The opening being towards the north-east, and corresponding with a gorge in the hills opposite, running in the same direction, none but cold winds could reach the mouth of the grotto. Moreover, the soil above was so thickly covered with trees and brushwood, that the rays of the sun could not reach the earth, much less the rock below. Credible persons a.s.serted that since some of the trees had been felled, there had not been so much ice in the cave.
In order to test the presence of salt, M. des Boz melted some of the ice, and evaporated the resulting water, but found no taste of salt in the matter which remained.[183] He denied the existence of the spring of water which previous accounts had mentioned, and believed that the water which formed the ice came solely from melted snow, and from the fissures of the rock.
In 1727, the Duc de Levi caused the whole of the ice to be removed from the cave, for the use of the army of the Saone, which he commanded. In 1743 the ice had formed again, and the grotto was subjected to a very careful investigation by M. de Cossigny, chief engineer of Besancon, in the months of August and October.[183] The thermometer he used had been presented to him by the Academy, and was very probably constructed by M.
de Reaumur himself, for de Cossigny"s account was sent through M. de Reaumur to the Academy, but still the observations made with it cannot be considered very trustworthy. On the 8th of August, at 7.30 A.M., the temperature in the cave was 1/2 above the zero point of this thermometer, and at 11.30 A.M. it had risen to 1 above zero. On the 17th of October, at 7 A.M., the thermometer stood at 1/2, and at 4 P.M.
it gave the same register.
M. de Cossigny found that the entrance to the cave was rather more than 150 feet above the Abbey of Grace-Dieu, and about half a league distant by the ordinary path. A great part of his account is occupied by contradictions of previous accounts, especially in the matter of dimensions,[184] The people of Besancon had urged him to stay only a short time in the cave, because of the sulphureous and nitrous exhalations, but he detected no symptoms of anything of that kind. The most curious thing which he saw was the soft earth which lay, and still lies, at the bottom of the long slope of ice by which the descent is made; and he subjected this to various chemical tests and processes, but could not find that it contained anything different from ordinary earth.[185]
When M. de Cossigny visited the cave, there were thirteen or fourteen columns of ice, from 6 to 8 feet high, and he was in consequence inclined to doubt the accuracy of the statement of M. Billerez, that in his time (1711) there were three columns only, from 15 to 20 feet high.
But my own observation of the shape of the columns suggested that the largest of all was probably an amalgamation of several others; so that it is not unreasonable to suppose that after the Duc de Levi removed the large columns seen by M. Billerez, a number of smaller columns were formed on the old site, and that these had not become large enough to amalgamate in 1743.
Not satisfied with these visits of August and October, M. de Cossigny visited the cave in April 1745. He found the temperature at 5 A.M. to be exactly at the freezing point, and at noon it had risen 1. From this he concluded that the stories of the greater cold in the cave during the summer, as compared with the winter, were false.
In 1769, M. Prevost, of Geneva, visited the cave, as a young man; and in 1789, he wrote an account of his visit in the _Journal de Geneve_ (March), which was afterwards inserted as an additional chapter in his book on Heat.[186] He believed that one or two hundred _toises_ was the utmost that could be allowed for the height of the hill in which the glaciere lies,--a sufficiently vague approximation. He rejected the idea of salt as the cause of ice, and came to the conclusion that the cave was in fact nothing more than a good natural ice-house, being protected by dense trees, and a thick roof of rock, while its opening towards the north sheltered it from all warm winds. He accounted for the original presence of ice as follows:--In the winter, stalact.i.tes form at the edges of various fissures in the roof, and snow is drifted on to the floor of the cave by the north winds down the entrance-slope. When the warmer weather comes, the stalact.i.tes fall by their own weight, and, lying in the drifted and congealed snow, form nuclei round which the snow is still further congealed, and the water which results from the partial thaw of portions of the snow is also converted into ice. Thus, a larger collection of ice forms in winter than the heat of summer can destroy; and if none of it were removed, it might, in the course of years, almost fill the cave. At the time of his visit (August), M.
Prevost found only one column, from 6 to 8 feet high.
In 1783 (August 6), M. Girod-Chantrans visited the Glaciere of Chaux (so called from a village near the glaciere, on the opposite side from the Abbey of Grace-Dieu), and his account of the visit appeared in the _Journal des Mines_[187] of Prairial, an iv., by which time the writer had become the Citizen Girod-Chantrans. He found a ma.s.s of stalact.i.tes of ice hanging from the roof, as if seeking to join themselves with corresponding stalagmites on the floor of the cave; the latter, five in number, being not more than 3 or 4 feet high, and standing on a thick sheet of ice. There was a sensible interval between this bas.e.m.e.nt of ice and the rock and stones on which it reposed: it was, moreover, full of holes containing water, and the lower parts of the cave were unapproachable by reason of the large quant.i.ty of water which lay there. The thermometer stood at 359 F.
two feet above the floor, and at 78 F. in the shade outside. M.
