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TRANSACTIONS OF THE SECTIONS. il 
almost magical. When the negative is removed, we see only a blank sheet of 
white paper; and our surprise is very great when, upon lifting this sheet, we dis- 
_ cover beneath it a perfect picture, which seems as it were to have passed through 
the opake and impervious screen. Sir D. Brewster exhibited specimens of portraits 
produced in this manner, and also specimens produced by the transmission of light 
through two perfectly coincident negatives of different degrees of strength; together 
with specimens of positives produced by placing the positive paper between two 
perfectly coincident negatives, and acted upon by light incident on both sides of the 
picture. Sir D. Brewster mentioned some unexpected theoretical results which these 
experiments indicated, but which required further investigation. 
On the Elementary Laws of Statical Electricity. 
By Wii.1aM Tuomson, B.A., St. Peter's College. 
The author, after noticing the labours of Coulomb, Poisson, and Green, on this 
subject, observes that of late years some eminent experimentalists, and especially 
Harris and Faraday, have begun to doubt, to a certain extent, the truth of Cou- 
lomb’s laws, and have entered upon the investigation of various phenomena which 
appeared to be incompatible with them. The principal subject of this paper is an 
attempt to show that almost all the results adduced in their memoirs, which refer 
to electricity in equilibrium, are necessary consequences of the mathematical theory, 
and that none are at variance with it. 
In the first part of the paper a number of laws of a very simple nature, arrived 
at by Harris, are shown to be approximate results of Coulomb’s theory. There is, 
however, one part of Mr. Harris’s investigations, that which refers to the “ striking 
distance” or to the insulating power of the atmosphere, which, though it does not 
bear directly upon the laws of statical electricity, is yet of great importance in enabling 
us to fix upon an absolute standard of electrical density or intensity. The result 
which Mr. Harris arrives at is, that the intensity necessary to produce a spark de- 
pends solely upon the density of the air, being otherwise independent of the pressure 
and temperature. 
In the second part of the paper Faraday’s researches on electro-statical induction 
are considered, and it is attempted to show that the theory there developed is one of 
two elementary methods of viewing the phenomena of statical electricity, or in fact 
generally, of attraction varying inversely as the square of the distance. Either of 
these views, one of which has presented itself to Coulomb, and the other to Faraday, 
may be made the foundation of the present mathematical theory, and therefore, as 
far as this is concerned, they may be adopted indifferently. It must be admitted, 
however, that for simplicity of conception the elementary laws of Coulomb have 
great advantage, and from them, by very simple analysis, given first by Green, we 
arrive at the elementary laws of Faraday, as theorems. 
Faraday’s memoir also contains the account of an investigation which brings to 
light a very remarkable electrical action, which he terms that of dielectrics, hitherto 
entirely unknown (if we except the observation of Nicholson, that the dissimulating 
power of a Leyden phial depends on the kind of glass of which it is made, as well as 
on the thickness). In the present paper a short account of the results of these re- 
searches of Faraday is given, and their relation with the general theory explained. 
The laws of the dielectric action have not yet been fully determined by experiment, 
but it seems probable that it may be perfectly assimilated to that of soft iron when 
under the influence of magnetic bodies. An extensive and rigorous series of expe- 
riments and measurements would however be required to establish this or any other 
hypothesis on the subject, but still the idea might be adopted to indicate the na- 
ture of the experiments from which it would receive its most decided test. 
There are, besides, some rerharkable questions relative to the physical state of 
dielectrics, which present themselves as objects for experimental inquiry. Thus it 
may be conceived that a dielectric in motion might present properties analogous to 
those discovered by Arago in magnetism, and exhibited in his experiment of the re- 
volving disc. As however a very distinct element, that of electrical currents, enters 
in the latter case in a way which could probably have no analogy in the former, it 
could hardly be expected that any remarkable agreement of the phenomena pre 
sented by the bodies in motion should be found to exist. 
a 
