108 Dr. A. Waller's Microscopic Observations on Hail. 



the time we examine it. Hail, as we know, is abundantly 

 formed at the elevation of Mont Blanc, and probably much 

 higher. Supposing the air in our observations had been con- 

 fined at that height, its volume would have been about double 

 that which it presented at the time it was liberated, supposing 

 the temperature on both occasions to be the same. The in- 

 fluence of temperature would no doubt operate in a different 

 direction, as hailstones must be considered to take their origin 

 at a temperature below zero ; and when examined, even after 

 their fall, they have been found generally, by M. Pouillet, —3° 

 or — 4° below 0* Cent., whereas when the air escapes, they 

 may be considered exactly at 0' Cent. But admitting the 

 temperature to have been —10° Cent, when they were formed, 

 the dilatation of the air would be about ^\ih of the volume, 

 which is far from compensating for the decreased pressure. 

 By subjecting hailstones while kept several degrees below the 

 freezing-point to a decreased pressure or a vacuum in the air- 

 pump, we might decide whether the air has any communica- 

 tion with the external atmosphere, and with what force it is 

 retained. 



In observation May 10, the air appeared to be fixed within 

 the hailstone, under the form of globules, very distinct in 

 their appearance from the solid spherules. Their diameters 

 varied from the one-fifth of a millimetre to sizes consider- 

 ably smaller. As they disappeared they seemed to form the 

 air-bubbles, which are invariably to be seen escaping. In 

 the other cases I could only detect, under the highest power, 

 minute dark points at most of the size of mm, 005, which ap- 

 peared to be on the surface of the spherules. I can only con- 

 jecture them to consist of air, because after fusion I could 

 perceive nothing like them in the drop of water. If it should 

 result from future observation with the air-pump, as I have 

 suggested, that the particles of ice merely retain air from the 

 same cause as other porous bodies, the transparent condition 

 of ice may enable us to form some idea of the manner in which 

 porous bodies, such as charcoal, act in fixing gaseous sub- 

 stances within them ; and whether the gases are reduced to a 

 state of liquid by their molecular attraction with the solid. 



Solid Globular Particles. 

 In the observations given above, it is found that, with the 

 exception of the first, all the hailstones have been seen under 

 the microscope to be composed entirely of minute spherules 

 of ice agglutinated together; and even the discrepancy in that 

 instance may be accounted for, from my attention having been 

 principally engaged with the numerous air-bubbles that were 



