1869.] in liberating Vapour from Boiling Liquids. 



245 



It has been maintained that air is a powerful nucleus in separating salt 

 from a supersaturated solution, that it is the air alone, as carried down by 

 the solid, that acts as a nucleus in separating gases from solution, and that 

 if air be absent from a liquid it cannot boil, because there is nothing for 

 the vapour to expand upon. 



I have shown in former experiments that, in the case of supersaturated 

 saline solutions, air is not a nucleus ; but that when it appears to be so, it 

 is merely acting the part of a carrier of some chemically unclean mote or 

 speck of dust. I have also shown that masses of air may be introduced 

 into soda-water without any separation of the gas, provided the conditions 

 of chemical purity be observed. A wire-gauze cage, for example, full of air 

 can be lowered into soda water without producing any discharge of gas into 

 the cage, or any separation of gas from the surface of the cage, so long as 

 it is chemically clean; when unclean, there is an abundant separation of 

 gas from the surface of the cage, but the enclosed air remains purely passive 

 all the time. 



A similar result may be obtained in the case of a liquid at or near the 

 boiling-point, if precautions be taken to raise the cage to the temperature 

 of the liquid before introducing it. 



The cage used in these experiments was smaller than that used in the 

 soda-water experiments. It was five-eighths of an inch in diameter, and an 

 inch and a half in length, and made of fine iron-wire gauze, such as is 

 used by millers in bolting meal. Two of these cages were prepared. One 

 was cleaned by being put into boiling spirits of wine ; it was rinsed in clean 

 water, and so held in the steam of pure water boiling in a test-tube, so that 

 the cage and the enclosed air might be adjusted to the temperature of the 

 w r ater. The cage was gently lowered into the water the moment the spirit- 

 lamp was withdrawn. There was no escape of vapour ; there was no violent 

 boiling up, which must have ensued had air been a nucleus. But here was 

 a mass of air in the midst of the liquid, and yet the steam did not expand 

 into it. The openings into the cage must have been very much larger than 

 the diameter of the globules of air which are supposed always to be present 

 when a liquid is boiling, and yet there was no separation of vapour. This 

 clean cage was removed, and the other cage, just as it had left the hands of 

 the maker, was held in the steam of the water of the same tube, and the 

 moment the lamp was removed gently lowered into the water. It was 

 instantly and completely covered with bubbles of steam ; but there was no 

 expansion of steam into the cage, and no escape upwards either of steam or 

 of air. 



A good result was obtained with parafhne oil boiling at 320°. While 

 the cage was being lowered, it became filled about one-half with the liquid, 

 but when completely submerged there was no action whatever. But, per- 

 haps, it may be said that the liquid was now so far below the boiling-point 

 as to be incapable of giving off vapour to any nucleus, clean or unclean. 

 To test this, a small pellet of paper was thrown in ; the liquid immediately 



