94 NATURE OF THE RADIATIONS [CH. 
plate, and in causing phosphorescent or fluorescent effects in certain 
substances. In each of these methods the fraction of the rays which 
is absorbed and transformed into another form of energy is different 
for each type of ray. Even when one specific kind of ray is under 
observation, comparative measurements are rendered difficult by 
the complexity of that type of rays. For example, the 8 rays from 
radium consist of negatively charged particles. projected with a 
wide range of velocity, and, in consequence, they are absorbed 
in different amounts in passing through a definite thickness of 
matter. In each case, only a fraction of the energy absorbed 
is transformed into the particular type of energy, whether ionic, 
chemical, or luminous, which serves as a means of measurement. 
The rays which are the most active electrically are the least 
active photographically. Under ordinary conditions most of the 
photographic action of uranium, thorium, and radium, is due to the 
8 or cathodic rays. The @ rays from uranium and thorium, on 
account of their weak action, have not yet been detected photo- 
graphically. With active substances like radium and polonium, 
the a rays readily produce a photographic impression. So far the 
y rays have been detected photographically from radium only. 
That no photographic action of these rays has yet been established 
for uranium and thorium is probably due merely to the fact that 
the effect sought for is very small, and during exposures for long 
intervals it is very difficult to avoid fogging of the plates owing to 
other causes. Considering the similarity of the radiations in other 
respects, there can be little doubt that the y rays do produce some 
photographic action, though it is too small to observe with certainty. 
These differences in the photographic and ionizing properties 
of the radiations must always be taken into account in comparing 
results obtained by the two methods. The apparent contradiction 
of results obtained by different observers using these two methods 
is found to be due to their differences in relative photographic 
and ionizing action. For example, with the unscreened active 
material, the ionization observed by the electrical method is due 
almost entirely to a rays, while the photographic action under the 
same condition is due almost entirely to the @ rays. 
It is often convenient to know what thickness of matter is 
sufficient to absorb a specific type of radiation. A thickness of 
