For although a given weight of hydrogen will give off more
heat when it burns than carbon will, yet acetylene will give off more
heat than either of its elements or both of them when they are separate.
This is because acetylene has stored up heat in its formation instead of
giving it off as in most reactions, or to put it in chemical language,
acetylene is an endothermic compound. It has required energy to bring
the H and the C together, therefore it does not require energy to
separate them, but, on the contrary, energy is released when they are
separated. That is to say, acetylene is explosive not only when mixed
with air as coal gas is but by itself. Under a suitable impulse
acetylene will break up into its original carbon and hydrogen with great
violence. It explodes with twice as much force without air as ordinary
coal gas with air. It forms an explosive compound with copper, so it has
to be kept out of contact with brass tubes and stopcocks. But compressed
in steel cylinders and dissolved in acetone, it is safe and commonly
used for welding and melting.
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