It is shown that the spectrum of x-rays is altered by transmission through matter. The mean quantum energy is increased by removal of the soft component. It is possible to use the narrow-beam attenuation coefficient as an integral characteristic of the radiation instead of spectra. This coefficient as a function of absorber thickness decreases as the thickness increases, the measured rate of decrease being proportional to the rate of increase in the mean energy calculated from spectra recorded with a scintillation counter. The broad-beam attenuation coefficient increases with the thickness of the absorber because such a beam accumulates secondary radiation more rapidly than it loses primary radiation. Hence the mean energy may decrease as the thickness increases.