cid must be greatly
raised, indeed, to produce such a violent combustion.

MRS. B.

There is, however, a peculiarity in this combustion, which is, that the
oxygen, instead of being derived from the atmosphere alone, is
principally supplied by the acid itself.

EMILY.

And are not all combustions instances of the change of temperature
produced by the chemical combination of two bodies?

MRS. B.

Undoubtedly; when oxygen loses its gaseous form, in order to combine
with a solid body, it becomes condensed, and the caloric evolved
produces the elevation of temperature. The specific gravity of bodies is
at the same time altered by chemical combination; for in consequence of
a change of capacity for heat, a change of density must be produced.

CAROLINE.

That was the case with the sulphuric acid and water, which, by being
mixed together, gave out a great deal of heat, and increased in density.

MRS. B.

The 5th law of chemical attraction is, that _the properties which
characterise bodies, when separate, are altered or destroyed by their
combination_.

CAROLINE.

Certainly; what, for instance, can be so different from water as the
hydrogen and oxygen gases?

EMILY.

Or what more unlike sulphat of iron than iron or sulphuric acid?

MRS. B.

Every chemical combination is an illustration of this rule. But let us
proceed--

The 6th law is, that _the force of chemical affinity between the
constituents of a body is estimated by that which is required for their
separation_. This force is not always proportional to the facility with
which bodies unite; for manganese, for instance, which, you know, is so
much disposed to unite with oxygen that it is never found in a metallic
state, yields it more easily than any other metal.

EMILY.

But, Mrs. B., you speak of estimating the force of attraction between
bodies, by the force required to separate them; how can you measure
these forces?

MRS. B.

They cannot be precisely measured, but they are comparatively
ascertained by experiment, and can be represented by numbers which
express the relative degrees of attraction.

The 7th law is, that _bodies have amongst themselves different degrees
of attraction_. Upon this law, (which you may have discovered yourselves
long since,) the whole science of chemistry depends; for it is by means
of the various degrees of affinity which bodies have for each other,
that all the chemical compositions and decompositions are effe