SULPHATE AND PHOSPHATES OF ALUMINA, ALUM. 205 



phuric acid, it readily dissolves, and forms a solution of sulphate of 

 alumina. This solution is characterized by a remarkable and almost 

 peculiar sweetish astringent taste. When evaporated to dryness it yields 

 a white salt, which dissolves in twice its weight of water only, and when 

 exposed to the air, attracts moisture rapidly, and spontaneously runs to 

 a liquid. This salt exists in some soils, especially in those of wet, 

 marshy, and peaty lands. 



No experiments have yet been made with the view of determining its 

 direct influence upon vegetation. 



4°. Phosphates of Alumina. — In combination with phosphoric acid, 

 alumina forms one compound well known to mineralogists, by the name 

 o( wavetlite. This mineral, however, occurs in too small quantity to be 

 an object of interest to the agriculturist. 



Phosphoric acid is disseminated in some form or other throughout our 

 clayey soils, though in very small and variable quantity. It is most 

 probable that in these soils a portion of the acid at least is in combina- 

 tion with the alumina in the state of phosphate. One of the most diffi- 

 cult problems in analytical chemistry is toefTect a perfect separation of a 

 small proportion of phosphoric acid from alumina, and rigorously to esti- 

 mate its quantity ; hence in the greater part of the analyses of soils hitherto 

 published, this most important ingredient in a fertile soil (the phosphoric 

 acid), when in combination with, or in presence of alumina, has either 

 been altogether neglected, or rudely guessed at, or indicated by a rough 

 approximation only. We have no direct proof, therefore, of the extent 

 to which the phosphates of alumina exist in different soils. 



5°. Alum. — The common alum of the shops owes its well known 

 sweetish astringent taste to the presence of the above sulphate of alumi- 

 na. It consists in 100 parts of about 40 of sulphate of alumina, 14^ of 

 potash, [described p. 189,] and 45j of water. Alum is formed naturally 

 on many parts of the earth's surface, especially as an efHorescence on 

 certain soils, and on some rocks when exposed to the air, — as on the 

 alum shales of the Yorkshire coast. It is largely manufactured by cal- 

 cining, and afterwards washing these alum shales. 



Alum has not been extensively tried as a manure. Its composition, 

 liowever, would lead us to expect it to exert a beneficial influence on the 

 growth of many plants — while the price, especially of the less pure va- 

 rieties, is such as to admit of its being applied to the land at a compara- 

 tively small cost. From some experiments made on a small scale, 

 Sprengel considers it highly worthy the attention of the practical agri 

 culturist. 



X. SILICA, SILICON, SILICATES OF POTASH, OF SODA, OF LIME, OF 



MAGNESIA, AND OF ALUMINA. 



1°. Silica. — The chief ingredient in all sand-stones and in nearly all 

 sands and sandy soils, is known to chemists by the name of silica. Flints 

 are nearly pure silex or silica — common quartz rock is another form of 

 the same substance — while the colourless and more or less transparent 

 varieties of rock crystal and chalcedony present it in a state of almost 

 perfect purity- It exists abundantly in almost all soils, constituting 

 what is called their siliceous portion, and is found in the ashes of all 

 plants without exception, but especially in those of the grasses. Silica 



