224 PHYTOCHEMY. 



of plants, which increase the quantity of carbonic acid in the 

 moisture of the soil, as well as in the atmosphere, we shall 

 find ourselves obliged to admit that this substance is the pro- 

 per nourishment of vegetables. To these circumstances belong 

 dew and rain, which convey more carbonic acid to plants, 

 than that which is supplied by spring water ; on which account, 

 this latter fluid is always inferior to rain-water for the sprink- 

 ling of plants. From the same cause arises the uncommon 

 fertility of volcanic countries, which, according to Gagliardo, 

 give out a peculiarly great quantity of carbonic acid. Our 

 own black mould is also so productive for the same reason, 

 namely, because the extractive matter which it contains is in- 

 cessantly attracting oxygen from the atmosphere, to form car- 

 bonic acid. Hence also arises the advantage in horticulture 

 of screens for fruit trees, because the surface of the earth, 

 when overshadowed by great leaves, attracts carbonic acid 

 vapours more strongly, and these also can less readily escape 

 than they must do from a bare field. On the same account, 

 Vetch-Oats and Summer-Rye succeed better when sowed 

 among Peas. Beans also are sowed among Oats, and this 

 mixture is called Rough-cast. Clover is commonly sowed 

 with some other crop, in particular with Rape, Flax, Peas, 

 and even with winter-crops. 



It is hence that heath is so little injurious to forests, that 

 it rather affords to the young shoots the advantage of shade, 

 and a more powerful attraction of carbonic acid. 



But every just theory of the effects of manure, is in the high- 

 est degree favourable to this assertion. Manure consists com- 

 monly of intermingled animal and vegetable matter ; in the 

 excrements of swine in particular, we find this latter sub- 

 stance, and especially seeds, so unchanged, that they readily 

 vegetate. But the excrements of animals are extremely apt 

 to become putrid, and it is necessary, in order to subject 

 them to a slower fermentation, during which carbonic acid 

 is produced, to mix them with straw, and other refuse of 

 vegetables. The manure of sheep, when mixed with 

 straw, is therefore much more lasting than the excrement 

 by itself ; and the refuse of the fold, unless it is very 



