440 Mr. C. Tomlinson on some Phenomena 



the exhaustion was made as perfect as possible. Not a single 

 bubble was visible either by daylight or by candlelight. Had 

 there been a bubble in any part of the tube it must have been 

 detected, since the bubbles have the lustre of mercury. On 

 again holding the tube over the flame, the charcoal gave a few 

 crackling sounds and then emitted vapour in abundance, and the 

 boiling was as easy as at first. Next morning abundance of air- 

 bubbles were given off during the boiling, showing that air had 

 been absorbed during the night. 



I do not see any a priori reason why the charcoal, as a nu- 

 cleus, should be exhausted. Its absorptive powers remain, 

 whether for vapour or gas, whatever number of times the same 

 specimen be used. In Mr. Hunter's experiments the air was 

 expelled by raising the charcoal to a red heat and quenching 

 it under mercury before introducing it into the vapour. When 

 used as a nucleus in boiling, the air is gradually displaced by 

 the vapour; it is continually absorbing vapour and emitting it 

 under the continued action of the heat. Its action resembles 

 those catalytic processes in which a body remaining apparently 

 unchanged brings about continuous changes in another body — 

 as, for example, where the presence of a small quantity of black 

 oxide of manganese enables chlorate of potash to liberate its 

 oxygen at a moderate heat. There is probably a continuous 

 formation of permanganic acid at the expense of the chlorate, 

 and a continuous decomposition under the continued action of 

 the heat, the function of the oxide being to transfer oxygen from 

 the state of combination to the free state. So also the charcoal, 

 by its strong absorptive power, transfers vapour from the liquid, 

 but cannot retain it at the high temperature, so that there is a 

 continual absorption and a continual emission. When after re^ 

 peated boilings the liquid is left to cool, a moderate heat is suffi- 

 cient to start the charcoal as a nucleus ; and if left during twelve 

 or fourteen hours, air is absorbed, and the action is then the 

 same as with a fresh piece of charcoal. 



I give one or two more examples of permanent nuclei. A 

 bit of gutta percha was boiled with water in a clean tube ; it 

 softened under the heat ; and a glass rod was introduced so as to 

 press it against the side of the tube near the bottom and leave 

 a smear, while the greater portion was removed adhering to the 

 rod. This smear was singularly active in liberating bubbles of 

 vapour ; and after the lamp was removed, bubbles were dragged 

 upwards from it so as to become pear-shaped. The tube was 

 boiled eight times, at intervals of from 5 to 20 minutes ; but the 

 smear continued to maintain its activity. Next morning the 

 tube was boiled again, and the smear was active as before. 



The inside of a tube was extensively smeared with lead soap. 



