442 Mr. C. Toralinson on some Phenomena 



5. It is well known in the laboratory that in distilling ether, 

 alcohol, and similar liquids in glass vessels, the vapour is given 

 off with difficulty. At one moment the liquid does not boil at 

 all, at another it bursts into a mass of vapour and liquid which 

 fills the vessel and occasions such a bumping of the retort as to 

 endanger its safety, and, indeed, sometimes to break it. The 

 vapour forms, in fact, with a sort of dull explosion which is very 

 marked in distilling sulphuric acid. These bumpings are called 

 soubresautshy the French. They are mitigated if not prevented 

 by introducing into the retort some solid matter not acted upon 

 by the liquid. It is recommended to use sharp, or angular, or 

 rough pieces of metal, glass &c, the points being, it is supposed, 

 favourable to the generation of steam. Silver, platinum, or 

 copper (in the form of foil or wire or filings), or bits of cork or 

 cartridge paper are recommended by Dr. Faraday as " promoters 

 of vaporization"*. 



It seems to me in such cases as the above that the various 

 substances act by the adhesion to them of the vapour and 

 its consequent separation under the continued action of the 

 heat, and that they cease to act as soon as they become chemi- 

 cally clean, in which case the vaporous solution adheres to them 

 as a whole f. 



Throughout his memoir M. Gernez avoids the use of the term 

 chemically clean, and attributes the varied behaviour of solid 

 nuclei simply and solely to the adhering air. Air and air only 

 is the means by which bubbles of vapour can escape from a liquid 

 at or above the boiling-point; and for this purpose the minutest 

 speck is all-sufficient. Get rid of this speck of air, and boiling be- 

 comes impossible; the liquid becomes more and more superheated, 

 and then suddenly goes off with an explosion. But to get rid of 

 this speck of air is the difficulty. It persists in remaining ; and 

 so efficacious is it that, although it may not be more than a mil- 

 limetre in diameter, it will continue its action during twenty- 

 four hours and liberate upwards of half a million of bubbles of 

 vapour, each bubble about five millimetres in diameter. How- 

 ever marvellous this result may appear, the proposition at the 

 head of the subdivision 3° on the next page (p. 380) is still 

 more so, namely that " each bubble of vapour is formed at the 



* Chemical Manipulation, 1830, p. 199. 



f A friend, writing from the laboratory of a manufacture (May 6, 18/5), 

 says: — "We have had some difficulty in determining our boiling-points. We 

 were very careful at first in cleaning our tubes and thermometers ; and the 

 result was that we did not get water to boil under 216° or 217° F. Now, 

 after cleaning all the parts thoroughly, we rub the thermometer-stem 

 and bulb with a duster in use in the laboratory, and get the boiling-points 

 all right." I recommended him to put a bit of cocoa-nut-shell charcoal into 

 the liquid. 



