ABSORPTION BY SOILS. 277 



COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE BY VARIOUS POWDERS. 



Discussion of the Table. It will be observed that in this table, the 

 largest amount of total gas given off by equal weights of any one sub- 

 stance was in the case of carbonate of magnesia ; but it is quite probable 

 that in part, at least, this large amount of gas was due to the evolution 

 of carbonic gas from the easily decomposable carbonate ; the more as 

 the analysis of the gases shows over 29^ of carbonic gas. But the 

 highest absorption by equal volumes of any substance is shown by the 

 ferric hydrate ; next to this by the light poplar charcoal, and next by 

 the carbonate of magnesia. The high absorptive power here shown by 

 the ferric hydrate is of great interest in connection with the facts already 

 stated regarding the absorption of moisture and ammonia by ferruginous 

 soils (see page 274, this chapter) ; and the fact that the larger proportion 

 of the gas as much as 70% in one case consisted of carbonic gas, is 

 particularly interesting in the same connection. Both in the amount of 

 gas contained, and in the proportion of carbonic gas therein, the ferric 

 hydrate exceeds even peat, the representative of humus in soils. It 

 will, however, be noted that in the garden soil, also, the proportion of 

 carbonic gas is very large, while that of oxygen is very low. It is curious 

 to note that in very few cases the proportion of oxygen to nitrogen is 

 the same as in the atmosphere ; in most cases the nitrogen predominates 

 considerably beyond its normal proportion, and in two cases, that of 



