278 ANNUAL OF SCIENTIFIC DISCOVERY. 



wretched a grass as Poa annua exhales in 120 days (too low a compu- 

 tation) of active vegetation, 6,000,000 pounds of water. To supply 

 the exigencies of the plants, therefore, it is only necessary for the 

 meadow to imbibe 3 grains of carbonic acid with every pound of 

 water. 



Mr. Lawes has found, also, that in a plant of any one of our ordi- 

 nary crops, more than 200 grains of water must pass through it for a 

 single grain of solid substance to accumulate within it. He states the 

 evaporation from an acre of wheat during the period of its growth to 

 be 114,860 gallons, or 73,510,000 gallons per square mile. With clo- 

 ver, it is rather more; with peas and barley, less. "When we apply 

 these calculations to a county or a kingdom, we are lost in the magni- 

 tude of the processes by which nature works ; but we see the more 

 clearly that, on such a scale, the quantity of material supplied by the 

 air, though minute to the individual, becomes vast in the aggregate. 

 "We see, moreover, the necessity for understanding the relations be- 

 tween evaporation and rate of growth, and the laws and effects of 

 absorption in soils. A thousand pounds of dry calcareous sand will 

 gain two pounds in weight in twelve hours when the air is moist, while 

 pure agricultural clay will gain thirty-seven pounds. 



The source of nitrogen comes next to be considered ; and this also 

 is seen to be independent of manures. Hereupon, it is observed that 

 " our domestic plants do not require a greater supply than in a state 

 of nature. A water-meadow which has never received any dung, 

 yields yearly from 40 to 50 pounds of nitrogen, while the best ploughed 

 land yields only about 31 pounds. The plants for which most dung is 

 used, as potatoes and turnips, are in fact proportionally the poorest in 

 nitrogen." That there is a supply independent of the soil, is further 

 seen in the millions of hides furnished every year by the cattle of the 

 Pampas without any diminution of produce; and in the great quan- 

 tities of nitrogenous matters, hay, butter and cheese, carried off' from 

 pasture-land; far more than is returned by the animals fed thei eon. 

 Experiments with various kinds of plants on various soils have satis- 

 factorily demonstrated that increase of nitrogen in the land and in the 

 crop does take place quite irrespective of supplies of manure. 



With respect to ammonia, "it appears that one-thirteenth of a grain 

 in every pound of water is sufficient for the exigencies of vegetation, 

 and there is perhaps no spring-water in the universe which contains 

 so little." Then as to sulphur and phosphorus, which are also among 

 the constituents of plants, the quantity needed in proportion to the 

 time of vegetation is so small, that one-540,000th of a grain of sul- 

 phuretted hydrogen per cubic foot diffused through the atmosphere to 

 a height of *3000 feet is all that is required. 



The consideration that cereals would soon disappear from the north 

 of Europe, if not cultivated, and perhaps from nearly the whole of 

 this quarter of the globe, adds weight to the arguments in favor of 

 enlightened attention to the inorganic-constituents of plants. The 

 point is to bring the soil into harmony with the conditions by which 

 growth may best be promoted. Much depends on the nature of tlie 



