CARBON AND SOME OF ITS COMPOUNDS 171 



Carbon dioxide is also produced during fermentation, the " rising " 

 of bread being due to the escape of the gas which is generated by the 

 fermentation, under the influence of the yeast, of the saccharine matters 

 formed from the starch. 



Carbon dioxide will not support life, and is sometimes used for 

 suffocating stray dogs and cats. 



By cold and pressure carbon dioxide may be liquefied and also 

 solidified, forming a soft white substance, which when mixed with 

 ether forms a powerful freezing mixture, the temperature sinking to 

 about -100C. 



Occurrence of carbon dioxide. Carbon dioxide has been already 

 stated to exist in the atmosphere, and to be produced by the oxidation 

 of animal and vegetable tissues ; also that under the influence of sunlight 

 it is reconverted by the green parts of plants into its constituents, of 

 which the carbon is used by the plant in the formation of new tissue. 

 The gas is found in many natural gaseous emanations, and is frequently 

 present to a large extent in the gases of caves and underground passages, 

 where, owing to its high density, it tends to accumulate if formed by 

 fermentation or other natural processes. In expired air, carbon 

 dioxide is present to the extent of about 4*7 per cent. Although such 

 air is not again respirable, this is probably due to the diminution of 

 the oxygen, for it is doubtful whether carbon dioxide has any direct 

 poisonous effect. The proportion of carbon dioxide may be increased 

 even to 20 per cent, without immediate serious effects if only the 

 quantity of oxygen be increased simultaneously. 



82. PRODUCTION OF CARBON DIOXIDE BY 

 HEATING CHALK. 



i. Change produced by heating chalk. Place a little powdered chalk 

 (not blackboard chalk) on a piece of platinum foil, and heat it strongly for 

 some minutes in the flame of a laboratory burner. If platinum foil is not 

 at hand, heat a lump of chalk on a piece of coarse wire gauze for some time. 

 After heating, shake the powder on to a damp red litmus paper. Observe 

 that the red litmus paper is in places changed to a blue colour. 



ii. Action of lime upon litmus. Test some wet lime with litmus paper. 

 Observe that it changes the colour of red litmus to blue. 



iii. Powder obtained by heating chalk. Shake up in ordinary tap- 

 water some of the powder obtained in Expt. 82 i. by heating chalk on 

 platinum foil. Filter, or allow the powder to settle ; then taste and notice 

 the peculiar taste of lime-water. 



iv. Loss of weight when chalk is heated. Weigh a crucible ; add 

 a gram of finely powdered chalk. Place it in a muffle furnace for about 

 an hour at a bright red heat, allow to cool and weigh. Heat the crucible 



