86 SCIENTIFIC AMUSEMENTS. 



is visible ; if, however, the liquid be well stirred, or if a rough 

 object (such as a crust of bread or lump of sugar or stone) be 

 dropped in, a further disengagement of bubbles of gas takes 

 place ; similarly a glass of sparkling wine that has stood awhile, 

 and apparently become "still," can be made to recommence 

 bubbling by stirring it up with a spoon or crust of bread. 

 This arises from the circumstance that the surplus gas origi- 

 nally contained in the aerated water or wine does not all 

 escape on first pouring out, but a portion is retained for awhile, 

 forming a "supersaturated" solution, just as a supersaturated 

 solution of a solid may be prepared by dissolving in a hot fluid 

 more solid than it can permanently retain dissolved when cold, 

 and then cooling without shaking (Expt. 63). In each case the 

 surplus dissolved matter gradually separates, if the solution is left 

 to itself, whilst the separation is brought about far more rapidly 

 by shaking or stirring up, and especially by contact with por- 

 tions of foreign solid matters that can act as a nucleus. Most 

 gaseous solutions behave in the same way ; whilst if the solution 

 is not supersaturated to begin with, it can often be made so by 

 cautiously warming it; the warmer fluid being unable to retain 

 as much gas dissolved as it could when cooler, becomes super- 

 saturated for the time being, so that on stirring it up, or bring- 

 ing foreign solid matter in contact with it, bubbles of gas are 

 disengaged. 



Many natural spring waters issue from the soil comparatively 

 cool, and these contain dissolved no more gas (usually carbonic 

 acid) than suffices to saturate them, or not quite so much ; but if a 

 tumbler of such water be allowed to stand a few minutes on a warm 

 summer's day, bubbles of gas will begin to form here and there 

 on the glass, especially at the points where it is a little roughened ; 

 owing to the slight rise of temperature brought about by contact 

 with the warmer air, the liquid is unable to retain in solution all 

 the gas that was completely dissolved whilst cooler, so that it first 

 becomes slightly supersaturated as regards the higher temperature 

 attained, and then allows the surplus dissolved gas to escape in 

 bubbles. The peculiar sensation on the tongue and palate felt 

 when the mouth is filled with effervescent water or wine, &c., is 

 largely due to the same cause ; the liquid becomes warmed in the 

 mouth, and consequently bubbles of gas form in contact with the 

 tongue, chiefly on the "papillae," or little roughnesses on its 

 surface, producing a sensation different from that due to the mere 

 taste of the fluid. This sensation is still felt, on filling the mouth 

 with aerated fluid that has stood just long enough to be no longer 

 visibly effervescent ; bubbles of gas form on the papillae precisely 



