The specific heat of any particular substance is, like its specific gravity, constant; but like the specific gravity it depends upon the temperature, as in the case of water, to which reference has already been made. To obtain a deeper insight into the behaviour of solid and liquid bodies, instead of using the gram as the unit of mass in calculating specific heats, the gram-atom, or gram-molecule, has again been used. For diatomic gases the specific heats are greater by an amount required by the assumption that they possess two additional degrees of freedom. In 1875, Weber discovered that the low atomic heats of carbon, boron and silicon, increased with temperature so that their atomic heats approached those of other substances. The lower the temperature the more frequently will it happen that the energy given up by a colliding particle is less than the quantum of energy which the solid body can receive.