36 YALE AGRICULTURAL LECTURES. 



great French chemist, proved that only from carbonic acid was 

 it obtained, by the experiment of supplying to a plant under a 

 bell-glass, a weighed quantity of the gas, and noting the pro 

 portion abstracted by the plant. The weight of carbon in the 

 soil being absolutely known, as well as that in the plant itself, 

 the increase of quantity at an advanced stage of growth was 

 found to have been attained at the expense of the carbon in 

 the gas, and not of that in the soil. 



Mr. Johnson stated it as the practice of some nurserymen to 

 place a piece of carbonate of ammonia, as large as a walnut, 

 upon the steam-pipes of the hothouse. The ammonia thus evap 

 orated produces in the leaves of all the plants with which it 

 comes in contact a splendid deep-green color, and greatly pro 

 motes the growth of the plants. 



To-day he treated on the ashes of plants, and in the course 

 of his lecture uttered some doctrines which sadly conflict with 

 the received notions which are to be found floating through 

 our agricultural papers. For instance : he said that, chemi 

 cally, magnesia is not injurious to crops when added in excess 

 to the field. The noxious effect of strong magnesian lime, if 

 any, was due simply to a mechanical action in the soil ; this 

 particular lime acting in some wise as a cement when moisten 

 ed. Again : he said that the stiffness of straw is most de 

 cidedly not owing to an abundance of silica on the outside, but 

 to " the denseness of cellular tissue in the stalk." This he con 

 sidered proved in the fact that we get from the leaves of the oat 

 and other plants a greater proportion of silica than from the 

 stalk, and yet all leaves are pliant and soft. And the addition 

 of wood-ashes, caustic-lime, and other alkalies with the view to 

 making soluble silicates for the use of the plants, is a piece of 

 useless folly, for " ah 1 water found in the soil contains silicates 

 and silica in excess beyond the wants of plants. The addition 

 of alkaline silicates to the soil would be unavailing, for the sili 

 cates would be decomposed and the silica rendered insoluble." 

 As an example, he stated that in marshy lands, where sedge 



