FIXATION OF FREE NITROGEN. 141 



or soil. The specially-made pots were ignited before use, and Plan of 

 cooled over sulphuric acid under cover. Each pot with its l ^ e ot e ^ ly 

 plants was enclosed under a glass-shade, which rested in the sted trials. 

 groove of a specially-made, hard-baked, glazed stoneware lute- 

 vessel, mercury being the luting material. Under the shade, 

 through the mercury, passed one tube for the admission of 

 air, another for its exit, and another for the supply of water 

 or solutions to the soil ; and there was an outlet at the bot- 

 tom of the lute- vessel for the escape of the condensed water 

 into a bottle affixed for that purpose, from which it could be 

 removed and returned to the soil at pleasure. 



A stream of water being allowed to flow from a tank into 

 a large stoneware Woulff's bottle of more than 20 gallons 

 capacity, the air passed from it by a tube through two small 

 glass Woulff's bottles containing sulphuric acid, and then 

 through a long tube filled with fragments of pumice saturated 

 with sulphuric acid, and lastly through a Woulff's bottle 

 containing a saturated solution of ignited carbonate of soda ; 

 and, after being so washed, the air entered the glass-shade, 

 from which it passed by the exit tube through an eight- 

 bulbed apparatus containing sulphuric acid, by which com- 

 munication with the unwashed external air was prevented. 

 Carbonic acid was supplied as required, by adding a measured 

 quantity of hydrochloric acid to a bottle containing fragments 

 of marble, the evolved gas passing through one of the bottles 

 of sulphuric acid, through the long tube, and through the 

 carbonate of soda solution, before entering the shade. 



In 1857 twelve sets of such apparatus were employed ; in 

 1858 a larger number, some with larger lute- vessels and 

 shades ; in 1859 six, and in 1860 also six. Each year the 

 whole were arranged side by side on stands of brickwork in 

 the open air. 



The numerical results obtained in the experiments of 1857 

 and 1858 are summarised in Table 45 (p. 142). 



The upper part of the table shows the results obtained, in Noasdmil- 

 1857 and 1858, in the experiments in which no combined < f^\^ r0 . 

 nitrogen was supplied beyond that contained in the seed gen. 

 sown. The growth was extremely restricted under these 

 conditions ; and the figures show that neither with the 

 Graminese, the Leguminosee, nor the Polygonace£e (buck- 

 wheat), was there in any case a gain of three milligrams of 

 nitrogen. In most cases there was much less gain than this, 

 or a slight loss. There was, in fact, nothing in the results 

 to lead to the conclusion that either of these different descrip- 

 tions of plant had assimilated free nitrogen. 



The lower part of the table shows the results obtained in 

 the experiments in which the plants were supplied with 



