1869.] in liberating Vapour from Boiling Liquids. 



243 



object being to keep the liquid in the tube at or near the boiling-point, but 

 not actually boiling. The temperature of the liquid in the tube may be 

 taken from time to time, and the thermometer, when not in use, may be kept 

 in a tall narrow glass containing a little of the liquid under examination. 



Carbonic disulphide, from its low boiling-point, gives off vapour with 

 facility, and is consequently well adapted to show the action of solid nuclei. 

 When the tube was placed in the hot water, a quantity of dense white 

 vapour ascended from the surface of the liquid to the top of the tube ; but 

 instead of overflowing it condensed in copious tears, which fell back into 

 the liquid and caused a strong descending current. On touching the sur- 

 face of the liquid with the end of a brass wire, violent ebullition set in, the 

 bubbles rising to near the mouth of the tube. The boiling ceased alto- 

 gether as soon as the wire was removed ; but when the surface was touched 

 with a strip of paper, it set in as violently as before. 



Now in these two experiments no air could have been carried down, since 

 the surface only was touched, and the boiling continued only while the solid 

 was kept in contact with such surface. Iron wire also liberated vapour 

 abundantly. The end of a glass rod was active at two small points, libe- 

 rating from each a rapid stream of bubbles, the remaining portions being 

 clean, or having soon become so by the action of the hot liquid, since glass 

 is readily cleansed by liquids near the boiling-point. 



But it is said that rough bodies are most favourable to the liberation of 

 vapour. The hot carbonic disulphide was touched with a rat's-tail file, 

 and it produced furious boiling. The file was then held in the flame of a 

 spirit-lamp, and while hot placed in the upper part of the tube, so that it 

 might cool down to about the temperature of the liquid, and yet be shel- 

 tered from the air. On touching the surface of the disulphide with the 

 end of the file, there was no liberation of vapour ; and the file was slowly 

 passed to the bottom of the liquid, but still there was no action. The file 

 was now taken out and waved in the air ; on reinserting it into the liquid, 

 there was a burst of vapour arising from some mote or speck of. dust caught 

 by the file from the air. The file was quickly cleaned by the liquid, and 

 it became inactive as before. It was again taken out and waved in the air, 

 and on once more putting it into the liquid boiling set in again. 



A tube containing ether was put into the hot-water bath ; it quickly 

 reached the boiling-point, and two specks in the tube became active in 

 discharging rapid streams of bubbles. Specks of this kind are often 

 powerful as nuclei in separating gas from soda-water &c, and in causing 

 the sudden crystallization of supersaturated saline solutions. Such specks 

 in the bottoms of flasks, beakers, and retorts are powerful nuclei in sepa- 

 rating vapour from a liquid during the boiling. The vapour seems to be 

 generated by these points, and to proceed from them to the surface in 

 rapidly enlarging bubbles. These specks consist of iron, carbon, or some 

 other material which is not so readily cleaned as the glass, or they present 

 a porous point to the vapour. 



