2 Prof. Magnus on the Propagation of Heat in Gases. 



stitution of the light gases which makes them capable of with- 

 drawing caloric from these surfaces in a manner different to what 

 other gases would do, which doubtless depends on the mobility 

 of their particles." Such a mobility is met with in other cases. 

 I have shown* that hydrogen passes through fine clefts and 

 apertures more easily than atmospheric air; and in fact the 

 different degree of diffusion in gases mainly depends on their 

 greater or less readiness to penetrate into capillary apertures. 

 Along with this difference in the mobility of the particles, which, 

 as follows from experiments on the transpiration of gases through 

 narrow apertures, depends on the specific gravity of the gas, 

 there may also be a different degree of friction within the gas 

 itself. It is, however, difficult to assume that this friction alone 

 occasions the great differences observed on cooling in different 

 gases. I hope to show in the following pages that the conduc- 

 tibility of heat by gases exercises an essential influence on the 

 internal cooling. 



In 1792 Count Kumford published a memoir on heatf, in which 

 lie contended that the propagation of heat in gases and vapours 

 is only produced by the motion of the gaseous particles, and 

 that a communication of heat from particle to particle — a con- 

 duction — does not take place in gases. Subsequently, in the 

 seventh of his ' Essays J/ he has extended his researches to 

 liquids, and has denied that they possess the property of conduct- 

 ing heat. 



This assertion was soon opposed by John Dalton§, while 

 Murray || and Dr. Thomas Thomson^! afterwards endeavoured 

 to refute it by comprehensive investigations. Biot also**, in 

 reporting on the memoir of the latter, observed that Rumford's 

 experiments only justified the conclusion that liquids conduct 

 heat to a very small extent, and not that the property is entirely 

 absent. 



It is only necessary to dip the hand in mercury to be con- 

 vinced that this liquid is a good conductor; for the insupportable 

 cold which the hand experiences, and which is also perceptible 

 when it is laid on the surface, in which case certainly no currents 

 are formed, can only depend on conduction. But if this fluid 

 can conduct heat, may not others do the same, even though in 

 a smaller degree ? 



* Poggendorff's Annalen, vol. x. p. 153. 



t Phil. Trans, for 1/92, part 1. p. 48. 



X Experimental Essays, vol. hi. p. 1. 



§ Memoirs of the Literary and Philosophical Society of Manchester, 

 vol. v. part. 2. p. 3/2. 



|| Nicholson's Journal, vol. i. pp. 165 and 241. Gilbert's Annalen, 

 vol. xiv. p. 158. 



% Nicholson's Journal, vol. iv. p. 529. Gilbert's Annalen, vol. xiv. p. 129. 



** Bulletin des Sciences par la Societe PhilomatiquedeParis,\o\.'ui. , p.'66. 



