THE ROOT 75 
may be shown by the fact that the vapour given off 
will tarnish a silver spoon much in the same way as 
it is tarnished by the sulphur of an egg. | 
Elements.—Chemical analysis of a plant shows that 
it contains the elements, carbon, oxygen, hydrogen, 
nitrogen, sulphur, phosphorus, potassium, calcium, 
magnesium, iron, and, in some eases, silicon and 
chlorine. Of these, if we leave out of consideration the 
oxygen used in respiration, all, with the exception of 
earbon, come from the earth—the oxygen and hydro- 
gen from the soil-water; nitrogen from the nitrates and 
ammonia compounds; potassium, calcium, magnesium 
and iron from nitrates, sulphates and phosphates; 
sulphur from sulphates; phosphorus from phosphates ; 
ehlorine from chlorides; and silicon from silicates. The 
carbon, which, as we have seen, forms the bulk of the 
plant, comes, of eourse, from the ecarbon-dioxide of 
the air. 
Nitrogen is an inactive gas forming four-fifths of 
the volume of the air. By far the simplest way of ex- 
tracting this gas from air is to moisten with ammonium 
ehloride solution and sprinkle the inside of a 
bell jar with iron filings, and then stand the 
jar in a dish of water and firmly stopper it. 
In the eourse of a few days the oxygen in 
the jar combines with the iron to form iron 
oxide or rust, and the water rises one-fifth of the 
way up on the inside to take its place. The remaining 
four-fifths is nitrogen. If water be now poured into 
the dish till the liquid stands at the same level inside 
as outside the jar, and a lghted taper inserted, the 
taper will at once be extinguished. The level of the 
water outside must be raised to prevent the entry of 
air into the jar when the stopper is removed. The 
result of this experiment is to show that the air is 
about four-fifths nitrogen and one-fifth oxygen, and 
that nitrogen will not support combustion. Nitrate of 
