77 VISCOSITY OF GASES 183 



I could, however, deduce in all strictness the proof that the 

 viscosity of gases is really independent of the pressure from 

 experiments carried out by T. Graham 1 ; these related to 

 a phenomenon which he called transpiration, viz. the slow 

 flow through a long, fine capillary tube, or through a bundle 

 of such tubes, or finally through the fine pores of a porous 

 substance. The theory of these experiments, which are 

 obviously similar to those carried out by Poiseuille 2 and 

 Hagen 3 on the behaviour of water and other liquids in 

 passing through a capillary tube, may be easily developed 

 by the analogy of the theory of these hydraulic experiments 

 worked out by Stokes 4 and others, now that the inde- 

 pendence of the viscosity of air on pressure has been 

 rendered at least very probable ; for, in spite of the variation 

 of pressure along the tube through which the flow takes 

 place, the friction exerted may be assumed to be the same 

 everywhere. This theory, which I published in a second 

 memoir, 5 led in the case of gases to a law of speed of flow 

 which corresponds fully with that found by Poiseuille 

 for liquids, and, like the latter, can be used to calculate the 

 numerical value of the coefficient of viscosity from the 

 measured amount of the gas transpired. Since the deduction 

 of this law rests on the hypothesis of a friction which is not 

 variable with the pressure, an experimental confirmation of 

 it is also a proof of the correctness of the hypothesis. This 

 confirmation was actually given by the experiments, as the 

 following numbers show, provided that the tubes used in 

 the experiments were sufficiently long. 



The following table 6 contains the calculation of the 

 results given by Graham in his tables XV. and XVI. 

 Here t denotes the time in seconds required for the flow of 



1 Phil. Trans. 1846, cxxxvi. p. 573 ; 1849, cxxxix. p. 349. 



2 Soc. Philomath. 1838, p. 77 ; Comptes rendus, 1840, xi. pp 961, 1041 ; 

 1841, xii. p. 112 ; Ann. Chim. [3] 1843, vii. p. 50 ; 1847, xxi. 76 ; M&n. de 

 Savants Strangers, 1846, ix. p. 433. 



3 Pogg. Ann. 1839, xlvi. p. 423 ; Abh. d. Berl. AJcad. 1854, p. 17. 



4 Cambridge Phil. Trans. 1851-6, ix. pt. 2, p. 8. 



5 Pogg. Ann. 1866, cxxvii. pp. 253, 353. 



6 Ibid. 1866, cxxvii. pp. 279, 365. In this memoir I have given a slightly 

 different meaning to these numbers ; this, however, has no influence on the 

 meaning now assigned. 



