X 
24 G. Jounstonz Sronzy on the Penetration of Heat across Layers of Gas. 
16. The observations previously made by Dulong and Petit on the rate of 
cooling in hydrogen come here to our aid. The interval between the walls of 
their receiver and the bulb was about 133 cm., and they found that, after allowing 
for radiation, the rest of the escape of heat within this receiver charged with 
hydrogen was proportional to p**’, whereas in the other gases examined it was 
proportional to p in a power which differed slightly from one gas to another, but 
was in none far from 4. The escape of heat with which they were here dealing, 
Dulong and Petit supposed to be wholly due to convection, but we have now reason 
to suppose that, when the gas was hydrogen, penetration contributed largely to it, 
and 1s the reason why the index of p in Dulong and Petit’s empirical formula was 
so different with hydrogen from its value with other gases. Accordingly, if we 
plot down the curve— 
y=kp®, : 
in the upper line of fig, 9, to represent the observed excess of the escape of heat, 
over what was accounted for by radiation, we may draw the dotted parabola crossing 
it somewhere to the right of A, and conclude that the convection, if it could be 
separated from penetration, would be represented by the ordinates of such a curve, 
and that the shaded interval between the two curves is due to heat having leaked 
away by penetration, across even so much as 133 cm. of hydrogen. Although we 
cannot assign the exact position of the dotted line until observations shall have 
been made in larger receivers, we can already see the general shape of the shaded 
space, which is at present the object of our search. 
17. In a mixture of air and hydrogen within the cylinder, De la Provostaye and 
Desains found that at a tension of 60 mm. the rate of cooling was “much less” 
than if the hydrogen alone had been present. From this, and from the observations 
plotted down in fig. 8, it follows that the Crookes’s layer in such a mixture is 
narrower than if all the molecules present had been hydrogen, This accords with 
