AGRICULTURE AND RURAL ECONOMY. 545 



air in the wood was almost twice as rich in carbonic anhy- 

 dride as in the open at least in summer; (2) the wooded 

 soil, on the contrary, had much less carbonic anhydride in 

 summer; the meadow land, which was in good condition, 

 having six times as much at a depth of half a meter, and 

 five times as much at one meter as the wood-soil ; (3) with in- 

 crease of temperature the carbonic anhydride increased much 

 more in meadow than in forest land; and (4) the carbon- 

 ic anhydride appears to diffuse very slowly, that from soil 

 under acacia bushes having little more than half as much as 

 uncovered soil close by. These various results are ascribed 

 to a difference in the temperature of the soils in summer, the 

 greater porosity of the meadow soil, and the more intimate 

 mixture of vegetable matter in the cultivated soil. Since 

 the carbonic anhydride is one of the most efficient agents in 

 disintefxratino: the materials of the soil, it is evident that the 

 preparation of plant -food will go on much more slowly in 

 wood than in cultivated ground. 



is 



Fertility of Yolcanic Soils. 



The cases in which chemical analysis of a soil accords with 

 its observed fertility are not so many as to be uninteresting 

 when they occur. Truchot has lately analyzed some volcan- 

 ic rocks and soils which come from them, and with these a 

 granite and an alluvial soil. The lava soil was the most fer- 

 tile. The soil and the rock it came from were both very 

 rich in lime, potash, and phosphoric acid. The granite soil 

 was less fertile. Both it and the granite rock had extremely 

 little lime and phosphoric acid, and not much potash. The 

 alluvial soil, which was the richest, had likewise the largest 

 percentages of these ingredients. 



Analysis of Soils. 



The questions to which decided and satisfactory answers 

 are frequently given by analyses of soils may, according to 

 Dr. Voelcker's experience, be summed up as follows: 



" 1. Whether or not barrenness is caused by the presence 

 of an injurious substance, such as sulphate of iron or sulphide 

 of iron, occasionally occurring in peaty and clayey soils? 



"2. Whether soils contain common salt (land flooded by 

 sea-water), nitrates, or other soluble salts, that are useful to 



