ordinary Atmospheric Tensions. 461 



contact with the glowing mass ; and accordingly it receives 

 heat from below only by diffusion and radiation, in quantities 

 far short of what it would receive from actual contact ; and as 

 it loses much heat by radiation upwards, it may be able for a 

 considerable time to maintain a sufficiently low temperature to 

 continue floating. 



5. On the same principles we are to explain the safety of ex- 

 ploits that are occasionally performed ; viz. the licking of a 

 white-hot poker, the dipping of the fingers into molten metal, 

 and the plunging of the hand into boiling water. In all these 

 cases the Crookes's layers that intervene prevent that contact 

 which would cause a dangerous scald or burn. 



It is usual before performing these two latter experiments 

 to moisten the hand with soapy water, ether, turpentine, or 

 liquid ammonia. All of these would have the useful effect of 

 lowering the surface-tension of the hot liquid, and thus dimi- 

 nishing the extent to which it would compress the Crookes's 

 layer. 



6. But the most splendid example I have yet seen of a 

 Crookes's layer is one which was first noticed by M. Boutigny, 

 and which was shown by Professor Barrett at the Brighton Meet- 

 ing 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 6 centims. 

 in diameter, furnished with a staple by which it can be lifted, 

 was brought to a bright red heat, and while glowing was low- 

 ered 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 diffusion as well as by radiation ; accordingly the 

 intervening air becomes intensely polarized, 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 depression in the water is now nearly hemi- 

 spherical, but not quite so, since the interposed layer of po- 

 larized 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 con- 

 stant stress P nearly equal to the tension of the open atmo- 

 sphere, acting equally in all directions, along with a variable 

 Crookes's stress p, acting for the most part nearly in the direc- 

 tion 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 wall vary with the depth, being 



