OF CERTAIN ELECTRICAL SUBSTANCES. 
171 
until I tried a small bowl of the best flint glass that I could succeed in obtaining 
anything like a comparative experiment. This bowl, however, which fortunately was 
of the same thickness, or very nearly so, as the other substances tried, charged very 
freely when six inches of coating were given to it, as in the other cases, and evinced a 
high inductive capacity. 
15. With respect to fluid dielectric bodies, although I have thought it worth while 
to advert briefly to the method I employed in subjecting them to experiment, yet I 
am obliged to admit that it was attended by no positive result whatever. I found 
all the fluid bodies I examined, viz. oil of turpentine, common oil, naphtha, &c., quite 
incapable of assuming the charged state, or at least if they did so, it became instantly 
destroyed. I could not hence arrive at any conclusion relative to their capacities 
for sustaining electrical induction, and in this failure I am not alone. Faraday 
found, not only these, but a great many solid bodies quite unfit for experiment on ac- 
count of their incapacity to sustain a sufficient charge (1279.) (1280.). That this 
incapacity is not dependent on the fluid condition of the body is quite evident, since 
the thick plate of glass just mentioned equally failed in its power of receiving a 
charge. I am however not without hopes, that by varying the temperature of these 
substances, or by another form of experiment, or otherwise by a more complete pre- 
paration of them, their inductive capacities, however low, may be relatively disco- 
vered. 
16. I have now merely to offer, in conclusion, a few observations on the experimental 
processes which have been employed in the above investigation. First, it is essential 
that the substances to be examined should have perfect solidity, and be well and 
evenly cast, so as not to present any small fissures or cracks. The coatings should 
be closely attached to their surfaces by a little stout paste, and well rubbed over, in 
order to completely exclude the particles of air, which otherwise are liable to detach 
the coating from the surface, and vitiate the experiment. When a substance thus 
coated charges and discharges freely, and on being charged with 1, 2, 3, & c. mea- 
sures successively, evinces by the electrometer intensities which are as the squares 
of these quantities, it may be taken as being in a fit state for experiment on its induc- 
tive capacity. Secondly, to avoid dissipation or loss from charges which with some 
substances evince a high intensity, it is desirable to work with one half or one quarter 
the whole charge in such cases, and deduce the comparative intensity from these. 
Thus in comparing shell-lac and air, we find that a quantity which with shell-lac 
only affects the electrometer by 1°, will with air as the dielectric medium affect the 
electrometer 25° ; and as it is most important to obtain the full measure of induction 
through air, it is safer in certain cases to find the intensity of half the quantity, and 
then by the law of the intensity which is as the square of the quantity, deduce the 
intensity due to the full charge. If we required, for example, to compare the result 
of twenty measures on shell-lac with twenty measures on air, and that the intensity 
of twenty measures on air was so great as to cause dissipation between the plates, 
z 2 
