SCIENCE IN SECONDARY SCHOOLS, 195 
fact would seem to indicate that the same constituent of the air is concerned 
in both cases—try to place this beyond doubt by experiment. 
How does the phosphorus act—does it associate with the active gas in air— 
is the white snow-like product a rust? How will you ascertain? You must 
prevent the smoke from escaping, must you not, if you wish to contrast its 
weight with that of the phosphorus—how will you do this—how is smoke to 
be held back or screened off—what is a respirator used for? Very well, then; 
fit up a suitable respirator to prevent the smoke from escaping from a tube in 
which phosphorus is burnt, 
From the result, it is clear, you see, that the phosphorus and iron behave 
alike towards air, withdrawing and combining with the same proportion—very 
nearly one-fifth; and it seems probable, does it not, that this one-fifth about 
(the Fire air, as we have called it) is a special constituent present to this extent 
in air? You have thus discovered what of air—that air is a mixture of at 
least two kinds of air, have you not? 
Where does the fire come from? It seems to have its origin in the act of 
association, does it not? What becomes of the fire or heat produced on 
associating phosphorus with Fire air? It escapes, does it not? The flask in 
which the phosphorus is burnt becomes unbearably hot in places but soon cools— 
the heat is soon lost : does it, the heat that is lost, weigh anything? Try! 
You have thus made the discovery—the wonderful discovery—that fire is 
weightless—something unsubstantial, unmaterial—but consider what strange 
changes attend its production: the metal iron and Fire gas give rise to the 
earth-like rust ; the phosphorus and the Fire air to phosphorus snow; the various 
ordinary combustibles, whether gaseous, liquid or solid, seem to afford water 
and something which has escaped our notice hitherto and which probably there- 
fore is an air-like or gaseous substance : but if so, it must be quite soluble in 
water, must it not, as nearly one-fifth of the air disappears when the various 
substances are burnt in it? But stay, do you know that all substances burn at the 
expense of one and the same constituent of air? Will it not be well to try 
whether, in all cases, the inactive four-fifths left after exposing iron or 
phosphorus in air be inactive also towards all ordinary combustibles? In this 
work, nothing must be taken for granted. And do you know that when iron and 
phosphorus ‘rust’ in air heat is produced as when phosphorus actually burns 
in air? Is heat given out when the phosphorus is merely exposed in air? 
Make the experiment in a really warm room, using a thin rod of phosphorus 
lashed to a wooden rod. 
You thus obtain evidence that even when the Fire air is absorbed slowly, 
heat is produced; and you can believe that whether the phosphorus burn 
visibly or not is merely a question of the rate at which the change takes place— 
whether the heat have time to get away or not. 
You may ask : Is the rusting of iron a case of slow burning? ‘The reply is— 
Can iron burn? How were fires lighted before matches were known—how 
were guns fired before percussion caps were invented? With the aid of a 
flint and steel. Try the effect of striking pieces of flint and of iron together. 
If you can find a smithy, watch the blacksmith at work at his forge; or still 
better, go to a steelworks where iron is rolled into bars and plates. Examine 
a new horseshoe and contrast its surface with that of one which has been in 
use. Examine the ground near the smith’s anvil. Heat a piece of bright 
iron to redness for some time and notice the effect; or prepare some coarse 
iron filings and heat them in a muffle furnace on a clay support, weighing them 
before and after heating. 
Having thus ascertained that iron can be burnt, you will be prepared to 
regard rusting also as a case of slow burning—whether it rust slowly or burn 
rapidly, it equally combines with Fire air and becomes converted into a 
pulverulent, earthy substance: a red earth in the one case, a black earth 
in the other. 
You will perhaps ask—do other metals burn? Do other metals give earths 
when burnt? Metals are so commonly used in household practice that it will 
be well to know something about them. Copper vessels are commonly used— 
does copper combine with Fire air and burn? Try! Does lead, does zinc, does 
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