PECULIAR FOG IN ICELAND. 159 



remain suspended a long time in the air and cause other 

 meteoric results, I think it better to leave them under the 

 head of vapours, and to call them concrete vapour." 



201. " As to the hollow spheres, which I have called 

 vesicular, it appears that conjecture has been in advance of 

 observation ; and their existence has been supposed in order 

 to account for the formation of vapour before knowing that 

 they could be made to fall before the senses. Desaguiliers, 

 in his ' Cours de Physique Experimentale/ torn. ii. lecon x., 

 opposes, by abstract reasoning, the physicists of his time 

 who admitted the existence of vesicles, and endeavours to 

 refute this opinion as a purely gratuitous hypothesis, or at 

 least one which does not depend on direct observation. 

 One may now make these vesicles visible to the least-expe- 

 rienced eyes. 



" This I consider the best way to observe them. Expose 

 to the rays of the sun, or at least to full daylight and 

 where the air is still, a cup of very hot water of a black or 

 dark colour — for example, coffee, or water mixed with ink. 

 From this will arise fumes more or less dense, which will 

 rise for a little and disappear. An observant eye will 

 perceive that this vapour is composed of rounded granules, 

 whitish and detached. But to see them better, we must 

 use a lens of an inch or an inch-and-a-half focus ; and with 

 this the surface of the liquid ought to be observed ; but 

 care must be taken to hold the lens out of the current of 

 the rising vapours, as they impair its transparency. 



"Observing attentively what passes on the surface of 

 the liquid, we see little spherical balls of different sizes, 

 and moving with different rapidities. The freest rise 

 rapidly and vanish ; but the coarser fall back into the cup 

 without mixing with the liquid from which they escaped, 



and roll on the surface like light dust, and so on 



It is sufficient to see them to be convinced that these are 

 hollow spheres." 



