TRANSACTIONS OF THE SECTIONS. 
55 
It would seem as if the process of polishing insulates the points to 
which the particles of vapour attach themselves, and prevents them 
from running into each other ; but though the vapour condensed on 
the polished surface thus becomes more sensible, it is not increased 
in quantity, as is easily proved by continuing to breathe on the pane 
of a window till streams of water run down on the unpolished surface, 
and on that only; and it is not a little remarkable, if the polish be 
also carried horizontally along the lower part of the pane, to observe 
the streams dammed up where they meet the polished part, and 
drops of water left along that portion of the line. 
The observation of these facts led the author to apply some prac¬ 
tical corrections to the ordinary method of ascertaining the dew¬ 
point : he adopts the direct process of Dr. Dalton, reducing the 
scale of the operation, and substituting metal for glass. A highly 
polished metallic vessel, not more than 4ths of an inch wide and 
14 inch long, is nearly filled with water ; some crushed sal ammo¬ 
niac is introduced; the salt is stirred up and mixed with the water 
by the bulb of a small thermometer, which falls in consequence 
very gradually, and when the dew appears the thermometer is in 
contact with the surface on which it is deposited. If a consider¬ 
able depression of temperature is required, the vessel may be 
cooled down previously to the experiment by a similar process. This 
instrument, from the small quantity and cheapness of the cooling 
material, may be used constantly at little cost, and from the con¬ 
ducting and radiating properties of the vessel, as well as the pre¬ 
cision with which it indicates the first deposition of dew, may pro¬ 
bably be found to be uniform in its results. 
Mr. Moore exhibited a leaden pipe which had served for about 
twenty years as the worm of a still for the distillation of medicated 
waters and spirits; at length it began to leak, and on examina¬ 
tion it was found to be supported at .various points by bars of 
wood crossing it, and to be tied at others with twine. Wherever 
it thus came in contact with either wood or twine, it was deeply 
corroded, and the lead appeared to be converted into a dark powder, 
which, when examined, was found to contain oxide and chloride 
of lead ; at all other points the pipe was perfectly sound. The ap¬ 
pearance of the corroded parts did not admit of the effect being 
attributed to mechanical action. The presence of chloride and 
oxide in the powder, established, he thought, that the corrosion 
was not entirely, if at all, caused by acids formed by the decaying 
organic matter; it appeared to him that it ought rather to be attri¬ 
buted to galvanic action, developed by the contact of the metal, and 
wood, or twine, which cause, acting for such a length of time, might 
be sufficient to accomplish the destruction of the pipe at the points 
of contact. 
Mr. Ettrick, referring most of the unfortunate explosions which 
have happened in collieries where the “ Davy lamp” is used, to the 
