460 Mr. G. J. Stoney on Crookes's Layers at 



only half-past nine, the sun had been shining brightly on the 

 exposed beach. The upper surface of each of the little pebbles 

 was perfectly dry; and the groups which they formed were 

 slightly depressed in curved hollows of the liquid. The tide 

 was rapidly rising ; and, owing to the narrowness of the channel 

 at the point where I made my observations, the sheets of float- 

 ing sand were swiftly drifting further up the river into brack- 

 ish and fresh water. On closely watching the rising tide at 

 the edge of the strand, I noticed that the particles of sand, 

 shells, and small flat pebbles, which had become perfectly dry 

 and sensibly warm under the rays of the sun, were gently up- 

 lifted by the calm, steadily rising water, and then floated as 

 readily as chips or straws." 



The calm air, tranquil water, hot sun, and warm stones 

 predicted from the theory are all recorded in these observa- 

 tions. 



This rare phenomenon must not be confounded with the 

 familiar one in which patches of fine sand float upon water 

 in consequence of its surface-tension. The surface-tension of 

 water in contact with air will not support flakes of stone of 

 above a certain size ; and those described by Professor Hen- 

 nessy are at or beyond the limit of size* that could, even if 

 separate, be floated by surface-tension. Hence they could not 

 be supported by that agency in 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 proper- 

 ties 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. 



4. 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 



* Taking the surface-tension of water in contact with air at 8-25 

 grammes per metre as determined at 20° 0. by M. Quincke, and assuming 

 2-5 as the specific gravity of the stone, it follows that a circular disk, 16 

 millims. in diameter and 085 of a millim. in thickness, would be the ex- 

 treme theoretic limit that could be supported by surface-tension. This is 

 about the size of a fourpenny bit. 



