Analyses of Books. 
155 
When however the change of temperature affected the supports of any of the mi- 
croscopes, the case was different ; and this shows the importance of securing the mi- 
croscopes in the most unexceptionable manner. Captain Kater suggests that in small 
instruments they should be fixed to a solid plate of metal ; or if attached to radii or 
supports, that these should be covered with flannel, so as to prevent sudden and par- 
tial changes of temperature. 
Another source of error is mentioned by Captain Kater, which is familiar to us, 
and which we have often had occasion to observe. It arises from the spring or re- 
sistance of the parts connected with the motion of the circle on its centre. It was 
found, for instance, that when the tangent screw was moved in opposite directions, a 
difference of 3"was found in the readings. And it was only by a kind of shaking 
motion, the screw being moved backwards and forwards by little jerks, that a proper 
consistency in the readings was established. This is the error no doubt that has 
been attributed, in instruments of reflection, to thebendingof the index — a thing we 
will venture to say impossible. 
We have already noticed that Captain Kater considers the reality of lateral re- 
fraction as having been fully established. “ The angle between the same objects 
would differ under the most favourable circumstances about 5" on different days, 
and perhaps I' d or 2"may be considered as the error which may affect an angle 
front lateral refraction in the ordinary state of the atmosphere.” 
The last source of error treated of is the same as that which occasions what is 
called the variation of “ the run” of the microscopes. Captain Kater proposes, in- 
stead of moving the wire of the microscope, to have the wire itself fixed and the body 
of the microscope moved by the micrometer screw. In this case no error can arise 
from a variation of the distance between the limb of the instrument, and the object 
glass of the microscope. It possesses also this further advantage, that the object 
glass may be changed and the power of the microscope varied at pleasure without 
affecting the scale. " Any error too, arising from the want of strict perpendicularity 
of the plane of the circle to the axis of motion, would be obviated. 
The paper terminates with tables of all the observations both of angles and azi- 
muths ; and a plate is given showing the disposition of the triangles. 
X. On the phenomena of volcanoes. By Sir Humphry Davy, Bart F. R. S. pp. 241 
to 250. 
The grand discovery which has immortalized the name of Davy — the metallic na- 
ture of the bases of the alkalis ; must have suggested to many the great support 
Which these views afford to the geological theory of Hutton. Combined with the 
beautiful results which Sir James Hall has established, we think the fact of the com- 
bustible nature of the bases of the earths and alkalis throws a powerful light on 
what were considered tire obscure or the weak points of that theory. In particular, 
the difficulty of the central fire which in the usual spirit of controversy was so much 
exaggerated has now ceased to be urged as an objection; and when we sec potassium 
burning on the surface of water, we can be at no loss to account for the heat, the ex- 
istence" of which, at some particular period, the geological appearances sufficiently 
proved. But neither this proof, nor the ocular demonstnatiou which the phenomena 
of volcanoes afforded, could convince the devotees of system ; and till it could be 
shown how and whence the heat should originate, they were determined to uphold 
their objections. Neither is the want of air to support such a combustion any longer a 
difficulty, when we know of metals which have such an attraction for oxygen as to 
bike it from water, and with such rapidity, as to become intensely heated from 
the violent chemical action. These objections which, though not vital to the theory, 
(for it was sufficient to show that we had proofs of such intense igneous action hav- 
ing actually occurred, without being obliged to show how it originated) were yet so 
often and so imposingly brought forward, that we must feel it is a great step to have 
completely silenced them. We now find it received as anestablished position, that the 
oxydation of the several metalloids by contact with water is not only sufficient 
to account for the phenomena of volcanoes, but even for the formation of the trap 
rocks ; supposing observation to have established the probability of their igneous 
origin. This great discovery, which we owe to Sir Humphry Davy, throws a beauti- 
ful light on the whole compass of geological phenomena, and lends probability and a 
coherence to what was till then supposed by many to be the mere wanderings of 
imagination. 
“ The metals of the alkalis and those of such of the earths as I had decomposed, 
were found to be highly combustible, and altered by air and water even at the 
visual temperatures of the atmosphere : it was not possible, consequently, that they 
