Professor Knapp, who published in 1858 a short paper (_Natur und Wesen der Gerberei und des Leders_) which is a model of clear explanation and practical experiment. Knapp, however, deals mainly with the changes in the condition of the fibre which are necessary to convert it into leather, and not with their physical causes; and before we can explain the means by which these changes are brought about, we must be acquainted with certain facts and theories about solutions which have become much clearer since he wrote. The particles (molecules) of all substances are drawn together by attractive forces somewhat of the same character as the attraction of gravitation which holds together the solar system, and which is the cause of weight. It is indeed even possible that these forces are identical. Like gravitation, these molecular attractions increase rapidly in intensity as the distance of the attracting bodies diminishes, so that in solids and liquids, where the molecules are near together, they are immensely powerful, while in gases and vapours they are barely perceptible. These attractions are opposed by the motion of heat, which takes the same part in molecular physics which the energy of planetary motion does in the solar system. In solids, the attractive forces hold the molecules rigidly in position, the motion of heat being limited to short vibrations round a fixed point, the effects of which are visible in the expansion caused by rising temperature. If the temperature is increased, most substances become liquid, a condition in which the particles can roll round each other, but are still held together by their mutual attractions, as the sun holds the earth from flying off into space. If the temperature goes on rising, the orbits of the molecules become greater, the liquid expands, and finally molecules fly off at a tangent out of reach of the attractions of the mass of liquid, and are only diverted from their course by colliding with solids or with other flying molecules, from which they rebound. This constitutes the state of vapour or gas.