635. By the viscosity or internal friction of a gas, is meant the resistance it offers to the gliding of one portion over another. In a paper read before the British Association in 1859, Maxwell† gives the following explanation of the internal friction of gases:— “Particles having the mean velocity of translation belonging to one layer of the gas, pass out of it into another layer having a different velocity of translation, and by striking against the particles of the second layer exert upon it a tangential force which constitutes the internal friction of the gas. The whole friction between two portions of gas separated by a plane surface, depends upon the total action between all the layers on the one side of that surface upon all the layers on the other side.”