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it in compost, or manure heaps, tlic}^ give up their silex in a soluble form^ 

 capable of again being taken up in the nourishment of other plants. 



As this substance may, and does exist in two conditions in the soil — 

 soluble and insoluble — it performs a double office, in both parts of which 

 it is of vast importance. As an insoluble portion of the soil, it acts 

 mechanically, in giving porosity to it, so that both the fruitful showers 

 and the invigorating atmospheres, loaded with the peculiar food of vegeta- 

 bles, may permeate its substance. If soils contain too little of it, they are 

 compact, close and impervious ; if too much of it, water runs through 

 them too rapidly, thus leaching them and carrying off their fertile ingre- 

 dients to lower depths. Soil is not thought to be overdosed with silex when 

 it amounts to 65 or 70 per cent. Above that proportion, it becomes too 

 loose, and not sufficiently retentive of fertile matters, and is capable of 

 producing crops only by annual and heavy additions of manure. 



As a soluble portion of the soil it becomes an important ingredient in 

 the composition of plants, and here, as in the soil where it is insoluble, it 

 acts mechanically in giving strength to the stems of all our cereals — 

 wheat, rye, oats, corn, (fee, and to all the grasses. Thus it is that in the 

 straw of all these plants, a very large portion is silex, nearly 70 per cent, 

 while in the seeds or grain of these plants it is only from two to three 

 per cent. Where it is wanting in the soils the stems of crops growing- 

 upon them are weak and unable to support themselves, like the soft limbs 

 of a young child, or the new bones of any animal. We see that silex, 

 in this state, is the sustaining and protecting agent in the tissues of vege- 

 tables, and, in the other, furnishing them with a suitable footing and 

 medium through which nutritious matters are brought to their roots. 



Lime. — Pure caustic lime, or quick lime, never exists for any length 

 of time in nature, as its affinity* for carbonic acid will cause it to take it 

 from the atmosphere, and thus form the common limestone, or carbonate 

 of lime. As a carbonate of lime, it acts only mechanically in the soil, 

 the same as silex, rendering it generally more porous. In this state of 

 combination it is insoluble in pure water, the same as is silex. Nature, 

 in this case also, however, has made abundant provision for its solution. 

 Water charged with carbonic acid dissolves it quite rapidly, and rain 

 water, having a great affinity for carbonic acid, as it is passing through 

 the atmosphere, becomes charged with it and is thus prepared to dissolve 



* By affinity is meant the tendency of bodies to unite and remain united. 



