AGRICULTURAL CHEMISTRY. 169 



bonated water, (i. e., water saturated with carbonic acid.) Thus, to 

 use a well-known instance, carbonate of lime is as good as insoluble 

 in pure water, but in carbonated water it dissolves quite readily. 

 Salts of ammonia dissolve phosphate of lime to a very appreciable 

 extent, as has long been known, and as Liebig has recently shown by 

 quantitative trials. Silica is not absent from natural waters, although 

 the conditions of its solution are not well understood. The chemist 

 has succeeded in preparing strong solutions of silica in pure water, 

 artificially, and the so-called infusoria of all fresh water streams, 

 which sometimes have accumulated to form beds of many miles in ex- 

 tent and many feet in depth, are but the silicious skeletons of micro- 

 scopic vegetable organisms that collected their silica from the clearest 

 and purest water. Phosphate of iron is soluble, or at least yields its 

 phosphoric acid, under the conjoint action of carbonate or silicate of 

 lime and carbonated water. Sulphate of baryta, even, is decomposed 

 in the soil, and yields its sulphuric acid to a growing plant. 



Allusion has already been made to the importance of those matters 

 which, originally belonging to the atmosphere, have become a portion 

 of the soil. 



Pulverized rocks do not constitute a good soil until they have be- 

 come weathered — i. e., chemically decomposed, so as to contain a 

 portion of soluble matters, and also acquire a certain content of car- 

 bon and nitrogen. It happens that these two effects are conjointly 

 brought about. The neighborhood of a volcano affords opportunity 

 for tracing the formation of a fertile soil in a manner analogous 

 to, or identical with what occured over all the land before the 

 human epoch. The lava that lies on the slopes or fills the con- 

 tiguous valleys, once melted rock, remains after cooling, almost 

 bare for years. Then lichens begin to cover its surface. These 

 succeed each other for generations, slowly increasing in number 

 and size, hastening by their decay the disintegration of the rock, 

 and causing the accumulation of humus and nitrates. So the weath- 

 ering of the rock, the use and enrichment of the sparse soil, goes 

 on, perhaps, for centuries before the earth is deep and fertile 

 enough to produce low shrubs. After another similar period a forest 

 is formed, with a soil rich in all that is needed for agriculture, being 

 stored with the fixed minerals that have been detached or solved 

 from the original lava, and having gathered during these ages mate- 

 rials from the atmosphere to make up the complement of fertility. 



We often see railroad cuttings through beds of gravel or clay which 

 perfectly resemble the adjacent productive soil, but which remain for 

 years perfectly naked and barren, and only after a long period of time 

 assume a state of tolerable fertility. 



The humus of the fertile soil, as already stated, does not, perhaps, 

 act to much extent in directly feeding vegetation, although we have 

 no positive evidence against the assumption that it is thus useful in 

 some degree. It does, however, in many indirect ways contribute to 

 the welfare of the plant. Its influence on the physical characters of 

 the soil, its mediating agency in maintaining the proper consistency, 

 moisture, and warmth of the earth, has been already noticed. The 



