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 described in his beautiful and important investigations contained in the Philosophical Transactions; namely that induction is the same in rare and dense air, and that the divergence of an electrometer under such variations of the air continues the same, provided no electricity pass away from it.  The effect is one entirely independent of that power which dense air has of causing a higher charge to be retained upon the surface of conductors in it than can be retained by the same conductors in rare air; a point I propose considering hereafter.

  [A] Philosophical Transactions, 1834, pp. 223, 224, 237, 244.

1288.  I then compared hot and cold air together, by raising the temperature of one of the inductive apparatus as high as it could be without injury, and then dividing charges between it and the other apparatus containing cold air.  The temperatures were about 50 deg. and 200 deg., Still the power or capacity appeared to be unchanged; and when I endeavoured to vary the experiment, by charging a cold apparatus and then warming it by a spirit lamp, I could obtain no proof that the inductive capacity underwent any alteration.