[303] SCIENTIFIC MANUAL. 17 



The so-called soda water contains no soda; the efferves- 

 cence being produced by the escape of the carbonic acid 

 gas, which is forced by great pressure into water and held 

 there by the soda fountain. As soon as the water is al- 

 lowed to flow from the fountain into the glasses into which 

 the syrups have been placed, the gas, being no longer un- 

 der pressure, escapes, and produces the foaming. 



The carbonic acid is generated on a large scale for soda 

 fountains, by treating some form of carbonate of lime with 

 sulphuric acid. The sulphuric acid combines with the 

 lime, forming sulphate of lime, and setting the carbonic 

 acid free. This is collected in a reservoir, and then forced 

 into water, for use as the so-called soda water. 



This effervescence is utilized by housewives in making 

 bread. Yeast powders (which are composed of carbonate 

 of soda and a little tartaric acid) or carbonate of soda and 

 sour milk, are mixed with flour for making bread. By the 

 action of heat the carbonic acid is set free, causing the ex- 

 pansion of the dough, or the "rising of the bread." 



Carbonic acid gas is exhaled from the lungs of all ani- 

 mals. It is estimated that 140 gallons of this gas are 

 thrown off from the lungs of a grown person in 24 hours. 

 At this rate, the air in a close room, occupied by a number 

 of persons, will soon become charged with this poisonous 

 gas. The importance of proper ventilation is, therefore, 

 apparent. The office of carbonic acid in plant nutrition, 

 and the reciprocal supply and consumption of carbonic acid 

 and oxygen, will be discussed in their appropriate places. 



Combustion, in the popular definition of the term, is the 

 decomposition of a substance, attended with light and heat. 

 In the true sense of the term, these phenomena do not al- 

 ways attend it. 



Combustion proper is the union of oxygen with another 

 substance, forming a new compound. 



Combustion may be rapid, as is the case when coal or 

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