THE PROTOCOCCUS NIVALIS.

43

to give a faint crimson tinge to the paper. Placed under the microscope, these granules resolve themselves into spherical purple cells, from the Tooth to the both part of an inch in diameter. Each of these cells has an opening, surrounded by serrated or indented lines, whose smallest diameter does not exceed the tooth part of an inch! When perfect, the plant is not unlike a red-currant berry; as it decays, the red colouring matter fades into a deep orange, and the deep orange changes into a dull brown. The thickness of the wall of the cell does not exceed the zooooth part of an inch! Each cell may be considered a distinct individual plant, since it is perfectly independent of others with which it may be aggregated, and performs for and by itself all the functions of growth and reproduction, having a containing membrane which absorbs liquids and gases from the surrounding matrix or elements, a contained fluid of peculiar character, formed out of these materials, and a number of excessively minute granules, equivalent to spores, or, as some would say, to cellular buds, which are to become the genus of new plants. There is something, adds Mr Macmillan, extremely mysterious in the performance of these

widely different func

tions, by an organism

O

FIG. 8.-Protococcus nivalis.

which appears so excessively simple. That one and the same primitive cell should thus minister equally to absorption, nutrition, and reproduction, is an extraordinary illustration of the fact, that the smallest and simplest organised object is, in itself, and for the part it was created to perform in the ope

44

ETERNAL SNOWS.

rations of nature, as admirably adapted as the largest and

most complicated.

THE ETERNAL SNOWS.

The epithet "eternal" or "perpetual," applied to snow, would appear to savour of the ambitious, if not of the profane. Can we say of anything which belongs to earth that it is "eternal?" Assuredly not. The earth has not always worn the aspect which it now wears, and, at a period not far distant from its origin, could not in any region have been covered with snow. Now, whatever has had a beginning cannot be eternal. Many authors have, for this reason, substituted for the word eternal the word perpetual. But the latter is equally inapplicable. Who will venture to affirm that our globe or its system will endure perpetually?

This difficulty, however, is one which need not particularly embarrass us. We have been long accustomed to look upon language as a simple mask, or, at least, as very dubious interpreter of thought. And we shall, therefore, continue to use indifferently the words "eternal" and "perpetual."

Let us suppose that two travellers set out from the equator, that plane of separation between the northern and the southern hemispheres. Let us also suppose that each proceeds in a diametrically opposite direction to the other, pursuing his route along one of those meridian lines which divide the earth into longitudinal portions, like the slices of a melon (to compare

THE FRIGID ZONES.

45

great things with small). The following will be their climatic stages :—

At first the two travellers will each traverse a moiety of the torrid zone, limited below and above the Equator by two parallel circles, in the northern hemisphere by the Tropic of Cancer, and in the southern by the Tropic of Capricorn. Do not let these appellations alarm you: they show, once more, the narrow connexity of the heaven with the earth; tropic, coming from the Greek rgorý, signifies a return-the sun returns from his apparent excursions, after having passed from the tropics to the zenith. For these circles form the extreme limits of the sun's march towards the north and towards the south: they are the two solstices-the summer solstice, when the sun enters the zodiacal sign of Cancer, and the winter solstice, when it enters the sign of Capricorn. The torrid zone is the only one which is thus divided into two portions by the Equinoctial, and which the sun passes twice a year to the zenith, that is, to the point directly above the heads of the inhabitants.

After having crossed the tropics, one of our two travellers will enter the North Temperate Zone, bounded by the Arctic Polar Circle, the other, the South Temperate Zone, bounded by the Antarctic Polar Circle. Having passed the polar circles, they will find themselves speedily arrested by ice and snow which never melt-by eternal ice and snow. These inhospitable regions compose the two frigid zones, which cover, like two immense hoods (forming the o 82 parts of the terrestrial

46

THE SNOW REGION.

surface), the one, the northern hemisphere, the other, the

In their progress through these various climates, our two travellers will arrive at a very curious comparative result,— that the southern hemisphere is colder than the northern. This difference becomes especially recognisable below the 50th degree of south latitude; so that, after passing the Antarctic Circle, the ice opposes the voyager's course with nearly insurmountable obstacles; while, in the northern hemisphere, the whaler frequently penetrates to Spitzbergen, situated much nearer to the Pole than to the Polar Circle. This is a general fact; we confine ourselves to putting it forward.

Let us now suppose that our two travellers, always ready to compare the results of their inquiries, accomplish the ascent of a very lofty mountain situated under the Equator, such as Chimborazo. In the course of their ascent, they will traverse the same climates and the same zones which had marked the stages of their journey from the Equator to the Poles: at their starting-point they will find themselves in the Torrid zone, then will come the Temperate and the Frigid zones, the latter rendered inaccessible by glaciers and eternal snows. These vertical zones of the mountain are characterised by vegetables and animals whose types are found in the corresponding horizontal zones of the terrestrial surface. But what is most

* The two temperate zones together represent perceptibly the half, or 0.520, and the torrid zone, two-fifths, or, more exactly, 0.398, of the terres trial surface.

CHANGES OF THE SNOW LINE.

47

remarkable is, that there exists between the northern and the southern slopes of the mountain the same difference as between the southern and northern hemispheres: the line of the eternal snows descends much lower on the northern than on the southern slope, in the same manner as, in the southern hemisphere, the polar ice advances much nearer the Equator than in the northern.

Such is the general view-point which we must adopt for the clearer comprehension of the details of observation. Of course, when speaking of the limit of the eternal snows, we refer only to the lower limit, that is to say, to the greatest elevation attained by the snow-line in the course of a single year. As for the upper limit, it entirely escapes us; for the summits of the loftiest mountains do not reach the atmospheric strata which, by virtue of their refraction, cannot contain any vesicular, aqueous, or condensable vapour.

The line of eternal snow which, at the poles, is found on the level of the ground, gradually rises as we approach the torrid zone, where it attains its maximum of elevation, from 13,000 to 17,000 feet. This phenomenon does not exclusively depend upon the geographical latitude, nor on the mean annual temperature of the locality: it is the result of an aggregate of diverse circumstances which we have not the space here to enumerate and discuss. We shall content ourselves with placing before the reader a table which will show the remarkable differences existing in the height of the perpetual snowline in various places.