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 ofa 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. 
