ON THE PENETRATION OF HEAT ACROSS LAYERS OF GAS. 
BY 
G. JOHNSTONE STONEY, m.a., r.r.s., &e., Secretary Royal Dublin Society. 
[Read 21st May, 1877.] 
Part I.—Tunory. 
1. Heat will pass between bodies at different temperatures by direct contact, by 
radiation and absorption, or by contact with a fluid and convection through it. 
That heat may be transported in these several ways has long been known, and the 
laws of the transfer have been made the subject of repeated and careful investigation 
by experiment and by the deductive method. And last year two papers* were 
published by the author of the present memoir, in which it was shown that heat will 
also escape, under new conditions, across a Crookes’s layer, if the layer be restricted 
in width. In those papers the mechanical actions that arise,and upon which Mr. 
Crookes had made many experiments, were made the subject of study; and the 
present communication aims at extending the investigation to the second branch of 
the subject, viz.:—The transfer of heat which accompanies those mechanical 
actions. 
2. When gas is in contact with a body A at a different temperature from 
itself, it is a familiar fact that convection currents rapidly set in. The first step 
of the process is the almost instantaneous formation of that layer which I have 
called Crookes’s layer—a layer of the gas of varying density and temperature, 
being on one side at the temperature of the body A, and on the other side at the 
temperature of the surrounding gas. It is because this layer has a different 
density from the rest of the gas, and because of the attraction of the earth, that 
those streams set in which are called convection currents; and accordingly, if the 
experiment could be made at a station where there is no gravity, these convection 
currents would not arise, although the Crookes’s layer would then also be fully 
developed. It will be convenient to inquire first what will occur under these 
simplified conditions, and afterwards to take into consideration whether any modifica- 
tion has to be made to allow for the effect of the neighbouring earth. To give to 
the problem definiteness and the utmost simplicity, I will suppose that a body A 
at temperature 6,, presents a large flat surface to an atmosphere of gas which is 
at a lower temperature @,, and exposed everywhere to a constant pressure, but 
which is uninfluenced by gravity. Let us further regard this gas as a perfect non- 
conductor of heat. 
3. If the excess of temperature is supposed to be suddenly imparted to A, there 
will be a brief interval of adjustment within the gas, after which the condition of 
the gas will settle down into the state in which the Crookes’s layer will have been 
* See Phil, Mag. for March and April, 1876. 
D 
