VII.] CHEMICAL CHANGES DURING DUNG-MAKING 187 



hydrate material as straw affords, and in the presence 

 of oxygen burn it up completely into carbon dioxide, 

 water, and inorganic ash. Such organisms, however, 

 do not play a very large part in manure-making, 

 because oxygen soon gets excluded from the mass ; 

 the work is taken up instead by other bacteria 

 capable of working in the absence of oxygen. Two 

 of these only have been as yet studied in any 

 detail ; they both rapidly attack carbohydrates like 

 cellulose, and give rise to carbon dioxide, marsh 

 gas or hydrogen respectively, certain fatty acids, of 

 which butyric is the chief, and the indefinite brown 

 acid substance known as "humus," which is richer in 

 carbon than the original carbohydrate. The evolution 

 of such gases can easily be demonstrated during the 

 making of dung, either by laboratory experiments or 

 by an analysis of the gases extracted from a dunghill. 



Table LI 1 1, shows the gases extracted from a fresh 

 dunghill by Deherain during one of his experiments at 

 Grignon. 



When the first sample was taken, the dungheap was 

 still in process of formation, and was in too dry a 

 condition. The hydrogen fermentation was most 

 prominent at this stage, and hydrogen and carbon 

 dioxide were the most prominent gases. On that 

 day the liquid manure was pumped up over the 

 whole mass, and fermentation became more active, as 

 seen by the very high temperatures reached on the 

 24th, when the formation of hydrogen had diminished, 

 while that of marsh gas had increased greatly. The 

 analyses on 30th August show the result of having 

 again let the heap get dry ; the top and middle were 

 full of air, as may be seen from the large proportions 

 of nitrogen and the presence of some oxygen; the 

 percentage of carbon dioxide had also become so 



