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XIII. The Bakerian Lecture. — On the Viscosity or Internal Friction of Air and other 

 Gases. By J. Clerk Maxwell, M.A., F.R.S. 



Received November 23, 1865,— Eead February 8, 1866. 



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 mole- 

 cules of bodies, when in the gaseous state, are less impeded by any complicated mecha- 

 nism 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 time. M. Kohlrausch* has shown 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. 



Professor W. THOMSONf has shown that something corresponding to internal friction 

 takes 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°, its elasticity was permanently diminished and 

 its internal friction increased. 



The viscosity of fluids has been investigated by passing them through capillary tubes J, 



• Ueber die elastische NacbwerkuBg bei der Torsion, Pogg. Ann. cxix. 1863. 

 t Proceedings of the Royal Society, May 18, 1865. 



t Liquids: Poiseuille, Mem. de Savants Strangers, 1846. Gases: Geaham, Philosophical Transactions, 

 1846 and 1849. 



MDCCCLXVI. 2 M 



