The second section discusses the properties of media, consisting of two or more gases, and arrives at the result that “in mixed media the mean square molecular velocity is inversely proportional to the specific weights of the molecules.” This was the great law rediscovered by Maxwell fifteen years later. With modern notation it may be put thus:--If _m_₁, _m_₂ be the masses of each molecule of two different sets of molecules mixed together, then, when a steady state has been reached, since the temperature is the same throughout, _m_₁ _v_₁² is equal to _m_₂ _v_₂². The average kinetic energy of each molecule is the same. From this Avogadros’ law follows at once--for if _p_₁, _p_₂ be the pressures, N₁, N₂ the numbers of molecules per unit volume-- _p_₁ = ⅓ N₁ _m_₁ _v_₁², _p_₂ = ⅓ N₂ _m_₂ _v_₂². Hence, if _p_₁, is equal to _p_₂, since _m_₁ _v_₁² is equal to _m_₂ _v_₂², we must have N₁ equal to N₂, or the number of molecules in equal volumes of two gases at the same pressure and temperature is the same. The proof of this proposition given by Waterston is not satisfactory. On this point, however, we shall have more to say. The third section of the paper deals with adiabatic expansion, and in it there is an error in calculation which prevented correct results from being attained. At the meeting of the British Association at Ipswich, in 1851, a paper by J. J. Waterston of Bombay, on “The General Theory of Gases,” was read. The following is an extract from the Proceedings:-- The author “conceives that the atoms of a gas, being perfectly elastic, are in continual motion in all directions, being constrained within a limited space by their collisions with each other, and with the particles of surrounding bodies. “The vis viva of these motions in a given portion of a gas constitutes the quantity of heat contained in it. “He shows that the result of this state of motion must be to give the gas an elasticity proportional to the mean square of the velocity of the molecular motions, and to the total mass of the atoms contained in unity of bulk” (unit of volume)--that is to say, to the density of the medium. “The elasticity in a given gas is the measure of temperature. Equilibrium of pressure and heat between two gases takes place when the number of atoms in unit of volume is equal and the vis viva of each atom equal. Temperature, therefore, in all gases is proportional to the mass of one atom multiplied by the mean square of the velocity of the molecular motions, being measured from an absolute zero 491° below the zero of Fahrenheit’s thermometer.”