The X rays falling on matter are thus subjected to transformations which may be compared to those which the phenomena of luminescence produce on the ultra-violet rays.
M.Benoist has founded on the transparency of matter to the rays a sure and practical method of allowing them to be distinguished, and has thus been enabled to define a specific character analogous to the colour of the rays of light.

It is probable also that the different rays do not transport individually the same quantity of energy.

We have not yet obtained on this point precise results, but it is roughly known, since the experiments of MM.

Rutherford and M'Clung, what quantity of energy corresponds to a pencil of X rays.

These physicists have found that this quantity would be, on an average, five hundred times larger than that brought by an analogous pencil of solar light to the surface of the earth.