* * * * *

1283. A most interesting class of substances, in relation to specific
inductive capacity, now came under review, namely, the gases or aeriform
bodies. These are so peculiarly constituted, and are bound together by so
many striking physical and chemical relations, that I expected some
remarkable results from them: air in various states was selected for the
first experiments.

1284. _Air, rare and dense._--Some experiments of division (1208.) seemed
to show that dense and rare air were alike in the property under
examination. A simple and better process was to attach one of the apparatus
to an air-pump, to charge it, and then examine the tension of the charge
when the air within was more or less rarefied. Under these circumstances it
was found, that commencing with a certain charge, that charge did not
change in its tension or force as the air was rarefied, until the
rarefaction was such that _discharge_ across the space _o_, _o_ (fig. 104.)
occurred. This discharge was proportionate to the rarefaction; but having
taken place, and lowered the tension to a certain degree, that degree was
not at all affected by restoring the pressure and density of the air to
their first quantities.

inches of mercury.
Thus at a pressure of 30 the charge was 88 deg.
Again 30 the charge was 88
Again 30 the charge was 87
Reduced to 11 the charge was 87
Raised again to 30 the charge was 86
Being now reduced to 3.4 the charge fell to 81
Raised again to 30 the charge was still 81

1285. The charges were low in these experiments, first that they might not
pass off at low pressure, and next that little loss by dissipation might
occur. I now reduced them still lower, that I might rarefy further, and for
this purpose in the following experiment used a measuring interval in the
electrometer of only 15 deg. (1185.). The pressure of air within the apparatus
being reduced to 1.9 inches of mercury, the charge was found to be 29 deg.;
then letting in air till the pressure was 30 inches, the charge was still
29 deg..

1286. These experiments were repeated with pure oxygen with the same
consequences.

1287. This result of _no variation_ in the electric tension being produced
by variation in the density or pressure of the air, agrees perfectly with
those obtained by Mr. Harris[A], and des