374 Notices of Books. [July, 
vours to demonstrate their presence in vapour experimentally, 
and to render them visible. 
The attention of Dr. R. A. Smith, from whose pamphlet the 
above sketch of opinion on this question is substantially taken, 
was drawn to this subject by a remarkable fog which he observed 
one afternoon in July, at Reikjavik, Iceland. ‘The peculiarity of 
the phenomenon consisted in ‘‘a larger size of particles than I 
have ever seen, and that the flatness with which it fell on the 
ground, and the lumbering mode of rolling, distinguished it 
from all fogs which I have seen.” ‘The people of the town, as 
well as Dr. Smith and his party, took it at first for dust, or 
smoke conveying unusually large particles. The wind had 
nothing to do with the matter, as the fog arose simultaneously 
from the sea and from a small lake ‘behind the town, and con- 
verged in the streets. The particles were found on examination 
to be at least ten times larger than those described by Saussure. 
They seemed, moreover, quite solid,—if the expression may be 
used for a liquid body,—without any hollow centre. This obser- 
vation accordingly led Dr.- Smith to examine the evidence 
offered by Saussure and others in favour of the vesicular theory. 
But he found nothing in the least conclusive. The notion of 
hollow particles ‘‘is a vague inference from certain of their 
qualities, and their supposed analogy to soap-bubbles.” The 
Author contends that the hypothesis of hollow particles, besides 
being unsupported by facts, is unnecessary, as fine but admit- 
tedly solid particles can remain for a very considerable time 
suspended in the air. As an instance of this kind, he remarks — 
«Times out of number I have observed, on calm summer 
evenings, a cloud of smoke from a steam-boat funnel lying for 
miles in length, at a height very little different from that of the 
funnel out of which it issued. . . . In these cases have we any- 
thing to look to but the size of the particles? They are so 
small that their resistance to the atmosphere is diminished to 
the utmost, as the resistance of the air is increased so much in 
proportion to the weight that they cannot fall rapidly.” 
The subject is one which demands more attention from phy- 
sicists and meteorologists than it has hitherto received. 
Principles of Metal Mining. By J. H. Cottins, F.G.S. London 
_and Glasgow: W. Collins, Sons, and Co. 
Tuis little work gives a general and intelligible view of mining 
operations. One of its greatest merits is that the technical 
language of the Cornish miners—often a source of no little 
mistification to geologists, mineralogists, and chemists—is fully 
explained. The Author insists, very justly, upon the importance 
of scientific training to practical miners. Referring to instances 
ee 
