vered the physiological
effects of "laughing gas," and made so considerable a reputation as
a chemist that at the age of twenty-two he was appointed lecturer,
and a year later professor, at the Royal Institution. For ten
years, from 1803, he was engaged in agricultural researches, and in
1813 published his "Elements of Agricultural Chemistry." During the
same decade he conducted important investigations into the nature
of chemical combination, and succeeded in isolating the elements
potassium, sodium, strontium, magnesium, and chlorine. In 1812 he
was knighted, and married Mrs. Apreece, _nee_ Jane Kerr. In 1815 he
investigated the nature of fire-damp and invented the Davy safety
lamp. In 1818 he received a baronetcy, and two years later was
elected President of the Royal Society. On May 29, 1829, he died at
Geneva. Davy's "Elements of Chemical Philosophy," of which a
summary is given here, was published in one volume in 1812, being
the substance of lectures delivered before the Board of
Agriculture.

_I.--Forms and Changes of Matter_

The forms and appearances of the beings and substances of the external
world are almost infinitely various, and they are in a state of
continued alteration. In general, matter is found in four forms, as (1)
solids, (2) fluids, (3) gases, (4) ethereal substances.

1. _Solids._ Solids retain whatever mechanical form is given to them;
their parts are separated with difficulty, and cannot readily be made to
unite after separation. They may be either elastic or non-elastic, and
differ in hardness, in colour, in opacity, in density, in weight, and,
if crystalline, in crystalline form.

2. _Fluids._ Fluids, when in small masses, assume the spherical form;
their parts possess freedom of motion; they differ in density and
tenacity, in colour, and in opacity. They are usually regarded as
incompressible; at least, a very great mechanical force is required to
compress them.

3. _Gases._ Gases exist free in the atmosphere, but may be confined.
Their parts are highly movable; they are compressible and expansible,
and their volumes are inversely as the weight compressing them. All
known gases are transparent, and present only two or three varieties of
colour; they differ materially in density.

4. _Ethereal Substances._ Ethereal substances are known to us only in
their states of motion when acting upon our organs of