or at a gross pressure of 30 lb. per
square inch; and however large that volume may be, unless it is contained
in tubes of very small diameter, as will appear hereafter, the
decomposition or dissociation into its elements will extend throughout
the whole of the gas. Equation (7) states that 2 volumes of acetylene
yield 2 volumes of hydrogen and a quantity of carbon which would measure
2 volumes were it obtained in the state of gas, but which, being a solid,
occupies a space that may be neglected. Apparently, therefore, the
dissociation of acetylene involves no alteration in volume, and should
not exhibit explosive effects. This is erroneous, because 2 volumes of
acetylene only yield exactly 2 volumes of hydrogen when both gases are
measured at the same temperature, and all gases increase in volume as
their temperature rises. As acetylene is endothermic and evolves much
heat on decomposition, and as that heat must primarily be communicated to
the hydrogen, it follows that the latter must be much hotter than the
original acetylene; the hydrogen accordingly strives to fill a much
larger space than that occupied by the undecomposed gas, and if that gas
is contained in a closed vessel, considerable internal pressure will be
set up, which may or may not cause the vessel to burst.
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