. Other important steps, in the same
direction, have been effected by investigations into the absorption of
radiant heat proceeding from different sources by solid, fluid, and
gaseous bodies. And it is a curious example of the interconnection of
the various branches of physical science, that some of the results
thus obtained have proved of great importance in meteorology.

[Sidenote: The spectroscope.]

The existence of numerous dark lines, constant in their number and
position in the various regions of the solar spectrum, was made out by
Fraunhofer in the early part of the present century, but more than
forty years elapsed before their causes were ascertained and their
importance recognised. Spectroscopy, which then took its rise, is
probably that employment of physical knowledge, already won, as a
means of further acquisition, which most impresses the imagination.
For it has suddenly and immensely enlarged our power of overcoming the
obstacles which almost infinite minuteness on the one hand, and almost
infinite distance on the other, have hitherto opposed to the
recognition of the presence and the condition of matter. One
eighteen-millionth of a grain of sodium in the flame of a spirit-lamp
may be detected by this instrument; and, at the same time, it gives
trust-worthy indications of the material constitution not only of the
sun, but of the farthest of those fixed stars and nebulae which afford
sufficient light to affect the eye, or the photographic plate, of the
inquirer.

[Sidenote: Electricity.]

The mathematical and experimental elucidation of the phenomena of
electricity, and the study of the relations of this form of energy
with chemical and thermal action, had made extensive progress before
1837. But the determination of the influence of magnetism on light,
the discovery of diamagnetism, of the influence of crystalline
structure on magnetism, and the completion of the mathematical theory
of electricity, all belong to the present epoch. To it also appertain
the practical execution and the working out of the results of the
great international system of observations on terrestrial magnetism,
suggested by Humboldt in 1836; and the invention of instruments of
infinite delicacy and precision for the quantitative determination of
electrical phenomena. The voltaic battery has received vast
improvements; while the invention of magneto-electric engines and of
improved means of producing ordinary electricity has pr