14 



Mr. Maxwell on the Viscosity of Air, ^c. [Feb. 8. 



February 8, 1866. 

 Lieut. -General SABINE, President^ in the Chair. 



The Bakerian Lecture was delivered by James Clerk Maxwell, 

 M.A., E.R.S., " On the Viscosity or Internal Friction of Air and 

 other Gases.^' The following is an abstract. 



All bodies which are capable of having their form indefinitely altered, 

 and which resist the change of form with a force depending on the rate of 

 deformation, may be called Viscous Bodies. Taking tar or treacle as an 

 instance in which both the change of form and the resistance opposed to 

 it are easily observed, we may pass in one direction through the series of 

 soft solids up to the materials commonly supposed to be most unyielding, 

 such as glass and steel, and in the other direction through the series of 

 liquids of various degrees of mobility to the gases, of which oxygen is the 

 most viscous, and hydrogen the least. 



The viscosity of elastic solids has been investigated by M. F. Kohl- 

 rausch* and Professor W. Thomson -f ; that of gases by Professor Stokes;}:, 

 M. O. E. Meyer §, and Mr. Graham |1. 



The author has investigated the laws of viscosity in air by causing three 

 horizontal glass disks, 10*56 inches diameter, to perform rotatory oscilla- 

 tions about a vertical axis by means of the elasticity of a steel suspension 

 wire about 4 feet long. The period of a complete oscillation was 72 

 seconds, and the maximum velocity of the edge of the disks was about 



inch per second. 



The three disks were placed at known intervals on the vertical axis, and 

 four larger fixed disks were so adjusted above and below them and in the 

 intervals between them, that strata of air of known thickness were inter- 

 cepted between the surfaces of the moving disks and the fixed disks. 

 During the oscillations of the moveable disks, the viscosity of the air in 

 these six strata caused a gradual diminution of the amplitude of oscilla- 

 tion, which was measured by means of the reflexion of a circular scale in a 

 mirror attached to the axis. 



The whole apparatus was enclosed in an air-tight case, so that the air 

 might be exhausted or exchanged for another gas, or heated by a current 

 of steam round the receiver. The observed diminution in the arc of oscil- 

 lation is in part due to the viscosity of the suspending wire. To eliminate 

 the effect of the wire from that of the air, the arrangement of the disks 

 was altered, and the three disks, placed in contact, were made to oscillate 

 midway between two fixed glass disks, at distances sometimes of 1 inch, 

 and sometimes of '5 inch. 



* Pogg. Ann. cxix. (1863). f Proceedings of the Royal Society, May 18, 1865. 



I Cambridge Philosophical Transactions, 1850. § Pogg. Ann. cxiii. (1861). 



II Phil. Trans. 1846 & 1849. 



