Volume I Part 29 (1/2)
_While round dark crags imprison'd waters bend Through rifted ice, in ivory veins descend._
CANTO III. l. 113.
The common heat of the interior parts of the earth being always 48 degrees, both in winter and summer, the snow which lies in contact with it is always in a thawing state; Hence in ice-houses the external parts of the collection of ice is perpetually thawing and thus preserves the internal part of it; so that it is necessary to lay up many tons for the preservation of one ton. Hence in Italy considerable rivers have their source from beneath the eternal glaciers, or mountains of snow and ice.
In our country when the air in the course of a frost continues a day or two at very near 32 degrees, the common heat of the earth thaws the ice on its surface, while the thermometer remains at the freezing point.
This circ.u.mstance is often observable in the rimy mornings of spring; the thermometer shall continue at the freezing point, yet all the rime will vanish, except that which happens to lie on a bridge, a board, or on a cake of cow-dung, which being thus as it were insulated or cut off from so free a communication with the common heat of the earth by means of the air under the bridge, or wood, or dung, which are bad conductors of heat, continues some time longer unthawed. Hence when the ground is covered thick with snow, though the frost continues, and the sun does not s.h.i.+ne, yet the snow is observed to decrease very sensibly. For the common heat of the earth melts the under surface of it, and the upper one evaporates by its solution in the air. The great evaporation of ice was observed by Mr. Boyle, which experiment I repeated some time ago.
Having suspended a piece of ice by a wire and weighed it with care without touching it with my hand, I hung it out the whole of a clear frosty night, and found in the morning it had lost nearly a fifth of its weight. Mr. N. Wallerius has since observed that ice at the time of its congelation evaporates faster than water in its fluid form; which may be accounted for from the heat given out at the instant of freezing; (Saussure's Essais sur Hygromet. p. 249.) but this effect is only momentary.
Thus the vegetables that are covered with snow are seldom injured; since, as they lie between the thawing snow, which has 32 degrees of heat, and the covered earth which has 48, they are preserved in a degree of heat between these; viz. in 40 degrees of heat. Whence the moss on which the rein-deer feed in the northern lat.i.tudes vegetates beneath the snow; (See note on Muschus, Vol. II.) and hence many Lapland and Alpine plants perished through cold in the botanic garden at Upsal, for in their native situations, though the cold is much more intense, yet at its very commencement they are covered deep with snow, which remains till late in the spring. For this fact see Amaenit. Academ. Vol. I. No.
48. In our climate such plants do well covered with dried fern, under which they will grow, and even flower, till the severe vernal frosts cease. For the increase of glaciers see Note on Canto I. l. 529.
NOTE x.x.xIII.--WINDS.
_While southern gales o'er western oceans roll, And Eurus steals his ice-winds from the pole._
CANTO IV. l. 15.
The theory of the winds is yet very imperfect, in part perhaps owing to the want of observations sufficiently numerous of the exact times and places where they begin and cease to blow, but chiefly to our yet imperfect knowledge of the means by which great regions of air are either suddenly produced or suddenly destroyed.
The air is perpetually subject to increase or diminution from its combination with other bodies, or its evolution from them. The vital part of the air, called oxygene, is continually produced in this climate from the perspiration of vegetables in the suns.h.i.+ne, and probably from the action of light on clouds or on water in the tropical climates, where the sun has greater power, and may exert some yet unknown laws of luminous combination. Another part of the atmosphere, which is called azote, is perpetually set at liberty from animal and vegetable bodies by putrefaction or combustion, from many springs of water, from volatile alcali, and probably from fixed alcali, of which there is an exhaustless source in the water of the ocean. Both these component parts of the air are perpetually again diminished by their contact with the soil, which covers the surface of the earth, producing nitre. The oxygene is diminished in the production of all acids, of which the carbonic and muriatic exist in great abundance. The azote is diminished in the growth of animal bodies, of which it const.i.tutes an important part, and in its combinations with many other natural productions.
They are both probably diminished in immense quant.i.ties by uniting with the inflammable air, which arises from the mud of rivers and lakes at some seasons, when the atmosphere is light: the oxygene of the air producing water, and the azote producing volatile alcali by their combinations with this inflammable air. At other seasons of the year these principles may again change their combinations, and the atmospheric air be reproduced.
Mr. Lavoisier found that one pound of charcoal in burning consumed two pounds nine ounces of vital air, or oxygene. The consumption of vital air in the process of making red lead may readily be reduced to calculation; a small barrel contains about twelve hundred weight of this commodity, 1200 pounds of lead by calcination absorb about 144 pounds of vital air; now as a cubic foot of water weighs 1000 averdupois ounces, and as vital air is above 800 times lighter than water, it follows that every barrel of red lead contains nearly 2000 cubic feet of vital air.
If this can be performed in miniature in a small oven, what may not be done in the immense elaboratories of nature!
These great elaboratories of nature include almost all her fossil as well as her animal and vegetable productions. Dr. Priestley obtained air of greater or less purity, both vital and azotic, from almost all the fossil substances he subjected to experiment. Four ounce-weight of lava from Iceland heated in an earthen retort yielded twenty ounce-measures of air.
4 ounce-weight of lava gave 20 ounce measures of air.
7 ............... basaltes .... 104 ......................
2 ............... toadstone .... 40 ......................
11/2 ............... granite .... 20 ......................
1 ............... elvain .... 30 ......................
7 ............... gypsum .... 230 ......................
4 ............... blue slate .... 230 ......................
4 ............... clay .... 20 ......................
4 ............... limestone-spar .... 830 ......................
5 ............... limestone .... 1160 ......................
3 ............... chalk .... 630 ......................
31/2 ............... white iron-ore .... 560 ......................
4 ............... dark iron-ore .... 410 ......................