1869.] in liberating Vapour from Boiling Liquids. 



251 



tion of the absorbed gas might either be driven out or further condensed. A 

 somewhat similar action may be noticed by transferring a piece of charcoal 

 from one boiling liquid to another. For example, a small piece of well- 

 burnt charcoal from the centre of a lump was held in the flame of a spirit- 

 lamp until it was red-hot, and so put into boiling water. It was not very 

 active at first, but it soon became so, and continued so as long as the heat 

 was kept up. After about half an hour's action the charcoal was taken out, 

 dried in a cloth, and put into boiling turpentine ; here it was amazingly 

 active, and continued so during some minutes after the lamp had been 

 removed. The charcoal was dried on filtering- paper and put into spirits of 

 wine ; it was now much less active than fresh charcoal would have been ; 

 and in ether its activity was still more diminished. The charcoal was next 

 put into hot water, and it at once started into activity ; it was far more 

 vigorous than clean charcoal is in water under any of the circumstances 

 that have come under my notice. The charcoal w r as doubly active, not 

 only from its porosity, but also from its want of chemical purity. On this 

 latter account charcoal that has been used iu boiling turpentine is singu- 

 larly active in boiling water. And this sufficiently accounts for the fact 

 noticed by Dufour, that when globules of w r ater in hot oil came into con- 

 tact with the thermometer or the sides of the vessel, they at once exploded 

 into steam ; but I believe the globules of water were in the spheroidal state 

 in all the cases of very high temperature cited by him. 



The diminished activity of charcoal and other porous bodies depends on 

 the order in which they are introduced into liquids of different boiling- 

 points. If transferred from a liquid with a high into one with a low boil- 

 ing-point, the charcoal is more or less inactive, its absorptive powers being 

 already satisfied ; but if transferred from a liquid with a low r into one with 

 a high boiling-point its activity is increased, not only by the expulsion of 

 the liquid absorbed, but also by the want of chemical purity that accom- 

 panies the process. Thus meerschaum or coke that is very active in tur- 

 pentine becomes inactive when transferred to spirits of wine \ but after a 

 time a single point in the solid may become active, and produce a rapidly 

 rising inverted cone of vapour that has a very striking effect. 



Conclusions. — The conclusions to which the foregoing details seem to 

 lead are : — 



(1) That a liquid at or near the boiling-point is a supersaturated solution 

 of its own vapour. 



(2) That a solid non-porous nucleus either is or is not efficient in libe- 

 rating vapour, according as it is chemically unclean or clean. 



(3) That as porous bodies do not become inactive, the proper nucleus for 

 liberating vapour in the operations of boiling and distilling liquids, and for 

 preventing soubresauts, is charcoal, coke, or some other porous body. 



P.S. (Jan. 21, 1869). — As it seemed probable that some numerical re- 

 sults as to the action of porous nuclei in increasing the amount of the 



t 2 



