Composition of Waters of Land-Drainarje and of Rain. 141 



and we see in this instance how very much we may be misled if 

 we neglect these circumstances in forming an opinion.* 



The same remark holds good with regard to phosphoiic acid. 

 The largest amount of this substance which, in any of the in- 

 stances, would be found in the drainage of the year, is 4 lbs., 

 which is equivalent to about 8 lbs. of phosphate of lime, a 

 quantity which the decimal just given for potash would equally 

 represent. 



It may be thought that this reasoning should be extended to 

 nitric acid ; but apart from tlie fact that the quantity of this 

 substance is so enormously large, as compared with the potash 

 and phosphoric acid, it is to be remembered that nitric acid does 

 not, like them, exist in the soil, but is in all probability a pro- 

 duct of the manure applied, and as such a direct loss. 



We turn now to tlie subject wliicli forms the next division of 

 our inquiry — 



3rd. IVlience are these substances derived ? 



Tliat all the mineral matters found in drainage-water are 

 derived either from the soil or the manure added to it there can 

 be no question. With the exception of some one or two sub- 

 stances (such as common salt), which are carried meclianicallv 

 in the air from the ocean, we do not find in rain-water any 

 appreciable amount of mineral matter. But this is not so obvi- 

 ously the case with ammonia and nitric acid ; in fact, we very 

 well know at the present time tliat both these substances do exist 

 in rain-water ; and should it so happen that their quantity were 

 sufficient, we might readily attribute the presence of these com- 

 pounds of nitrogen in drainage-water to the rain. 



That the ammonia in rain equals in quantity that in 

 drainage-water seemed credible enough, but it would hardly 

 be anticipated that so great an amount of nitric acid should be 

 found in rain-water ; and indeed the circumstance that in one 

 sample of dramage-water there should be but half a grain or less 

 per gallon, and in another as much as 14 grains, seemed very 

 mucli op])osed to such a view. However, it was absolutely indis- 

 pensable that all doubt ontiiis point should be removed ; and for 

 this purpose it became necessary to institute a careful examina- 

 tion of rain-water itself. 



I must briefly recall to the memory of my readers some cir- 

 cumstances connected with this cpiestion of the comj)osition of 

 rain-water, which 1 brought before them on a previous occasion. 



♦ Wu must, however, in the case of highly cultivated soils be eqiiiilly pnarded, 

 on the other side, against niultipl} ing riinlitij In/ iiindiliti/. There are few soils in 

 England upon wliicii a calculation of the potash, or any otlur nianiirial substance 

 not specially abundant, could be assumed as constant, from the cultivated surface 

 to the (lei)tli acted on by 4-feet drainage. — C. W. II. 



