On Crookes's Layers at Atmospheric Tensions. 59 



A still more instructive illustration of these facts is afforded 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 experiment the hot iron, 

 when laid on the anvil, does not fit it accurately, 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 between 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 experiment 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 downwards 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 with 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 publicly 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. 



