Page:Elementary Text-book of Physics (Anthony, 1897).djvu/162

148 the gas follows Boyle's law quite closely, becoming, however, more compressible as the pressure increases, until the minimum value of $$pv$$ is reached. It then becomes gradually less compressible, and at high pressures its volume is much greater than that determined by Boyle's law. If the temperature be raised, the agreement with the law is closer, and the pressure at which the minimum value of $$pv$$ occurs is greater. Hydrogen seems to differ from the other gases, only in that the pressures at which the observations upon it were made were probably greater than the one at which its minimum value of $$pv$$ occurs. The volume of the compressed hydrogen is uniformly greater than that required by Boyle's law.

Important modifications are introduced into the behavior of gases under pressure by subjecting them to intense cold. It is then found that all gases, without exception, can be liquefied, and most of them solidified.

The subject is intimately connected with the subject of critical temperature, and will be again discussed under Heat.