80 SCIENTIFIC AMUSEMENTS. 



&c. Variations in pressure do not affect the solubility of solids in 

 liquids to anything like the same extent. 



Again, in the case of solids, the general rule is that the higher 

 the temperature the more substance can a given quantity of solvent 

 dissolve, although there are some exceptions to this rule (Chapter 

 V.) ; but with gases the opposite is uniformly the case ; the 

 warmer the solvent, the less gas can it dissolve. 



Expt. 74. To make a Cold Liquid apparently Boil without 

 becoming perceptibly Heated. Pour into a small flask enough 

 strong solution of ammonia (liquor ammonice fort., sp. gr. '880) to 

 fill it one-third full, and then gently warm it over a lamp ; if the 

 solution be strong enough, you will immediately see a torrent of 

 bubbles arise in this flask precisely as though it were boiling, 

 although the heating has hardly been continued long enough to 

 make the flask feel warmer to the hand than at first. The 

 bubbling arises from the fact that the liquid has really become a 

 little warmer, and in consequence cannot dissolve as much gas as 

 when cold, so that the surplus escapes with effervescence, much 

 as the bubbling is caused in soda-water released from a siphon, 

 except that in the latter case the inability of the water to retain 

 all the gas dissolved is due to diminution of pressure, and in the 

 former to slight increase of temperature. When water is saturated 

 with ammonia gas at the ordinary temperature, it dissolves several 

 hundred times its bulk of the gas, and increases somewhat in 

 volume itself in so doing. 



Expt. 75. To collect a Jar of Ammonia Gas. The bubbles 

 escaping in the last experiment consist of the gas ammonia, which 

 possesses a very strong odour, and produces a choking sensation when 

 inhaled in too large a proportion along with air ; it is not, however, 

 directly poisonous, except through the irritation it thus causes, and 

 when largely diluted with air is refreshing and gently stimulating, 

 being the essential constituent in the vapours given off by "spirits 

 of hartshorn" (somewhat diluted solution of ammonia) and 

 ordinary "smelling salts," Expt. 51; sal volatile, a medicinal 

 stimulant, also smells of ammonia, being in point of fact essentially 

 a solution of ammonia in alcohol. Ammonia gas is lighter than 

 air ; consequently, in order to collect some of the gas for experi- 

 ments, it suffices to fix in the flask used in the previous experiment 

 a cork with a round hole perforated therein, through which passes 

 tightly a piece of quill glass tubing a foot or so in length ; or to 

 close the flask with a tubulated india-rubber cap through which 

 the glass pipe passes (fig. 43). By holding a narrow jar (or piece 

 of wide glass-tubing with the end sealed up by the blowpipe or 

 tightly corked), mouth downwards, over the tube, any gas escaping 



