110 PRINXIPLES OF GARDENING. [CH. II. 



This partly arises from causes already explained 

 when treating of the influence of the colour of soils 

 upon vegetation. Some manures, as salt, protect 

 plants from suffering by sudden reductions of tem- 

 perature, by entering into their system, stimulating 

 and renderuig them more - vigorous, impregnating 

 their sap, and consequently rendering it less liable to 

 be congealed^. 



Every cultivator of the soil, by certain empirical 

 signs, may be able to determine that ceitain appli- 

 cations are required to render his land produc- 

 tive: for example, he knows when chalk may be 

 applied to advantage ; but no lengthened practice has 

 yet enabled any one to judge of the quality of a chalk 

 by its exterior appearance. ChemistiT alone can do 

 this. The fanners of a district in Yorkshire having 

 experienced the benefit of lime, procured some from 

 a neighbouiing kiln, and were astonished to behold 

 the injury it caused to their crops, and it remained 

 an anomaly of their experience, until chemistry 

 demonstrated that the lime near home contained a 

 very large proportion of magnesia, which, absorbing 

 carbonic acid very slowly, remained in a caustic state, 

 to the injuiT of the roots of the plants, and the diminu- 

 tion of benefit from the carbonic acid evolved by the 

 decomposing constituents of the soil. 



The experiments of Saussure demonstrate the 

 ^ Cuthbert Johnson's valuable work on Fertilizers. 



