18 GQ. Jounstone Stronuy on the Penetration of Heat across Layers of Gas. 
Part [].—InrerprReTATION oF EXPERIMENTS. 
8. Accordingly, I made a search of this kind last year, shortly after the public- 
ation of my two papers in the Philosophical Magazine, but without finding any 
records more to the purpose than those by Dulong and Petit of experiments with 
hydrogen, which will be cited below ; and as these, taken by themselves, did not 
seem sufficiently decisive, I postponed publishing further on the subject, until I 
should have leisure to make experiments myself. But before this leisure came, 
Mr. George F. Fitzgerald met with a brief notice in Jamin’s “ Physique,” of 
experiments by De la Provostaye and Desains, which appeared to him to contain’ 
observations on the penetration of heat; and on referring to the original memoirs, 
published more than thirty years ago in the Comptes Rendus, I had the pleasure 
of finding the record of two elaborate experimental investigations into what we 
now know to have been the penetration of heat. At that time the experimental 
results were regarded as anomalous, and the only conjecture which De la Provostaye 
and Desains put forward is that they may in some way depend on the swiftness 
of convection currents in attenuated gases.* It is, however, easy to see that no 
such increased swiftness as can exist will account for the observed phenomena. 
Mr. Fitzgerald was unable to spare the time necessary to follow up the subject, or he 
would have joined me in working out this part of the present memoir ; but to him is 
due the whole credit of having perceived the importance of these observations, and 
to his kindness I owe the advantage of having had my attention directed to them, 
and the permission to make use of them, as I now do. 
9. Dulong and Petit, experimenting with a large thermometer bulb placed at 
the centre of a hollow copper globe, 30 cm. in diameter, blackened on the 
inside and kept at a constant temperature, observed the rate at which the ther- 
mometer, after having been warmed, cooled in different gases, at different tensions, 
and with the bulb naked or coated in various ways. From these experiments they 
obtained their well-known empirical law for the escape of heat by radiation and 
convection. The expression which they give consists of two terms, of which one 
represents the velocity with which heat escapes by radiation, and the other the 
velocity with which it escapes by convection, or, as we shall presently see, in some 
cases by convection and penetration. We have here no concern with the first of 
these two terms, further than to observe that the escape by radiation is the same 
at all tensions of the gas, and for all dimensions of the receiver, and depends only 
on the character of the surfaces exposed, on 9, the temperature of the copper globe, 
and on 6,— 6, the excess of temperature of the thermometer. For given values 
of 6, and @, it was accordingly a constant at all the tensions, and with all the 
receivers which De la Provostaye and Desains used, and in the following diagrams 
* See note A. at the end of this memoir. 
