By this time, it was known that the main process by which mesotrons lose energy is ionisation energy loss, which is described by the Bethe formula, and is proportional to the mass per unit area of the layer of material traversed. If this were the only process, the intensity of the hard component passing through a layer of solid material would decrease by the same amount as in an equivalent layer of air. Rossi and his collaborators found that the decrease was significantly larger in the atmosphere than in a corresponding layer of solid carbon. Because the distance traversed in air was much larger than that in carbon, they interpreted this result as evidence for decay of the mesotron, and taking into account the effect of relativistic time dilation, estimated its mean life at rest as roughly 2 microseconds.