CHAPTER VII. NITROGEN.

28. Separation.

Experiment 18.--Fasten a piece of electric-light pencil, or of
crayon, to a wire, as in Experiment 15, and bend the wire so it
will reach half-way to the bottom of a receiver. Using forceps,
put into the crayon a small piece of phosphorus. Pass the wire up
through the orifice in the shelf of a p.t. (pneumatic trough),
having water at least l cm. above the shelf. Heat another wire,
touch it to the P, and quickly invert an empty receiver over the
P, having the mouth under water, so as to admit no air (Fig. 10).
Let the P burn as long as it will, then remove the wire and the
crayon, letting in no air. Note the color of the product, and
leave till it is tolerably clear, then remove the receiver with a
glass plate, leaving the water in the bottom.

Do the fumes resemble those of Experiment 16? Does it seem likely
(Fig 10.) that part of the air is O? Why a part only? Find what
proportion of the receiver is filled with water by measuring the
water with a graduate; then fill it with water and measure that;
compute the percentage which the former is of the latter. What
proportion of the air, then, is O? What was the only means of
escape for the P2O6, and P2O2 formed? These products are solids.
Are they soluble in water? Compute the percentage composition,
always by weight, of P2O2 and P2O5.

The gas left in the receiver is evidently not O. Experiment 19
will prove this conclusively, and show the properties of the new
gas.

29. Properties.

Experiment 19.--When the white cloud has disappeared, slide the
plate along, and insert a burning stick; try one that still
glows.

See whether the P and S on the end of a match will burn. Is the
gas a supporter of combustion? Since it does not unite with C,
S, or P, is it an active or a passive element? Compare it with
O. Air is about 14 1/2 times as heavy as H. Which is heavier, air
or N? See page 12. Air or O?

Write out the chief properties, physical and chemical, of N, as
found in this experiment.

30. Inactivity of N.--N will scarcely unite chemically except on
being set free from compounds. It has, however, an intense
affinity for boron, and will even go through a carbon crucible to
unite with it. It is not combined with O in the air; but the two
form a mixture (page 86), of which N makes up four-fifths, its
use being to dilute the O. What would be the effect, in case of a
fire, if air were pure O? What effect on the human system?

Growing plants need a great deal of N, but they are incapable of
making use of that in the air, on account of the chemical
inactivity of the element. Their supply comes from compounds in
earth, water, and air. By reason of its inertness N is very
easily set free from its compounds. For this reason it is a
constituent of most explosives, as gunpowder, nitro-glycerine,
dynamite, etc. These solids, by heat or concussion, are suddenly
changed to gases, which thereby occupy much more space, causing
an explosion.

Nitrogen exists in many compounds, such as the nitrates; but the
great source of it all is the atmosphere. See page 85.