IX.] GAS LIME 265 



are also added to the potash in the manure, but sinks 

 to 991 when all the alkaline salts are lacking. From 

 this low figure of 991 the addition of soda causes a 

 rise to 14-68, of magnesia to 14-87; the differences in 

 the yield of the plots are in fact reflected in the pro- 

 portions of potash in the ash, though the variations 

 are not so great. But though the addition of soda or 

 magnesia on Plots 12 and 14 causes an increase in the 

 proportion of potash in the ash, neither the magnesia 

 nor the soda in the ash are perceptibly raised. Hence, 

 we may conclude that the whole effect of either sulphate 

 of soda or sulphate of magnesia upon the crop is 

 indirect and due to their attack upon the potash 

 reserves in the soil. 



These results with wheat at Rothamsted are con- 

 firmed by the parallel experiments upon mangolds and 

 grass ; in each case sodium and magnesium salts add to 

 the effect of a potash dressing, and in the absence of 

 potash partially do its work, more in the earlier than in 

 later years of the experiment when the easily attacked 

 soil potash is becoming exhausted. 



Gas lime is a greenish yellow, evil smelling sub- 

 stance obtained during the purification of coal-gas by 

 its passage over trays of freshly slaked lime, which 

 absorbs sulphuretted hydrogen and other sulphur com- 

 pounds from the crude gas. Various sulphides and 

 partially oxidised sulphur compounds of calcium are 

 formed, as may be seen from the analyses set out below, 

 and these are to some extent attacked by the carbon 

 dioxide of the air with the liberation of the original 

 gaseous sulphur compounds. The main action, however, 

 on exposure to the air is one of oxidation, so that 

 eventually the material becomes little more than a 

 mixture of gypsum and calcium carbonate from the 

 uncombined lime. It is in this oxidised form only 



