xx, CARBOX AND SOMK OF ITS COMPOUNDS 21 



also produced during fermentation, the "rising" of bread being 

 due to the escape of carbon dioxide generated by the fermenta- 

 tion of the saccharine matters formed from the starch under the 

 influence of the yeast. 



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 - 100 C. 



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, while further, 

 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 tissue. It is also 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 fermenta- 

 tion 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 partly due to the diminution of 

 the oxygen, and it is doubtful whether carbon dioxide has 

 any direct poisonous effect. The proportion of carbon dioxide 

 may be increased to even 20 per cent, without immediate serious 

 effects if only the quantity of oxygen be simultaneously increased. 



Proof of formula C0 2 . The density of carbon dioxide should 

 be found in the manner indicated for hydrochloric acid gas, and 

 will be found to be 22. As, further, the combustion of carbon in 

 oxygen leads to no change of volume, the formula CO 2 for the 

 gas, with the atomic weight of carbon = 12, is indicated. 



Hardness of Water. EXPT. 299. To a solution of cal- 

 cium carbonate or chalk, in water containing carbon dioxide, 

 add some soap solution and shake. Observe that a great 

 quantity of soap must be added before a lather is obtained. 

 Boil the solution first, and again add the soap. Observe a 

 lather is readily formed. 



The solution is hence a very hard water, but becomes soft 

 again by boiling. The temporary hardness may then be due to 

 chalk dissolved in water by the aid of carbon dioxide, and the 

 reason for the loss of hardness on boiling is evident, since the 

 chalk is then precipitated and no longer exists dissolved in the 



u 2 



