rays upon a ])hotographic 

 plate depends principally upon two things — the penetrability of 

 the rays and the current sent through the tube. The more pene- 

 tratinjEj- rays are less efifective than the weaker, softer or less 

 i:»enetratino- rays. When the hardness of the rays remains the 

 same, the photographic effect is augmented by increasing the 

 current through the tube, so much so that it is possible to obtain 

 a " snapshot '" of the moving parts of the body, such as the 

 heart, etc. In this connection it is worthy of note that the emul- 

 sions, of which tlie film is made, play an important part. The 

 more absorljent the emulsion is, the more satisfactory would it 

 be for obtaining skiagrams of bodies. 



The hardness of these rays is classified according to arbi- 

 trary scales of i^enetrability, usually the penetrability relative to 

 thicknesses of aluminium. This enables the radiographer to 

 standardize the radiations ref|uired for the diagnosis for dif- 

 ferent parts of the body. 



Besides the chemical action on a photographic plate — these 

 rays also produce precipitating and other chemical effects, e.g., 

 iodine is precipitated from its solution in chloroform, and 

 ammonium oxalate-mercury bichloride mixture i:)recipitates 

 calomel. Barium, magnesium, and potassium platino-cyanides 

 change colour due to de-hydration. 



These chemical eft'ects enable one to measure the quantity 

 of the radiation, thus pastilles and emulsions on paper or jilates 

 are used to determine the dosage given to a patient. 



(r) Fluorescent Effect. 



Certain comi>ounds of uranium and alkali salts, and also 

 alkali earth metals, emit light when X-rays fall on them. The 

 intensity of tlie fluorescent light varies inversely as the sc|uare 

 of the distance of the .substance from the target of the tube. 

 This effect is ap])lied in screens for localising foreign bodies, 

 thus ob\-iating the necessity of photographing the affected parts. 

 This method, however, leaves no record for future reference. 



(d) Ionising Effect. 



When a gas is ex])osed to the action of these rays it breaks 

 up into electrically charged particles or ions ; that is to say, the 

 gas becomes ionised, and in that condition conducts electricity. 

 The amount of ionisation produced in a given time depends upon 

 the nature of the gas, its pressure, the potential difference across 

 the tube, the inverse scjuare of the distance from the target of 

 the tube, and the penetrability of the rays, but it seems to be 

 independent of the temperature of the gas. This principle is 

 utilised es])ecially bv phvsicists for making quantitive measure- 

 ments. 



Beifore going on to the physiological eff'ect, we shall say a 

 few words about secondary ravs. When X-ravs fall on sub- 



