THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 473 



C— The Distilled Water. 



The first two-fifths of the distillate from ordinary water was allowed to escape, and the 

 next two-fifths were collected for further treatment. The water so obtained retained 

 traces of ammonia. It was mixed with phosphoric acid, free from nitric acid and 

 ammonia, in such quantity that the amount of acid present exceeded that of the ammonia 

 several thousand times. It was then re-distilled from a copper vessel to which was 

 attached a large Liebig's condenser. 



Under these circumstances no ammonia could go over unless it were carried over 

 mechanically, in which case it would be accompanied with several thousand times its 

 own weight of phosphoric acid ; and, as no distilled water was used that gave any 

 evidence of the presence of this acid, the amount of ammonia in it, if any, must have 

 been several thousand times less than that to which the term "traces" is applied. 



The distilled water was so prepared only a few days prior to being required for 

 use. 



All parts of the apparatus, the presence of ammonia in which could possibly affect the 

 result, were, after thorough washing both with ordinary and with common distilled 

 water, finally well rinsed with this pure double-distilled water just before being used. 



D. — The Pots used to receive the Soil, Ash, Plant, &c. 



For the experiments of 1857 common flower-pots were used; their height, and 

 diameter at the top, were each 6 inches, and their diameter at the bottom 3*2 inches ; 

 their weight was about 1 lb. Small common white glazed earthenware plates were used 

 as the pans. 



Subsequent observation suggested, for the experiments of 1858, the kind of pot, and 

 pan beneath it, represented in Plate XII. figs. 1, 2 & 3. 



The Pot, of which fig. 2, Plate XII., represents the elevation, was made of the same 

 material as ordinary flower-pots. It was, however, made as light as possible, and was 

 not baked so hard as the latter generally are. The height, and diameter at the top, 

 were each 5 inches ; and the diameter at the bottom was 4 inches. The bottom is per- 

 forated with about twenty holes of nearly one-fourth of an inch diameter, as is shown 

 in figs. 1 & 2. There were also two rows of similar holes (A, B, fig. 2) round the sides 

 at a distance of 0'5 to 1 inch from the bottom. 



The Pan, represented with the pot placed in it in fig. 3, Plate XII., is made of hard- 

 baked and well-glazed stone-ware. It is 1*5 inch deep and 5 -2 inches in diameter at the 

 bottom. At the top it is curved inwards (A, B, fig. 3), so as to adapt its upper rim to 

 the sides of the pot. 



These arrangements of pot and pan afford several advantages, for the purposes of the 

 investigation, over those adopted in 1857. The surface for evaporation is less in pro- 

 portion to the volume of soil. The facilities for the exit of roots, and for the access of 

 air, are, on the other hand, greater. The pan affords room for an abundance of water, 



3t2 



