56 Mr. G. Johnstone Stoney, 



tension. Hence they could not be supported by that agency %n 

 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 ra.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. 



