258 YEAST OR FERMENT. 



crystallised pyrites is observed to form on the de- 

 caying roots. 



Now we know that in the putrefaction of wood 

 under water, when air therefore is excluded, a part 

 of its carbon combines with the oxygen of the 

 water, as well as with the oxygen which the wood 

 itself contains ; whilst its hydrogen and that of the 

 decomposed water are liberated either in a pure 

 state, or as carburetted hydrogen. The products of 

 this decomposition are therefore of the same kind 

 as those generated when steam is conducted over 

 red-hot charcoal. 



It is evident, that if with the water a substance 

 containing a large quantity of oxygen, such as sul- 

 phuric acid, be also present, the matters in the 

 state of putrefaction will make use of the oxygen of 

 that substance as well as that of the water, in order 

 to form carbonic acid ; and the sulphur and hydro- 

 gen being set free will combine whilst in the nas- 

 cent state, producing hydrosulphuric acid, which 

 will be again decomposed if metallic oxides be pre- 

 sent ; and the results of this second decomposition 

 will be water and metallic sulphurets. 



The putrefied leaves of woad (Isatis tinctoria), in 

 contact with indigo-blue, water, and alkalies, suffer 

 further decomposition, and the indigo is deoxidised 

 and dissolved. 



The mannite formed by the putrefaction of beet- 

 roots and other plants which contain sugar, con- 

 tains the same number of equivalents of carbon and 



