THE SOURCES OF THE NTTEOGEN OF VEGETATION, ETC. 533 



wheat (1858) would not be so easily explained, had not the Nitrogen in the drain- 

 water remaining at the end of the experiment been determined. Our object in doing 

 this was twofold : — 



1. To ascertain whether the luting at the bottom of the shade had allowed rain-water 

 to pass, thus affording a source of combined Nitrogen to the plants. 



2. To see if the plants growing in soil to which combined Nitrogen was added, had 

 evolved any ammonia. 



It was, of course, not possible to accomplish both these purposes. But the fact that 

 ammonia was found in the condensed water only in the cases where there was a loss in 

 the total quantity of combined Nitrogen would lead to the inference that both the pre- 

 sence of ammonia in this water, and the loss of combined Nitrogen in the experiment, 

 were due to the same cause. 



The condensed water showing the amount of combined Nitrogen recorded in the Table 

 (XIV.) was that which had been evaporated and condensed during the last four weeks of 

 growth (1858) ; and during this period the high temperature, and the advanced stage of 

 the plants, were favourable to the evaporation of ammoniacal water. A considerable part 

 would condense on the interior of the shade, owing to its comparatively low tempera- 

 ture ; but a certain quantity of that which was in the state of vapour during the passage 

 of the air through the apparatus would be borne forward into the sulphuric acid in the 

 bulb-apparatus M, and thus occasion a loss in the amount of combined Nitrogen deter- 

 mined in connexion with the plants. The reason why the loss is greater with the oats 

 (as it is in both experiments) than with the other cereals is not perfectly clear ; but the 

 circumstances of growth seemed to afford some explanation of the fact. In one case, at 

 least, they ripened at a much warmer period of the season, and they became much drier 

 in stem and leaf, and were therefore more liable to evolve ammonia. On these points, 

 the circumstances of growth detailed in the Appendix should be consulted. 



In considering the column of gain or loss of Nitrogen, it is very desirable to take into 

 account the total quantity of Nitrogen at the disposal of the plant, in the different series 

 of experiments. It is also important to consider the amount of growth in the experi- 

 ments made under the different conditions. The following Tables (XV. and XVI.) bring 

 out the character of the results in these respects more clearly than they can be gathered 

 from Tables XII. and XIV. Table XV. shows, for the plants grown without supply of 

 combined Nitrogen beyond that contained in the seed, and Table XVI. for those grown 

 with such supply, the dry matter, and the Nitrogen, per seed sown, — the dry matter, 

 and the Nitrogen, in the total produce of each seed that grew, — and the per cent, of the 

 total Nitrogen at the disposal of the plant which it appropriated. Finally, the last two 

 columns of Table XVI. show the amounts of dry matter, and of Nitrogen, in the pro- 

 duce grown with the extraneous supply of combined Nitrogen, in relation to those in 

 the produce grown without such supply. 



