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 β or cathodic rays. The α rays from uranium and thorium, on account of their weak action, have not yet been detected photographically. With active substances like radium and polonium, the α rays readily produce a photographic impression. So far the γ 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 γ 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 α rays, while the photographic action under the same condition is due almost entirely to the β rays.
It is often convenient to know what thickness of matter is sufficient to absorb a specific type of radiation. A thickness of aluminium or mica of ·01 cms. or a sheet of ordinary writing-paper is sufficient to absorb completely all the α rays. With such a screen over the active material, the effects are due only to the β and γ rays, which pass through with a very slight absorption. Most of the β rays are absorbed in 5 mms. of aluminium or 2 mms. of lead. The radiation passing through such screens consists very largely of the γ rays. As a rough working rule, it may be taken that a thickness of matter required to absorb any type of rays is inversely proportional to the density of the substance, i.e. the absorption is proportional to the density. This rule holds approximately for light substances, but, in heavy substances like mercury and lead, the radiations are about twice as readily absorbed as the density rule would lead