ON THE AIR AND WATER OF TOWNS. 69 



subjects the soil however to a loss by drainage. The use of nitrates as a 

 manure has long been known to be valuable ; our old English philosophers 

 have been well-aware of it, for example, Boyle and Bacon. Before the nitre 

 can be formed in the soil, the ammonia must be oxidized ; and before it can 

 be used as food for plants, the process must be reversed and it must become 

 deoxidized. The oxygen thus stored up may then readily be applied for rapid 

 oxidation when this is wanted, and in this way it becomes a kind of concen- 

 trated atmosphere, a source of air by which to form carbonic acid from the 

 mould of the soil. We can readily view it as a great stimulator of vegetable 

 life, besides being a source of nitrogen to feed the plants. In dry climates, 

 perhaps it might be viewed as a supply of water, as we might readily consider 

 it formed by the union of the oxygen with the hydrogen of substances in the 

 ground. The organic matter in a soil may be supposed to decompose in 

 two methods, by the formation of ammonia and of nitric acid. If the soil is 

 very alkaline and moist, the conversion of the organic matter into ammo- 

 niacal compounds is very rapid. I put some soil, not very rich in organic 

 matter, into this condition by the assistance of a little ammonia so as to make 

 it alkaline, and the consequence was the rapid occurrence of a very intense 

 putrefactive decomposition, not in any Avay differing, as far as could be per- 

 ceived, from that of ordinary putrefaction of animal and vegetable matter. 

 These nauseous and unwholesome odours are therefore possible from the 

 ordinary soil of our fields ; but any occurrence such as this on a large scale 

 would be disastrous, and the ground is protected from it by an almost con- 

 stant acidity, whicli sometimes increases so as to be injurious, forming what 

 is called sour land. This very acid state generally occurs in wet land, where 

 it is probable that alkalinity would be most injurious, but the soil may be 

 found alkaline in a well-manured garden and where the ground is dry without 

 apparent injury. The other mode in which the organic matter may decom- 

 pose is by the formation of nitric acid, the nitrogen obtaining oxygen indi- 

 rectly from the air, and so providing against the excess of ammonia which 

 might readily occur in certain soils, producing results which would be fatal 

 to animal life, if not provided for by an enormous vegetation. At the same time 

 it does seem reasonable to suppose that the cause of a diseased climate may 

 frequently be found in such a decomposition as that mentioned, when too 

 much ammonia has been formed, or too little nitrogen has been removed 

 by the formation of nitric acid. All the circum.stances may be found for the 

 purpose, abundance of nitrogen compounds and moisture ; even a warm 

 climate is not essential. And it also seems reasonable to suppose that the 

 formation of nitric acid is one of the means by which such evils are avoided, 

 whether in town drainage, \i here it becomes evident from the great amount 

 found, or in the imperfectly drained and rich, although not swampy lands of 

 tropical climates, where the large amount of nitrogen, if converted into am- 

 monia, would no doubt produce the worst eifects. 



Water alone on soil often becomes too saturated with organic matter for 



use, and either attacks also the living plants, or induces circumstances which 



forbid them to grow. Let us suppose the soil dried up ; the decomposition 



would cease, and the nitrates formed would come out in efflorescence. Let 



us suppose it not dried, but kept constantly moist and cold, and we have the 



1 ground in a state described in a very interesting manner by Bernardini Ra- 



i mazzini, producing a disease which is not unlike the potatoe disease with us, 



I and although perhaps not directly bearing on my subject, may yet come in 



I by a natural association. A wet and mild winter in the territory of Modena 



I was followed by a summer of a similar kind, and " the sound of the grass- 



! hopper gave way to the croaking of frogs." The crops were destroyed and 



