THE SOURCES OF THE XITEOGEN OF VEGETATION, ETC. 547 



Accuracy of the method for the determination of nitrogen by combustion 



with soda-lime, &c. 

 In order to ascertain the accuracy of the method before relying upon it for the pur- 

 poses of the investigation, a few preliminary experiments were made upon the determi- 

 nation of small and known quantities of nitrogen, mixed with large quantities of soil, 

 which had been previously freed from combined nitrogen as in the preparation of the 

 soils for the plant-experiments. The nitrogenous substance taken for the purpose was 



the powdered crystals of purified quadroxalate of ammonia, I .__ HO, (C 2 03) 4 -f7HO. 



The results were as follow — 



Experiment 1. — 50 grammes of the prepared soil were mixed with quadroxalate con- 

 taining by calculation 0-0024 gramme nitrogen; and on burning with soda-lime, and 

 determining as above described, 0-0027 gramme nitrogen was found. 



Experiment 2. — 100 grammes of the soil mixed with quadroxalate equal, by calcula- 

 tion, to 0-0035 gramme nitrogen, gave on combustion 0-0037 gramme nitrogen. 



The error of analysis was, therefore, three-tenths of a milligramme of nitrogen with the 

 50 grammes, and two-tenths with the 100 grammes of soil. These results were obtained 

 at the commencement of the inquiry, with comparatively large quantities of titrated acid, 

 and therefore before experience had suggested the precautions to be adopted to reduce 

 the errors of determination to the minimum. They may hence be taken as examples of 

 the maximum errors of analysis, but they are less than would affect the bearing of the 

 results hi the investigation on the question of assimilation. 



Testing for Nitric acid. 



The indigo test, as recently refined by Boussingault*, and the protosulphate-of-iron 

 test, were both employed. "When nitric acid was sought for and not found, if practicable 

 the negative result was always confirmed by the addition to some of the substance under 

 examination of a quantity of nitric acid (in the form of nitrate) less than could affect 

 any conclusions to be drawn from the fact of its presence or absence in the substance in 

 question. In all the cases of such addition the re-examination showed the presence of 

 nitric acid. 



The method of Boussingault was much more delicate than the protosulphate-of-iron 

 test ; but, on the other hand, the latter was much less liable to give deceptive indica- 

 tions, dependent on other circumstances than the presence of nitric acid. In using the 

 protosulphate test, the aqueous extract of the substance under examination was evapo- 

 rated to a small volume with excess of fixed alkali, then transferred to a test-tube, and 

 further evaporated till only a few drops remained. A considerable excess of concentrated 

 sulphuric acid was then added, and on the surface of the liquid a concentrated solution 

 of protosulphate of iron was carefully poured without agitation, by means of a small 

 pipette with a mouth of almost capillary fineness. The characteristic brown tinge 

 indicated the presence of nitric acid. 



* Abu. de Chim. et de Phya., toI. xkiii. (1856) p. 153 et seq. 



