On Crookes’s Layers at Atmospheric Tensions. 57 
was shown by Professor Barrett, at the Brighton meeting of the 
British Association, with the improvement of adding soap to 
the water, an addition which seems essential to the full success 
of the experiment. A copper ball, some six cm. in diameter, 
furnished with a staple by which it can be lifted, was brought to 
a bright red heat, and while glowing was lowered into a large 
beaker of soapy water. As the ball approaches the cold surface 
of the water heat passes from the ball to the water by conduction 
or penetration* as well as by radiation ; accordingly the interven- 
ing air becomes intensely polarzied, and the Crookes’s stress that 
accompanies the polarization makes a hollow in the surface of the 
water. Let the ball be lowered till it is half submerged, the de- 
pression in the water is now nearly hemispherical, but not quite 
so, since the interposed layer of polarized gas will be thinnest at 
the bottom, where, to withstand the pressure of the water, it must 
exert most force. The stresses at any point of this polarized 
layer consist of a constant stress P nearly equal to the tension of 
the open atmosphere, acting equally in all directions, along with 
a variable Crookes’s stress p, acting for the most part nearly in. 
the direction of a radius of the ball; the most marked deviation 
from this direction being close to the horizontal surface of the 
water, where the action of the upper hemisphere of the ball gives 
an inclined direction to the Crookes’s stresses, and helps to round 
off the surface of the water. The amount of the Crookes’s 
pressure acting on the water will vary with the depth, being such 
al each point that it gives a component equal and opposite to 
the resultant of the pressure of the water at that depth, and of 
the surface tensions round the point. Whenever the Crookes’s 
force is not quite in the direction of this resultant, there will be 
a free tangential component, and this must produce surface cur- 
rents in the water. These, however, cannot be observed in the 
present experiment because they are of small amount, and too 
much mixed up with convection currents arising from the heat 
that reaches the water by radiation and diffusion. 
When the ball is lowered until it is quite submerged it will be 
surrounded on all sides by its envelope of polarized air, thinnest at 
* The heat which diffuses across a layer of gas passes under what are known as the 
laws of conduction if the number of gaseous molecules present is sufficiently large. If 
fewer molecules are present the heat passes under other laws, which may be distinguished 
from the laws of conduction by calling them the laws of penetration. 
