and _liquid states_ of substances, and shows the connection between them. On this account, although van der Waals' formula cannot be considered as perfectly general and accurate, yet it is not only very much more exact than _pv_ = _RT_, but it is also more comprehensive, because it applies both to gases and liquids. Further research will naturally give a closer proximity to truth, and will show the connection between composition and the constants (_a_ and _b_); but a great scientific progress is seen in this form of the equation of state. Clausius (in 1880), taking into consideration the variability of _a_, in van der Waals' formula, with the temperature, gave the following equation of condition:-- (_p_ + _a_/(_T_(_v_ + _c_)^2)) (_v_-_b_) = _RT_. Sarrau applied this formula to Amagat's data for hydrogen, and found _a_ = 0·0551, _c_ =-0·00043, _b_ = 0·00089, and therefore calculated its absolute boiling point as -174°, and (_pc_) = 99 atmospheres. But as similar calculations for oxygen (-105°), nitrogen (-124°), and marsh gas (-76°) gave _tc_ higher than it really is, the absolute boiling point of hydrogen must lie below -174°. Although a substance which passes with great difficulty into a liquid state by the action of physico-mechanical forces, hydrogen loses its gaseous state (that is, its elasticity, or the physical energy of its molecules, or their rapid progressive motion) with comparative ease under the influence of chemical attraction,[35] which is not only shown from the fact that hydrogen and oxygen (two permanent gases) form liquid water, but also from many phenomena of the absorption of hydrogen. [35] This and a number of similar cases clearly show how great are the internal chemical forces compared with physical and mechanical forces.