164 ON A PECULIAR FOG IN ICELAND. 



doubt this unforming and reforming is very active at times. 

 Sometimes the unforming is seen clearly in a constant fall 

 of vapour which never goes below a certain level, some- 

 times in rising up a hill to one definite point. A very 

 curious and frequent occurrence is the formation of vapour 

 from a current of air as soon as it touches a peak, and the 

 complete evaporation of all as soon as it leaves the peak a 

 few hundred yards — this going on for hours, a constant 

 forming and reforming. So much is this the case that I 

 thought of assigning it as a reason for the suspension 

 generally of all clouds ; but I do not see it acting on all 

 occasions ; and at any rate the fact of the cloud of carbon 

 gave an analogy which showed that size was not to be for- 

 gotten in the matter. I can imagine that size in the 

 vapour of carbon from chimneys is not the only point, 

 but that the form of the carbon, light and feathery, and so 

 holding air, may diminish the specific gravity ; but, as in the 

 case of vesicles, it can never reduce the gravity to that of 

 the air. Besides, we see the matter illustrated by dust, 

 with equal precision, although, so far as I have observed, 

 not in such extreme cases. We see an example also in 

 sulphuric acid and sulphur-fumes mixed with various sub- 

 stances, arsenic &c, in copper-smoke. We see the same 

 in the smoke from iron-furnaces sending out lime and 

 other substances, which go like clouds to great distances. 

 Indeed the matter is illustrated in nature to extremes, and 

 any one can find out some point for himself. 



Of course we cannot deny the possibility of the air being 

 entangled in the condensed vapour and so assisting; but 

 how enormous must be the proportional amount of air to 

 keep the whole suspended ? Even the hydrogen little bal- 

 loons spoken of fall soon, although that light gas supports 

 them better than common air can do. 



