I wish, with you, that some chemist (who should, if possible, be at
the same time an electrician) would, in pursuance of the excellent
hints contained in your letter, undertake to work upon glass with
the view you have recommended. By means of a perfect knowledge of
this substance, with respect to its electrical qualities, we might
proceed with more certainty, as well in making our own experiments,
as in repeating those, which have been made by others in different
countries, which I believe have frequently been attended with
different success on account of differences in the glass employed,
thence occasioning frequent misunderstandings and contrariety of
opinions.
There is another circumstance much to be desired with respect to
glass, and that is, that it should not be subject to break when
highly charged in the Leyden experiment. I have known eight jars
broken out of twenty, and at another time, twelve out of thirty-five.
A similar loss would greatly discourage electricians desirous of
accumulating a great power for certain experiments.--We have never
been able hitherto to account for the cause of such misfortunes. The
first idea which occurs is, that the positive electricity, being
accumulated on one side of the glass, rushes violently through it, in
order to supply the deficiency on the other side and to restore the
equilibrium. This however I cannot conceive to be the true reason,
when I consider, that a great number of jars being united, so as to
be charged and discharged at the same time, the breaking of a single
jar will discharge the whole; for, if the accident proceeded from the
weakness of the glass, it is not probable, that eight of them should
be precisely of the same degree of weakness, as to break every one
at the same instant, it being more likely, that the weakest should
break first, and, by breaking, secure the rest; and again, when it is
necessary to produce a certain effect, by means of the whole charge
passing through a determined circle (as, for instance, to melt a
small wire) if the charge, instead of passing in this circle, rushed
through the sides of the jars, the intended effect would not be
produced; which, however, is contrary to fact. For these reasons, I
suspect, that there is, in the substance of the glass, either some
little globules of air, or some portions of unvitrified sand or salt,
into which a quantity of the electric fluid may be forced during the
charge, and there retained till the general discharge: and that the
force being suddenly withdrawn, the elasticity of the fluid acts upon
the glass in which it is inclosed, not being able to escape hastily
without breaking the glass. I offer this only as a conjecture, which
I leave to others to examine.
The globe which I had that could not be excited, though it was from
the same glass-house which furnished the other excellent globes
in my possession, was not of the same frit. The glass which was
usually manufactured there, was rather of the green kind, and chiefly
intended for drinking-glasses and bottles; but the proprietors being
desirous of attempting a trial of white glass, the globe in question
was of this frit. The glass not being of a perfect white, the
proprietors were dissatisfied with it, and abandoned their project.
I suspected that too great a quantity of salt was admitted into the
composition; but I am no judge of these matters.