[From the Philosophical Transactions, Vol. CLVI.] 



XXVII. THE BAKERIAN LECTURE. On the Viscosity or Internal Friction 



of Air and other Gases. 



Received November 23, 1865, Read 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 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 suf- 

 ficient 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 



