ordinary Atmospheric Tensions. 463 



bottle, and the neck of the bottle partially stopped by a cork 

 through which a thin tube passes. So long as the drop con- 

 tinues in the spheroidal state, a mixture of air and vapour 

 slowly escapes through the tube in the cork ; but the instant 

 the spheroidal state ceases, and the water comes into contact 

 with the copper, a sufficient portion of the water flashes off so 

 suddenly into steam that the cork is driven out with explosive 

 violence. 



10. A still more instructive illustration of these facts is af- 

 forded by the familiar experiment, known to every smith, that 

 an explosion will occur if a little water is dropped on an anvil, if 

 a white-hot strap of iron is laid over the drop, and if the iron 

 is then given a tap with the sledge-hammer. In this experi- 

 ment the hot iron, when laid on the anvil, does not fit it accu- 

 rately, but comes into contact only at a few points, and leaves 

 a chink elsewhere. While the iron is descending towards the 

 drop of water, a Crookes's layer of polarized air is formed be- 

 tween it and the cold water, which exerts a sufficient pressure 

 upon the drop, both to flatten it out, and to keep it from coming 

 into contact with the glowing iron. At this stage of the ex- 

 periment the lower portion of the chink is occupied by water, 

 and the upper portion by polarized air. The stratum of air 

 moderates the flow of heat towards the water, so that the water 

 is able to continue liquid by parting with as much heat down- 

 wards to the cold anvil as it receives from above, before it is 

 itself warmed beyond the boiling-point. But when the sledge- 

 hammer descends, the soft iron yields, the chink is obliterated 

 by a force greater than that which the Crookes's layer can 

 support, and the glowing mass comes, in many places, into 

 direct contact with the water. The vastly augmented flow of 

 heat which is consequent upon this direct contact, rushes 

 across the film of water with a speed equal to the velocity of 

 sound in water, which will carry it across a film the seventh 

 of a millimetre in thickness in the ten-millionth of a second. 

 Within this brief period of time the greater part of the water 

 is raised to a very high temperature ; and its sudden conversion 

 into red-hot steam causes the explosion. 



Before concluding this communication I wish to take the 

 opportunity of thanking my scientific friends for their kindness 

 in bringing such remarkable instances of Crookes's layers at 

 ordinary atmospheric tensions to my notice, and giving me 

 permission to publish an account of them, 



