^88 
Mr, Tennant on the Means of 
that means be distilled over without any additional fire ; and 
though this does not take place under the usual circumstances, 
yet it may be effected in the following manner. 
•The temperature required for converting any fluid into 
vapour is dependent on the pressure of the air upon its sur- 
face, and may therefore be lowered if that pressure is dimi- 
nished. If then the weight of the air was removed from water, 
it would rise into vapour below the common boiling point, and 
might therefore be distilled over by steam of the usual heat. 
In order to produce this effect, a vessel having a receiver 
connected with it should be made air tight, and the steam 
made to pass through the vessel along a worm or spiral tube 
of metal, in the manner represented in the annexed outline. 
The vacuum is now easily produced by applying heat to the 
vessel till the steam issues from the opening in it, and in the 
receiver, when they are to be immediately closed, and the heal 
removed. 
