ught down by the 'snow' from the higher
Martian atmosphere might produce the blue tinge in the great volumes of
heavy gas produced by its evaporation or liquefaction.

It may be noted that Mr. Lowell objects to the carbon-dioxide theory of
the formation of the snow-caps, that this gas at low pressures does not
liquefy, but passes at once from the solid to the gaseous state, and
that only water remains liquid sufficiently long to produce the blue
colour' which plays so large a part in his argument for the mild climate
essential for an inhabited planet. But this argument, as I have already
shown, is valueless. For only very deep water can possibly show a blue
colour by reflected light, while a dust-laden atmosphere--especially
with a layer of very dense gas at the bottom of it, as would be the case
with the newly evaporated carbon-dioxide from the diminishing snow-cap
--would provide the very conditions likely to produce this blue tinge of
colour.

It may be considered a support to this view that carbonic-acid gas
becomes liquid at--140 deg. F. and solid at--162 deg. F., temperatures far
higher than we should expect to prevail in the polar and north temperate
regions of Mars during a considerable part of the year, but such as
might be reached there during the summer solstice when the `snows' so
rapidly disappear, to be re-formed a few months later.

_The Double Canals._

The curious phenomena of the 'double canals' are undoubtedly the most
difficult to explain satisfactorily on any theory that has yet been
suggested. They vary in distance apart from about 100 to 400 miles. In
many cases they appear perfectly parallel, and Mr. Lowell gives us the
impression that they are almost always so. But his maps show, in some
cases, decided differences of width at the two extremities, indicating
considerable want of parallelism. A few of the curved canals are also
double.

There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or
starting points, of the Euphrates and Phison, two widely separated
double canals diverging at an angle of about 40 deg. from the same two
oases, so that the two inner canals cross each other. Now this suggests
two wide bands of weakness in the planet's crust radiating probably from
within the dark tract called the 'Mare Icarium,' and that some
widespread volcanic outburst initiated diverging cracks on either side
of these bands. Something of this kind may have been the cause of most
of the d