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THE gaseous form of matter is distinguished by the great simplification which occurs in the expression of the properties of matter when it passes into that state from the solid or liquid form. The simplicity of the relations between density, pressure, and temperature, and between the volume and the number of molecules, seems to indicate that the molecules of bodies, when in the gaseous state, are less impeded by any complicated mechanism than when they subside into the liquid or solid states. The investigation of other properties of matter is therefore likely to be more simple if we begin our research with matter in the form of a gas.

The viscosity of a body is the resistance which it offers to a continuous change of form, depending on the rate at which that change is effected.

All bodies are capable of having their form altered by the action of sufficient forces during a sufficient tune. M. Kohlrausch has shewn that torsion applied to glass fibres produces a permanent set which increases with the time of action of the force, and that when the force of torsion is removed the fibre slowly untwists, so as to do away with part of the set it had acquired. Softer solids exhibit the phenomena of plasticity in a greater degree; but the investigation of the relations between the forces and their effects is extremely difficult, as in most cases the state of the solid depends not only on the forces actually impressed on it, but on all the strains to which it has been subjected during its previous existence.

"Ueber die elastische Nachwerkung bei der Torsion," Pogg. Ann. cxix. 1863. VOL. II. 1



OK THE VISCOSITY OB INTERNAL FRICTION

Professor W. Thomson • has shewn that something corresponding to internal friction take place in the torsional vibrations of wires, but that it is much increased if the wire has been previously subjected to large vibrations. I have also found that, after heating a steel wire to a temperature below 120°, ita elasticity was permanently diminished and its internal friction increased.

The viscosity of fluids has been investigated by passing them through capillary tubest, by swinging pendulums in them, and by the torsional vibrations of an immersed disk§, and of a sphere filled with the fluid ||.

The method of transpiration through tubes is very convenient, especially for comparative measurements, and in the hands of Graham and Poiseuille it has given good results, but the measurement of the diameter of the tube is difficult, and on account of the smallness of the bore we cannot be certain that the action between the molecules of the gas and those of the substance of the tubes does not affect the result. The pendulum method is capable of great iiccuracy, and I believe that experiments are in progress by which its merits us a means of determining the properties of the resisting medium will be tested. The method of swinging a disk in the fluid is simple and direct. The chief difficulty is the determination of the motion of the fluid near the edge of the disk, which introduces very serious mathematical difficulties into the calculation of the result. The method with the sphere is free from the mathematical difficulty, but the weight of a properly constructed spherical shell makes it unsuitable for experiments on gases. ...

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