CARBON DIOXIDE. 107 



of temperature (Chapter XX.), they nevertheless tend to inter- 

 mix, an action of diffusion being set up precisely as with liquids 

 (Expt. 90), but in a much more marked way and far more rapidly, 

 owing to the much greater mobility of gases. If, as is usually the 

 case, the two gases are not of the same density, the heavier one 

 will rise gradually and intermix with the lighter one, which will 

 sink during the process, precisely as the liquids above described. 



Expt. 99. To generate Carbon Dioxide from Chalk. In 

 Expt. 72 we have seen that carbon dioxide is produced when 

 tartaric acid, carbonate of soda, and water are brought into contact 

 with one another in a " gasogene," or even in a tumbler. Chalk 

 and limestone are substances analogous to carbonate of soda in this 

 respect that when they are brought into contact with solutions of 

 acids of sufficient power, they are acted on in a similar fashion, 

 carbon dioxide gas being evolved. This result is brought about 

 by simply pouring some diluted hydrochloric acid upon some 

 fragments of limestone or lumps of chalk at the bottom of a 

 tumbler ; a vigorous effervescence is produced, and the air in the 

 tumbler soon becomes replaced by the evolved gas, which may 

 be tested, as in Expt. 71, by lowering into the tumbler a lighted 

 candle at the end of a wire, when the candle will be extinguished. 

 In the manufacture of aerated water, the gas is produced by acting 

 on chalk, &c., with a strong mineral acid (usually sulphuric acid, 

 for the sake of cheapness), and is then pumped into the water and 

 bottled under pressure. 



Expt. 100. To produce a Current of Carbon Dioxide Gas. 

 In experiments with gases it is requisite to produce them inside a 

 suitable vessel completely closed in, but furnished with an exit 

 pipe or tube, through which the evolved gas can pass off in a 

 current, so as to allow of its being collected in bottles or jars, &c., 

 or otherwise treated as may be required. A convenient form of 

 vessel, or gas generator, suitable for the production of gases where 

 the materials require heating, is that indicated by fig. 55 (some- 

 what similar to that used for the production of ammonia, Expt. 

 74) the exit pipe being shortened and bent downwards ; or an 

 india-rubber tube may be attached so that the liberated gas can be 

 led away through the flexible tube to wherever it may be required. 

 In order to introduce more fluid, if requisite, the cork should be 

 doubly perforated (fig. 56), or the flask closed by a two-necked cap 

 (fig. 30), a " thistle funnel " being introduced through one orifice 

 ( passing to the bottom of the flask), and the exit pipe or " delivery 

 tube" through the other perforation. When heat is not requisite, 

 the thin flask may be advantageously replaced by a stout flat- 

 bottomed bottle, such as a pickle-bottle, provided with a well-fitting 



