SCIENCE OF COMMON THINGS. 305 



Tanning of hides to form leather. Vinegar Alcohol. Yeast. 



Through the agency of the nitric acid contained in it. 



196 6 Do plants produce acids f 



Acids are formed in the vegetable kingdom in great 

 abundance / they especially exist in unripe fruits, im- 

 parting to them a sour taste. 



Acids formed from mineral substances are called "mineral acids;'' 

 acids formed by or from vegetable subsiunces are called " organic acids." 



196*7 Why does tanning hides convert them into leather f 



Hides are steeped in water, with ground bark of the 

 oak, hemlock, or other trees ; these barks contain large 

 quantities of tannic acid, which combines with the 

 skin of animals, and forms a combination which is in- 

 soluble in water and not subject to putrefaction viz. 

 leather. 



1968 What is ordinary vinegar ? 



An acid, called acetic acid, and water. 



I960 If wine or beer be imperfectly corked, why does it rapidly turn 

 sour? 



Because air gets into the liquor, and the oxygen of 

 the air combining with the alcohol of the liquor pro- 

 duces acetic acid, or vinegar. 



1970 What is alcohol? 



Alcohol is the spirit existing in wine, beer, cider, 

 etc., obtained in the process of fermentation. 



1971 What is a ferment ? 



A ferment is a substance containing nitrogen in a 

 state of decomposition, which is able to excite fermen- 

 tation in solutions of sugar ; old cheese, putrefying 

 flesh, blood, etc., all of them are ferments. 



1973 What is yeast? 



We apply the term yeast to a particular species of, 

 ferment ; the foam of beer (or of some similar liquor), 

 produced by fermentation. 



1973 Can you explain why it is that a body in a state of fermentation 

 or putrefaction should cause unlimited quantities of similar matter to pass 

 into the same state ? 



e only know the f act : the reason we are ignorant 

 of. The most minute portion of milk, paste, juice of 



