HEATED AIR NOT A CONDUCTOR OP ELECTRICITY. 
a given density is an accurate measure of 
tlie comparative quantity contained in a unit 
of space, or of the tension (by which is to be 
understood the elastic force of a given quari- 
tity accumulated in a given space, and is 
directly as the density of the ^stratum,) and 
the attractive force discovered by the elec- 
trometer, or the intensity is directly as the 
square of the quantity contained in a unit of 
space. 
5. The effect of an atmosphere varying in 
density and temperature in restraining electri- 
cal discharges, is as follows : _ 
1st. The respective quantities requisite to 
pass a given interval, varied in a simple ratio 
of the density of the air. When the density 
was one half as great, the discharge occurred 
with one half the quantity accumulated, that 
is to say, with one fourth of the attractive 
force indicated by the electrometer. 2nd 
The distance through which a given accumu- 
lation could discharge was found to be in an 
inverse simple ratio of the density of the air, 
the intensity or free action being constant. 
In air of one half the density, the discharge 
occurred at twice the distance, or the resist- 
ance of air to the passage of electricity is as 
the square of the density directly, and if 
the density of the air be decreased, the dis- 
tance between the points of action be in- 
creased, the electrical accumulation will still 
remain complete. 
6. Heated air is not as is frequently stated 
a conductor of electricity, and heat does not 
facilitate electrical transmission through air 
in any other way than by diminishing its den- 
sity. Supposing heat to be material, it is 
a non-conductor of electricity, because the 
incorporation of a conducting with a non- 
conducting substance is found to impair the 
insulating power of the latter as in the case 
of air charged with free vapour, whereas in 
the intimate union of two non-conductors the 
insulating power remains perfect. 
7. Sir Humphry Davy has well illustrated 
the effect of heat in imparing the conducting 
power of metals, and the same fact has been 
observed by Mr. Chiistie, Dr. Ritchie, 
however, has lately brought forward an ob- 
jection ; for, in transmitting electricity over 
a forked iron rod, one of the legs of which he 
heated to redness, he found that the electri- 
city passed in preference from the heated 
side rather than from the cool side. To make 
this experiment free from objection, it would 
be necessary to insert the heated iron rod in 
an exhausted receiver. Dr. Ritchie was 
aware of this, but conceives that the effect of 
a heated wire would be a species of electrical 
evaporation from its surface. His very inge- 
nious paper in the philosophical transactions 
has certainly not attracted that attention 
which it deserves. The objection stated to 
his experiment by Mr. Harris, does not 
appear to affect the result which he ob- 
tained. 
8. Volta observed that of two plane sur- 
faces of equal area, that which has the greatest 
extension has also the greatest capacity for 
electricity. Mr. Harris has prosecuted this 
fact and ascertained that the intensity varies 
in an inverse ratio of the perimeter of plates 
U 
which he employed, varying in shape from a 
circle through a square up to a long paral- 
lelogram. I'he following illustrates the 
results — 
DIMENSIONS,— AREA=75 SQUARE INCHES. 
Length . 
Breadth 
Perimeter. 
Intensity. 
12 5 
6 
37 inches 
99 
25- 
3 
5G „ 
6 
54-5 
1-4 
112 ,. 
3 
The extent of edge has no influence on the 
intensity. The intensities of conductors are 
therefore, it appears, inversely as their peri- 
meters, and the intensity varies in an inverse 
ratio of the area when the perimeters remain 
the same, from which, it follows that the 
intensity must vary inversely with those 
quantities jointly or calling I, intensity, A, 
area, P, perimei , we have 
I a — 
- AP 
P)Ut suppossing the quantity of electricity to 
vary, then the intensity being as the square of 
the quantity, the formula is 
X 
I a — 
- AP 
and the capcity of a conductor being measur- 
ed by thequanacty of electricity, it can receive 
under a given intensity, there follows x2 a 
I A P, or with a constant intensity, x repre- 
senting the cai)acity, we obtain capacity 
It appears that the intensity does not vary 
in an inverse ratio of the square of the sur- 
face according to the general law, except 
when the areas are so disposed that the whole 
perimeter of the various plates is as the res- 
pective surfaces. 
9. The operation of electricity on distant 
bodies, by induction, is quite independent of 
atmospheric pressure, and is exactly the same 
in vacuo as in air, the attractive force vary- 
ing as the squares of the respective distances 
inversely. 
1st. The attractive force exerted between 
an electrified and a neutral uninsulated con- 
ductor, is not at all influenced, by the form 
or disposition of the unopposed portions. 
2d. The force is as the number of attract- 
ing points in operation directly, and as the 
squares of the respective distances inversely, 
hence the attractive force between two paral- 
lel place circles being found, the force be- 
tvveen any other two similar planes will be 
given. 
Sd. The attractive force between two un- 
equal circular areas is no greater than that 
between tw'o similar areas each equal to the 
lesser. 
4th. The attractive force also of a mere 
ring and a circular area on each other, is no 
greater than that between two similar rings. 
5t//. The force between a sphere and an 
opposed spherical segment of the same cur- 
vature, is no greater than that of two similar 
segments, each equal to the given segment. 
It has been much agitated whether electri- 
city can pass through a vacuum, but the fact 
