284? Chemistry. [Lecture 34. 



into still smaller fibres. It is composed of oxy- 

 gen, carbon, hydrogen, nitrogen, and lime. 



23. Suber, or cork, is the bark of a species of 

 the oak. Nitric acid corrodes, dissolves, and 

 decomposes it, converting it partly into suberic 

 (a peculiar) acid, and partly into a substance re- 

 sembling wax. 



24. The only alkalies found in plants are 

 potass and soda. 



25. The only earths are lime, silica, magnesia, 

 and alumina. Silica exists in many of the grasses 

 and equisetums or horse-tails. That excellent 

 philosopher Sir H. Davy has demonstrated that 

 it forms a part of the epidermis, or outermost 

 bark, of those plants ; and that the Dutch rush 

 owes its property of polishing wood entirely to 

 this circumstance. 



26. The most common metal in plants is iron, 

 and sometimes manganese. 



Before I conclude this part of my lecture, I 

 must add a few words upon that spontaneous 

 change which some vegetable matters undergo, 

 and which is called fermentation* This change 

 generally takes place in those vegetable sub- 

 stances which contain a quantity of sugar and 

 of mucilage, with a proper quantity of water, 

 and a heat of from 60 to 70. It is called the 

 vinous fermentation, because in this way vinous 

 fluids, including beer and cider, are prepared. 



The old chemists established three stages of 



