240 HOW CROPS FEED, 
tested it forammonia. In but two instances did they find 
sufficient to weigh. In all cases, however, they were able 
to detect it, though it was present in very minute quanti- 
ty. The two experiments in which they were able to 
weigh the ammonia were made in a light, sandy soil from 
which potatoes had been lately harvested. On the 2d of 
September the field was manured with stable dung; on 
the 4th the first experiment was made, the air being taken, 
it must be inferred from the account given, at a depth of 
14 inches. In a million parts of air by weight were found 
82 parts of ammonia. Five days subsequently, after rainy 
weather, the air collected at the samc place contained but 
13 parts in a million. 
b. Ammonia physically condensed in the Svil,—Many 
porous bodies condense a large quantity of ammonia gas. 
Charcoal, which has an extreme porosity, serves to illus- 
trate this fact. De Saussure found that box-wood char- 
coal, freshly ignited, absorbed 98 times its volume of 
ammonia gas, Similar results have been obtained by Sten- 
house, Angus Smith, and others (p. 166). The soil cannot, 
however, ordinarily contain more than a minute quantity 
of physically absorbed ammonia, The reasons are, first, a 
porous body saturated with ammonia loses the greater share 
of this substance when other gases come in contact with 
it. It is only possible to condense in charcoal 98 times its 
volume of ammonia, by cooling the hot charcoal in mer- 
cury which does not penetrate it, or in a vacuum, and then 
bringing it directly into the pure ammonia gas. The 
charcoal thus saturated with ammonia loses the latter rap- 
id!y on exposure to the air, and Stenhouse has found by 
actual trial that charcoal exposed to ammonia and after- 
wards to air retains but minute traces of the former. 
Secondly, the soil when adapted for vegetable growth is 
moist or wet. The water of the soil which covers the 
particles of earth, rather than the particles themselves, 
must contain any absorbed ammonia. Thirdly, there are 
