92 LESSONS IN CHEmSTRY. 



a large amount of phosphate of lime, (53) the compotmd 

 from which bones derive their chief fertilizing qualities, and 

 might, therefore, be advantageously employed as manure. 



133. In the variety of composition presented by the rocks 

 upon which the soil rests, we have a striking instance of wise 

 arrangement ; did they consist of merely one kind of matter, it 

 would be impossible that our cultivated plants could grow, for 

 as you have seen, we are not acquainted with any of these plants 

 that does not require more than one kind of matter for its food.* 

 In every country, however, not only have we a great variety 

 of rocks, but few of these are absolutely pure; limestone does 

 not contain merely lime, nor the quartz rocks of Donegal 

 merely silica. In these apparently pure rocks chemistry dis- 

 covers minute quantities of other ingredients. The more 

 complex the composition of any rock, the greater probability 

 of the soils formed from it proving fertile. This is illustrated 

 by the well known productiveness of those districts where 

 several kinds of rock occur. 



1 34. Organic matter of the soil. It has already been stated 

 (115) that when a portion of soil is burned until all black- 

 ness has disappeared, it is found to have decreased in weight, 

 and that the incombustible ash which remains consists of the 

 saline and earthy inorganic matters which are contained in the 

 rocks by the decay of which it was produced, while the part 

 that consumes is composed of the remains of the vegetables 

 that have grown upon it, or have been mixed with it by the 

 farmer. In all cultivated soils in this country, we discover 

 a considerable proportion of this organic matter, and in peaty 

 soils it frequently amounts to 60 or 70 per cent. In tropical 

 countries the decay of the vegetable matter proceeds rapidly, 

 and all the carbon of dead plants is restored to the atmos- 

 phere in the form of carbonic acid, but in Ireland and other 

 cold countries this change is not so perfect, and the remains 

 of vegetables accumulate in the soil, and when moisture is 

 present frequently undergo a peculiar kind of decomposition, 

 by which they are converted into the substance termed peat. 



* The teacher will observe, that I have said that all our cultivated 

 plants require more than one kind of mineral food, but respecting the 

 exact number of substances which are essential to their growth, we are 

 not yet sufficiently informed to decide. The only plants which have yet 

 been discovered to contain no inorganic matter, are the little mould 

 plants, which form on the surface of solutions of sugar and other organic 

 substances. (Mulder.) 



