156 ARTIFICIAL ILLUMINATION. 
instance, silversmiths and persons dealing in bright metal goods prefer 
to have their gas lamps outside their windows in order to exclude the 
water vapour formed which might tend to rust their goods. There is 
another and a more important reason, however, why outside gas lamps 
are used; it is the unavoidable presence of a certain small amount of 
sulphur in the gas, which, on burning, forms sulphurous, and, 
ultimately, sulphuric acid, the destructive effects of which are well 
known. 
Now I would like to say a word or two about the right and the 
wrong way of using a gas burner. Never interfere with the natural 
processes which are going on in an open gas flame; otherwise some- 
thing worse than comparatively harmless water and carbonic acid 
may be evolved as the products of combustion. If, for instance, a cool 
body, such as a kettle of water, be held in the flame of a gas burner, 
the combustion will not be perfect, and the deadly carbonic oxide gas 
and the equally poisonous acetyline gas, which latter is one of the com- 
pounds of hydrogen and carbon, will be evolved. Put the kettle, 
therefore, over the gas flame, and not i it. A good gas stove, with a- 
flue to take away all the products of combustion, is convenient and 
economical, but a gas stove without a flue is unhealthy in a small 
room, even when it is properly used, on account of the carbonic acid 
gas which must inevitable be evolved ; and, if it is badly used through 
the cooking vessels being immersed in the flames, instead of being 
placed over them, the atmosphere may be made extremely poisonous, 
and serious results to health, or even risk to life, may follow. I give 
this warning because I think it is not generally known how deadly an 
enemy we may make of a gas stove which, if properly treated, should 
be a most useful friend. I have had personal experience of the saddest 
results arising from neglect of the simple laws governing the com- 
bustion of coal gas. If the use of the gas stove is attended by a 
pungent smell, however slight, imperfect combustion may be suspected 
and acetyline is, no doubt, being evolved. 
We have studied combustion a little in an elementary and gencral 
way, but there may be various rapidities of combustion. We may 
have, for instance, a very slow combustion, such as takes place when 
iron is slowly oxidised, or rusts in the open air. This, though it is a 
slow process, is as true a case of burning as any other kind of com- 
bustion is. We may have a more rapid, but still a moderate, rate of 
combustion, as in an-ordinary gas jet, where each particle of hydrogen 
or carbon does not burn till it comes into contact with the oxygen 
necessary to unite with it. If, however, the atoms of two substances 
are so intimately and accurately mixed that each atom of the one finds 
itself in the immediate neighbourhood of exactly the necessary number 
of atoms of the other substance, then, when combustion takes place, 
it ig instantaneous, and we have a rapid or explosive effect. It is 
hardly necessary for me to dilate upon explosions under the roof of 
the Royal Artillery Institution—the atmosphere is almost redolent of 
the subject—but I will give you just one little example. I have 
here an intimate mixture of hydrogen and oxygen atoms in their exact - 
combining proportion. On applying a light they unite instantaneously 
