154 ARTIFICIAL ILLUMINATION. 
Firstly, air is a mixture of oxygen and nitrogen in the proportion of 
about one to four by volume. The nitrogen is simply a diluent, the 
oxygen being the great supporter of combustion. Bodies which burn 
in air burn more violently in pure oxygen. You have before you an 
example of the violent combustion of the refractory body, iron wire, 
in pure oxygen, which would not, of course, burn as you see it, though 
it might slowly oxidize, in ordinary air. Now we must not suppose 
that this iron is really being destroyed. Combustion is not destruction ; 
it is construction. A simple substance when burning is uniting 
with something else; it is not being destroyed, but it is forming a 
compound substance. In the experiment before you, the compound 
which is being formed is iron oxide; it is the result of the chemical 
union of the iron with the oxygen, and, if I were to gather up the 
whole of the iron oxide, or rust as we call it, which is now being 
formed, I should find that its weight is exactly the sum of the 
weights of the iron and the oxygen which have been consumed. I 
have said that the iron has combined with the oxygen; I might just 
as well have said that the oxygen has burnt or combined with the 
iron ; either of the two combining substances is burning quite as much 
as the other. A 
To illustrate this point I call your attention to an experiment where 
I am showing you, not a jet of coal gas burning in air, as is usual, but 
a jet of air burning in an atmosphere of coal gas. The air flame is, 
under these circumstances, not a bright one; but still it is as much a 
case of burning as the ordinary gas jet is—the products of combustion 
being, of course, the same. 
The next element we will consider is hydrogen, the lightest of all 
gases. I plunge a lighted taper into this jar containing hydrogen, 
which jar, you notice, I hold mouth downwards in order to prevent the 
hydrogen from escaping, and you see that the hydrogen extinguishes 
the light, although it takes fire at the mouth of the jar where it meets 
the oxygen of the air, and burns with the pale blue, almost invisible, 
flame which is characteristic of this gas. ‘he yellow tinge to this 
flame which you may notice is due to a great extent to sodium in the 
dust of the atmosphere. ‘l'hus we see that hydrogen is not a supporter 
of combustion, but that it burns or combines with oxygen. ‘The sub- 
stance which is formed during this burning is water, or rather steam, 
the intense heat of combustion yapourising the molecules of water as 
fast as they are formed. Here we have a burning jet of hydrogen gas 
issuing from a bottle in which the gas is being generated by the action of 
dilute sulphuric acid on zinc. Although we cannot see the steam which 
is being formed, yet we can make it evident in the form of water by 
condensing it on this cold glass tumbler which I hold in the flame. 
You see that I can wipe the moisture off with my hand. 
The next element to discuss is carbon, familiar forms of which are 
coal, charcoal, lamp-black, etc., and, in the purest crystalised shape, 
the brilliant diamond. When carbon burns in a plentiful supply of 
oxygen, carbonic acid gas, or carbon di-oxide, is generated. ‘This is 
an invisible, heavy and asphixiating gas. I have here a tall jar full of 
it which you notice is standing now with its mouth upwards and un- 
