546 MR. J. B. LAWES, DR. GILBERT, AND DR. PUGH ON 
by a number of four or five digits, which, multiplied by the number of septems 
of alkali representing the product in an analysis, gave the actual quantity, in grammes, 
of the nitrogen to be estimated. 
Amount, and measurement, of the titrated acid used in Nitrogen determinations. 
It was desirable that at least three times as much acid should be used as would be 
neutralized by the ammonia formed. The acid being more concentrated than the 
alkali, it required a more exact method of measurement than was afforded by the burette 
used for the latter. Pipettes, of which the diameter at the point of reading off is com- 
paratively small, and which hence admit of a higher degree of accuracy, were therefore 
employed. In the construction of those to be used, care was taken to maintain the same 
relation of the diameter of the neck at the point of reading to the entire volume in 
instruments of different sizes—a condition seldom observed by makers of pipettes. 
When the quantity of nitrogen involved in an analysis was very small—as in the case of 
the soils and pots in the experiments without nitrogenous manure—only about six sep- 
tems of the titrated acid, measured in a small pipette with a very narrow neck, were used. 
The exact volume of the pipette-ful of acid was not a matter of any consequence. It 
was only essential to ascertain its exact value expressed in septems of the titrated alkali- 
solution. When the amount of nitrogen involved was larger, and more under control— 
as for example when grains were to be analysed—care was taken to operate on such a 
quantity of nitrogenous material that the number of septems of the alkali representing 
its nitrogen should be sufficiently large to render the constant errors of titrating, read- 
ing, &c., inappreciable. ‘This end was attained when the substance experimented upon 
contained 5 to 8 milligrammes, or more, of nitrogen. 
Combustion-tubes, bulbs, &c. 
The combustion-tubes used in the determinations of nitrogen in the soils, pots, &c., 
were about 3 feet long and about 1 inch in diameter. The bulb-apparatus was capable 
of holding two-and-a-half to three times as much fluid as that usually employed; but the 
central and lowest bulb, and particularly its tubular connexions with the other bulbs, 
were very small, so that a small quantity of liquid could close the passage. This arrange- 
ment was necessary owing to the small quantity of acid frequently used, and the large 
amount of water driven off in the combustion from the large quantities of soil and soda- 
lime. For the combustion of the experimentally grown plants smaller tubes were 
employed ; and for seeds, &c., ordinary combustion-tubing was used. 
The Soda-lime. 
Before use, the soda-lime was ignited with 2 per cent. of pure sugar, in order to 
ensure its freedom from ammonia-yielding matter. It was then slaked with pure 
distilled water, dried, and kept in well-corked bottles. 
