56 Mr. G. JOHNSTONE STONEY, 
tension. Hence they could not be supported by that agency i 
the groups which he describes. We are therefore forced to look 
elsewhere for the cause of the support of these groups; the 
thermal and mechanical properties of Crookes’s layers show that 
they will suffice: and we have seen that all the conditions were 
present which would call Crookes’s layers into existence. 
Mr. George F. Fitzgerald has pointed out another very striking 
example. A piece of cold iron may be made to float on melted 
cast iron, and will even float high like cork on water. Here 
the difference between the temperature of the glowing mass 
of molten metal and the cold piece of iron is so considerable 
that the stresses that are developed are able to support the 
weight of the piece of iron while it is still at such a distance 
from the fiery liquid that it seems to float high upon it. 
What it floats on is in reality a bath of polarized air, the 
stresses within which both support its weight and force down 
the surface of the molten metal. This air-bath keeps it out of 
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. 
On the same principles we are to explain the safety of exploits 
that are occasionally performed, viz—The licking of a white hot 
poker, the dipping of the fingers into molten metal, and the plung- - 
ing 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 diminishing the 
extent to which it would compress the Crookes’s layer. 
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 
stone, it follows that a circular disk, 16 m.m. in diameter and 0°85 of a m.m. in thickness 
would be the extreme theoretic limit that could be supported by surface tension. This is 
about the size of a fourpenny bit. 
