In 1884 his book Études de Dynamique chimique (Studies in dynamic chemistry) appeared, in which he entered for the first time the field of physical chemistry. Of great importance was his development of the general thermodynamic relationship between the heat of conversion and the displacement of the equilibrium as a result of temperature variation. At constant volume, the equilibrium in a system will tend to shift in such a direction as to oppose the temperature change which is imposed upon the system. Thus, lowering the temperature results in heat development while increasing the temperature results in heat absorption. This principle of mobile equilibrium was subsequently (1885) put in a general form by Le Chatelier, who extended the principle to include compensation, by change of volume, for imposed pressure changes - it is now known as the van 't Hoff-Le Chatelier principle.