20 G. Jounstone Sroney on the Penetration of Heat across Layers of Gas. 
reached 6 mm. of mercury ; but that after passing that tension, the rate of cooling, 
instead of continuing to decrease, remained sensibly constant between tensions of 
6 mm. of mercury and 2°8 mm., the lowest tension at which they experimented. 
This is represented on figure 2 by the hori- 
zontal line from m to n. In this figure the 
abscissas represent tensions In mm. of mercury, 
and the ordinates of the thick line represent the 
observed rates of cooling at different tensions, 
but with constant values of 6, and 6, The part 
of the ordinate between the horizontal lines 
represents the escape by radiation, its continuation 
up to the parabola represents the escape by con- 
vection, and the extension upwards into the shaded OMe PEA aN a " 
portion of the figure is due to the penetration of heat across the Crookes’s layer 
which evidently reached the walls of the receiver when the tension was reduced to 
about 6 mm., and was compressed when the exhaustion proceeded farther. 
Similarly with the receiver of intermediate size, 
in which a Crookes’s layer of a width of 6 cm. 
would reach the walls, the results obtained by De la 
Provostaye and Desains are represented graphically 
by figure 3. In this case heat leaked away by pene- 
tration in appreciable quantities at tensions under 
20 mm. of mercury, and kept the total escape of heat 
nearly constant between tensions of 20 and 4 mm. 
12. But the most decisive experiments were made with the cylindrical vessel, 
which was the smallest of the three receivers. In it the interval between the bulb 
and the walls of the vessel was probably only 13 cm. 
With this vessel the rate of cooling was slower than in the two larger receivers at 
all tensions from 760 mm., or the tension of an atmosphere, down to about 45 mm. 
This seems to indicate 
that the convection cur- 
rents were impeded by 
the form and small size 
of the cylinder ; so that 
if the ordinates of the 
parabola Or represent 
the rate of cooling which 
would result from con- 
vection ina large vessel, 
the ordinates of some 
Dicaceits Ly Se A O60. os 21 SLOILS. 
smaller curve such as Pt ee eee ee ee eee 
