THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 475 



Seedsmen to the Royal Agricultural Society of England, who bestowed much labour 

 and attention upon the selection. From the quantity of each kind received, the largest 

 and the smallest were picked out, as were also any that did not look quite healthy. 

 Given numbers of the remainder were then weighed, and the average weight, per seed, 

 was calculated. A few seeds, each weighing as nearly as possible the mean weight, were 

 then selected for planting. 



In order to estimate the quantity of Nitrogen in the seeds sown, in some cases a quan- 

 tity of seeds equal in weight and number to those sown was submitted to analysis. But 

 the difficulties of grinding, without loss, so small a quantity, and the consideration that 

 one small quantity might differ more in composition from another such quantity, than 

 either would from the average composition of a large number of well-selected seeds, led 

 us generally to estimate the Nitrogen in the seeds sown from the percentage of it found 

 in the mixture of a large number ground up together. 



The seeds selected for growing were sown in the pots of soil prepared as already 

 described, to the depth of about 1 inch below the surface. With large seeds, such as 

 Beans, it was necessary that care should be taken so to deposit them that the radicle 

 and plumule should each take its natural direction. If this precaution was neglected, 

 the seed was liable to be raised out of the soil after sprouting, which involved the incon- 

 venience of opening the apparatus in which the plant was enclosed, in order to re-bury 

 the seed. 



In some cases, as soon as the seeds were sown the pots were removed from over the 

 sulphuric acid, and placed at once beneath the large glass shades which were to serve as 

 the enclosing apparatus. In other instances, the pots were first placed under other 

 shades, luted by mercury or sulphuric acid, and standing in the laboratory, and then, 

 after a few days, they were removed to their final position. 



G. — The Atmosphere supplied to the Plants. 



As regards the essential conditions of growth, and the circumstances associated with 

 it, which must be kept within the control of our means of investigation, the same 

 remarks apply to the atmosphere, though with less force, as have already been made in 

 reference to the soil (subsection A, p. 470). 



It is true that the constitution of the atmosphere is less complicated, and that we are 

 much better acquainted with it than we are with that of ordinary soils ; yet the extreme 

 mobility of the atmosphere renders the presence in it of exceedingly small quantities 

 of substances calculated to influence vegetable growth much more dangerous in quan- 

 titative experiments on vegetation than would be their presence in the soil. Thus the 

 presence of gaseous impurities, and of solids mechanically suspended, in the atmosphere 

 cannot be overlooked. And hence, although it is not necessary to submit the natural 

 atmosphere to such radical changes as those to which the natural soil must be subjected, 

 some measures must be taken to exclude the sources of error to which allusion has just 

 been made. 



