on [Gaseous] Supersaturated Solutions. 187 



will be found to contain minute furrows, and if plunged into 

 aerated water will be immediately covered with innumerable 

 scarcely visible bubbles which arrange themselves in the furrows ; 

 while on the first wire less numerous and, on that account, more 

 quickly growing bubbles appear. In the same way behaves a 

 strongly oxidized zinc wire that has long been exposed to the 

 air, the minute roughnesses of which are easily detected by rub- 

 bing with the finger; only in this case the furrows are absent. 

 An oxidized brass wire, as compared with one free from oxide, 

 show r s, on the contrary, no great difference. 



It has been already remarked that the temperature has a great 

 influence on the quantity of gas held in solution. If of two 

 volumes of equally impregnated water one be warmed, there is 

 a diminution of attraction between the air and water particles, and 

 also in the expansive force of both ; supersaturation sets in; and if 

 this already existed, it is increased. Hence in air- or carbonic- 

 acid-impregnated water, it may happen that no bubbles can be 

 separated, but if gently heated by a small flame, or a warm metal 

 plate, the action sets in at once. By this simple means soda-water 

 or spring-water that has long been exposed to the air and appa- 

 rently of no further use, may again be brought into activity. In 

 water kept until the next day, in which no bubbles can be sepa- 

 rated (as is almost always the case), a gentle heating causes them 

 to appear anew and swell out in large numbers. 



The experiments with oxidized zinc wire and with scratched 

 brass wire plainly show how important is the mechanical con- 

 dition of the surface on the liberation of gas. It was very 

 interesting to notice the action of the parts of these sap-containing 

 plants the surface of which is slightly rough, such as the stems 

 of the strawberry, milfoil or common yarrow, birch leaves with 

 their peduncles, &c. When these were immersed in the air- or 

 carbonic-acid-impregnated water, their minute projecting points 

 were quickly covered with numerous bubbles, which took up an 

 enormous quantity. The phenomenon was very interesting in 

 water weakly impregnated with air and carbonic acid, when this 

 was gently warmed. As the bubbles gradually formed, they were 

 from the first easily seen ; the smallest appeared on the finest 

 hairs of the pine or Scotch fir, which were entirely covered with 

 them, and in sunshine displayed a brilliant show of colours. 



These oft-repeated experiments convinced me that the air- 

 bubbles at the moment of their appearance are so small as to 

 be invisible, and by slow growth attain a visible size. How the 

 cohesion of the liquid at any given point within it (as is required 

 by the origin of a bubble) is overcome, cannot be made a matter 

 of observation ; but that this takes place by means of the 

 slight roughnesses of the surface, fine points, &c, cannot, ac- 



