By W. SPRING.
The author asks in the first place, What is the cause of the different
specific gravities of one and the same metal according as it has been
cast, rolled, drawn into wire, or hammered? Does the difference observed
prove a real condensation of the matter under the action of pressure, or
is it merely due to the expulsion by pressure of gases which have been
occluded when the ingot was cast? According to well-known researches,
metals such as platinum, gold, silver, and copper, which have
been proved to occlude gases on fusion, and to let them escape,
_incompletely_, on solidification, are precisely those which are
most increased in their specific gravity by pressure. The author has
submitted to pressures of about 20,000 atmospheres metals which possess
this property, either not at all, or to a very trifling extent, and he
finds that though a first pressure produces a slight permanent increase
of density, its repetition makes little difference. Their density is
found to have reached a maximum. Hence the density of solids, like that
of liquids, is only really modified by temperature. Pressure effects
no permanent condensation of solid bodies, except they are capable of
assuming an allotropic condition of greater density. The author's former
researches tend to show that solid matter, in suitable conditions of
temperature, takes the state corresponding to the volume which it is
compelled to occupy. Hence there is an analogy between the allotropic
states of certain solids and the different states of aggregation of
matter. Possibly the different forms of matter may be due to a single
cause--polymerization. The limit of elasticity of a solid body is the
critical moment when the matter begins to flow under the action of the
pressure to which it is submitted, just as, e.g., ice at or below 0° may
be liquefied by strong pressure. A brittle body is simply one which does
not possess the property of flowing under the action of pressure.
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