168 now CROPS FEED. 



3. Oxygen is often nearly or quite wanting, as in char- 

 coal, oxide of iron, alumina, river silt, and whiting. 



4. Carbonic acid, though sometimes wanting entirely, 

 is usually abundant in the absorbed gases. 



5. In the pores of charcoal and of soils containing de- 

 caying organic matters, carbonic acid is often partially re= 

 placed by carbonic oxide. The experiments, however, do 

 not furnish proof that this substance is not formed under 

 the influence of the high temperature employed (284° F.) 

 in expelling the gases, rather than by incomplete oxidation 

 of organic matters at ordinary temperatures. 



6. A substance, wlien moist, absorbs less gas than when 

 dry. In accordance with this observation, De Saussure no- 

 ticed that dry charcoal saturated Avith various gases evolv- 

 ed a good share of them when moistened Avith water. 

 Ground (and burnt ?) coffee, as Babinet has lately stated, 

 evolves so much gas when drenched with water as to burst 

 a bottle in which it is confined. 



Tiie extremely variable figures obtained by Bluratritt 

 wlien operating with the same substance (the figures given 

 in the table are averages of two or three usually discordant 

 results), result from the general fact that the proportion 

 in which a number of gases are present in a mixture, in- 

 iluences the proportion of the individual gases absorbed. 

 Thus while charcoal or soil will absorb a large amount of 

 ammonia from the pure gas, it will take up but traces of 

 this substance from the atmosphere of which ammonia is 

 but an infinitesimal ingredient. 



So, too, charcoal or soil saturated with ammonia by ex- 

 posure to the unmixed gas, loses nearly all of it by stand- 

 ing in the air for some time. This is due to the fact that 

 gases attract each other, and the composition of the gas 

 condensed in a porous body varies perpetually with the 

 variations of composition in the surrounding atmosphere. 



It is especially the water-gas (vapor of water) which is 

 a fluctuating ingredient of the atmosphere, and one which 



