Mr. J. C. Maxwell on the Dynamical Theory of Gases. 129 



On the Piz Surley and the Nevado de Toluca there fell frozen 

 s!eet,or snowlike rice; and on the Jungfrau the snow fell hissinglike 

 hail, which would seem to point to sleet rather than snow having 

 fallen. No doubt v,e ought to take into consideration the higher 

 temperature of the valley, where hail about to fall turns to rain ; 

 but we think that the cases we have just pointed out, of rain in the 

 valleys and frozen sleet on the isolated mountain, show that the 

 condensation is not one taking place over the whole sky accord- 

 ing to the same law. Thus, in particular, during the observa- 

 tion of the 22nd of June, 1865, I saw on the horizon all the 

 needles, though far distant from each other, enveloped in dirty 

 sleet, which lasted for a very long time, whilst in the rest of the 

 sky all condensation had ceased, and in the valley there fell but 

 one heavy and short shower. 



In the other cases the phenomenon which took place around 

 the tops of the rocks was very different from that occurring in 

 the valleys. Around the pyramids there were columns of frozen 

 sleet, fine and very thinly scattered ; in the valley, rain heavy and 

 abundant, such as the thinly scattered sleet could not have pro- 

 duced if changed to rain. Around the pyramids by which the 

 electricity flowed off, the condensation had then the special cha- 

 racter of being less abundant, dust-like (fine frozen sleet), and was 

 also more persistent than in the rest of the sky. 



The electrical phenomenon which has here been described, and 

 which we may call the humming of mountains [bourdonnement 

 des montagnes), appears not to be uncommon though not very 

 frequent at high altitudes. Among the guides and hunters whom 

 I have questioned on the subject, some have never observed it, 

 others have heard it some once or twice in their lives; but it 

 is fair to add that it takes place on just those days when the 

 threatening appearance of the sky keeps travellers from the 

 highest peaks. 



XV. On the Dynamical Theory of Gases. 

 By J. Clerk Maxwell, F.R.S.L. § E.* 



THEORIES of the constitution of bodies suppose them 

 either to be continuous and homogeneous, or to be com- 

 posed of a finite number of distinct particles or molecules. 



In certain applications of mathematics to physical questions, 

 it is convenient to suppose bodies homogeneous in order to make 

 the quantity of matter in each differential element a function of 

 the coordinates, but I am not aware that any theory of this kind 

 has been proposed to account for the different properties of 



* From the Philosophical Transactions for 1867, Part I., having been 

 read May 1, 1866. 



Phil. May. S. 4. Vol. 35. No. 235. Feb. 1868, K 



