G. Jounstone Stoney on the Penetration of Heat across Layers of Gas. 19 
is represented by the interval between two horizontal lines. This interval is, more- 
over, small, because De la Provostaye and Desains reduced the loss by radiation 
to a very small amount by silvering or gilding the bulbs of their thermometers. 
The other term of Dulong and Petit’s expression which furnishes the rate of 
escape by convection is— 
K p’, 
where p is the tension measured in millimetres of mercury, K depends on the gas 
and on 6, and 6,, and c was found to be nearly } when thereceivercontained hydrogen, 
but was nearly } for the other gases experimented on and for atmospheric air. I 
will return to the case of hydrogen; but in the other gases the velocity of the 
escape of heat by convection with given temperatures of the bulb and receiver 
will be represented at different tensions of the gas by the ordinates of a curve not 
differing much from a parabola, since this would be the curve if the index were 
exactly 4; and accordingly, curves of this kind are laid down in the annexed 
diagrams. It is not material whether a large or a small portion of the parabola is 
introduced, because all parabolas are similar. 
10. By thus plotting down the results of the experiments upon diagrams, we 
obtain the means of seeing at a glance how much of the escape of heat observed 
by De la Provostaye and Desains can be accounted for by radiation and convection, 
and how much remains to be allotted to penetration. De la Provostaye and 
Desains made their observations in three receivers—-a hollow sphere of 24 cm. 
diameter, a hollow sphere of 15 cm. diameter, and a cylinder* 6 em. in diameter 
and 20 cm. in height. They, unfortunately, do not recordt the sizes of their ther- 
mometer bulbs, so that the exact intervals between them and the walls of the 
containing vessels cannot be known. But it is likely that the thermometers were 
of about the same size as those they used in other similar investigations, and we 
shall, probably, not be far wrong in attributing to them a diameter of 3 centimetres. 
If this may be assumed, the interval between the bulbs and the walls of ‘the 
receiver, would be— 
103 cm. in the largest receiver, 
6 cm. in that of intermediate size, and 
13 cm. in the cylinder. 
In Dulong and Petit’s experiments the diameter of the receiver was 30 cm., so 
that the interval was about 134 cm. 
11. With atmospheric air in their largest receiver (that with an interval 
between the bulb and the receiver of about 103 cm.), De la Provostaye and Desains 
found that the rate of cooling, or the escape of heat per second which is propor- 
tional to it, was represented by Dulong and Petit’s expression (which had been 
based on experiments made with an interval of about 134cm.), until the exhaustion 
* The direction in which the heat penetrates, and of the Crookes’s stress, will be perpendicular to the 
isothermal surfaces within the gas in the simple case which we have hithérto considered, where A. is 
parallel to B; but it will in general pierce the isothermal surfaces obliquely if one part of the Crookes’s 
layer is more curtailed than another. 
+ They give this information in the Annales de Chimie. See note B at the end of this memoir. 
