104 Notices of Books. (January, 
has proved that the spectrum of incandescent hydrogen existing 
under very great pressure is entirely continuous. For inter- 
mediate pressures we have intermediate spectra. ‘* We may 
state generally,” says the author, “that beginning with any one 
element in its most rarefied condition, and then following its 
spectrum, as the molecules come nearer together, so as at last to 
reach the solid form, we shall find that spectrum become more 
and more complicated as this approach takes place, until at last 
a vivid continuous spectrum is reached.” The latter part of 
this lecture treats of the application of the spectroscope to the 
investigation of the nature of various heavenly bodies, the sun, 
stars, and nebule, at the hands of various physicists and astro- 
nomers. The third and concluding lecture mainly discusses 
absorption-spectra. It is here shown that it easy to detect 
different substances by noticing their absorption ; by introducing 
the substance between the source of light and the prism, and 
noticing the resulting spectrum, side by side with a continuous 
spectrum. We may discriminate between two solutions, the 
one of blood, the other of magenta dye, exactly similar in 
colour and general appearance. Again, Mr. Sorby has proved 
that, by means of the spectrum-microscope, a blood spot so 
small that it contains only one-thousandth of a grain of blood 
may be detected. Some very interesting and comparatively new 
matter is given near the end of this lecture relating to the 
probably constitution of sun spots, the spectroscopic examination 
of which seems to prove that they are due to ‘ general absorp- 
tion, plus special absorption in some particular lines.” The 
observed widening of the sodium line in the spectrum of a sun 
spot is traced to difference of pressure :—‘‘ If we take a tube 
containing some metallic sodium sealed up in hydrogen, and 
pass a beam of light from the electric lamp through it, by 
decomposing this beam with our prisms we shall obtain an 
ordinary continuous spectrum without either bright or dark 
lines; but by heating the metallic sodium in the tube, which is 
placed in front of the slit, we really fill that tube with the vapour 
of sodium; and as the heating will be slow, the sodium vapour 
will rise very gently from the metal at the bottom, so that we 
shall get layers of different densities of sodium vapour filling 
the tube. Immediately the sodium begins to rise in vapour, a 
black absorption-line shows itself, in one spectrum, in precisely 
the same position as the yellow line of sodium, and you will find 
that the thickness of the sodium absorption line will vary with 
the density of the stratum of vapour through which it passes. 
Thus, from the upper part of the tube we obtain a fine delicate 
line, which gradually thickens as we approach the bottom of the 
tube, and thus we produce the appearance in the spectrum of the 
spot where the layers of sodium vapour are very dense, and the 
very fine delicate line of the sodium vapour when thrown up into 
the sun’s chromosphere.’ Thus, in fact, it seems to be undoubted 
