29 HOW CROPS FEED. 
cur or may occur in the air in minute and variable quanti- 
ties, viz.: 
Water, as.vapor...average proportion by weight, 1] 100 
Carbonic acid gas as jae ®|10-000 
Ammonia we ms ss 1150-000 000 2 
Ozone ga ue . ‘¢ minute traces, 
Nitric acid “ce ce “ce “ce “cc a“ 
Nitrous acid “ee “cc ae ‘73 «cs ae 
-Marsh gas a cc “ “ “ce “ 
In me of Carbonic oxide, “e cc ce “ec ce ce 
1 Sulphurous acid, iio s 3 ~ sg 
towns. Sulphydric acid cc «“c “cc 6“ er “ 
Miller gives for the air of England the following aver- 
age proportions by volume of the four most abundant in- 
eredients.—(Hlements of Chemistry, part IL., p. 30, 3d Ed.) 
OXSYPCH 2.062. 0 cass peepee beeen 20 61 
Mitropen ... 2 s.ea! ss sxsies sel ae 77.95 
Carbonic acid. ...: 5. is. csc -seanene mane .04 
Water-Vapor, . . .« i0a0 0000s sms Gam eee 1.40 
100.00 
We may now appropriately proceed to notice in order 
each of the ingredients of the atmosphere in reference to 
the question of vegetable nutrition. This is a subject re- 
garding which unaided observation can teach us little or 
nothing. The atmosphere is so intangible to the senses 
that, without some finer instruments of investigation, we 
should forever be in ignorance, even of the separate exist- 
ence of its two principal elements. Chemistry has, how- 
ever, set forth in a clear light many remarkable relations 
of the Atmosphere to the Plant, whose study forms one 
of the most instructive chapters of science. 
§ 2. ‘ 
RELATIONS OF OXYGEN GAS TO VEGETABLE NUTRITION. 
Absorption of Oxygen Essential to Growth.—The ele 
ment Oxygen is endowed with great chemical activity. 
This activity we find exhibited in the first act of vegeta 