Girod-Chantrans determined, from all he saw and heard, that the summer freezing and winter thaw were fables, and he believed that the cave was only an instance of Nature"s providing the same sort of receptacle for ice as men provide in artificial ice-houses. He was fortunate enough to obtain by chance the notes of a neighbouring physician, who had made careful observations and experiments in the glaciere at various seasons of the year, and a _precis_ of these notes forms the most valuable part of his account.
Dr. Oudot, the physician in question, found ten columns in January 1778, the largest of which was 5-1/2 feet high. The flooring of ice was nowhere more than 15 inches thick, and the parts of the rock which were not covered with ice were perfectly dry. The thermometer--M.
Girod-Chantrans used Reaumur, so I suppose that he gives Dr. Oudot"s observations in degrees of Reaumur, though some of the results of that supposition appear to be anomalous--gave 22 F. within the cave, and 21 F. outside.
In April of the same year, the large column had increased in height to the extent of 13 inches; and the floor of ice on which it stood was 1-1/2 inch thicker, and extended over a larger area than before; the thermometer stood at 36.5 F. and 52 F. respectively in the same positions as in the former case. In July, the large column had lost 6 inches of its height, and the thermometer gave 38.75 F. and 74.75 F.
In October, the large column was only 3 feet high, and many of the others had disappeared, while their pedestal had become much thinner than it had been in the preceding months. There was also a considerable amount of mud in the cave, brought down apparently by the heavy rains of autumn. The thermometer gave 37.6 F. and 63.5 F.
On the 8th of January, 1779, there were nine columns of very beautiful ice, and one of these, as before, was larger than the rest, being 5 feet high and 10 feet in circ.u.mference. The temperatures were 21 F. and 16.15 F. in the cave and in the open air respectively.
Tradition related that, before the removal of the ice in 1727, one of the columns reached the roof, (Prevost calculated the limits of the height of the cave at 90 and 60 feet,) and this suggested to Dr. Oudot the idea of placing stakes of wood in the heads of the columns he found in the cave, in the hope that ice would thus collect in greater quant.i.ties under the fissures of the roof. Accordingly, he made holes in three of the columns, and established stakes 4, 5, and 10 feet high, returning on the 22nd of February, after an interval of six weeks, to observe the result of his experiment. He found the two shorter stakes completely masked with ice, forming columns a foot in diameter; and the longest stake, though not entirely concealed by the ice which had collected upon it, was crowned with a beautiful capital of perfectly transparent ice. The columns which had no stakes fixed upon them had also increased somewhat in size, but not nearly in the same proportion as those which were the subject of Dr. Oudot"s experiment. The thermometer on this day gave 29.5 F. and 59 F. as the temperatures.
It may be remembered that I found one very beautiful column, far higher than any of those mentioned by Dr. Oudot, and higher than those which M.
Billerez saw, formed upon the trunk and branches of a fir-tree. I have now no doubt that the peculiar shape of another--the largest of the three columns which were in the cave at the time of my visit--is due to the fact of its being a collection of several smaller columns, which have in course of time flowed into one as they increased separately in bulk, and that its height has been augmented by a device similar to that adopted by Dr. Oudot. The two magnificent capitals which this column possessed, as well as the numerous smaller capitals which sprang from its sides, will thus be completely accounted for.
One more account may be mentioned, before I proceed to the theory which has found most favour in Switzerland of late years. M. Cadet published some _Conjectures_ on the formation of the ice in this cavern, in the _Annales de Chimie,_ Nivose, an XI.[188] He saw the cave in the end of September 1791, and found very little ice--not a third of what there had been a month before, according to the account of his guide. The _limonadier_ of a public garden in Besancon informed him that the people of that town resorted to the glaciere for ice when the supplies of the artificial ice-houses failed, and that they chose a hot day for this purpose, because on such days there was more ice in the cave. Ten _chars_ would have been sufficient to remove all the ice M. Cadet found, and the air inside the cave seemed to be not colder than the external air; but, nevertheless, M. Cadet believed the old story of the greater abundance of ice in summer than in winter, and he attempted to account for the phenomenon.
The ground above and near the cave is covered with beech and chestnut trees, and thus is protected from the rays of the sun. The leaves of these trees give forth abundant moisture, which has been pumped up from their roots; and as this moisture pa.s.ses from the liquid to the gaseous state, it absorbs a large quant.i.ty of caloric. Thus, throughout the summer, the atmosphere is incessantly refrigerated by the evaporation produced by the trees round the cave; whereas in winter no such process goes on, and the cave a.s.sumes a moderate temperature, such as is usually found in ordinary caves. Unfortunately for M. Cadet"s theory, the facts are not in accordance with his imaginary data, nor yet with his conclusions. He adds, on the authority of one of his friends, that the intendant of the province, M. de Vanolles, wishing to preserve a larger amount of ice in the cave, built up the entrance with a wall 20 feet high, in which a small door was made, and the keys were left in the hands of the authorities of the neighbouring village, with orders that no ice should be removed. The effect of this was, that the ice diminished considerably, and they were obliged to pull down the wall again. M. Cadet saw the remains of the wall, and the story was confirmed by the Brothers of Grace-Dieu. It would be very interesting to know at what season this wall was built, and when it was pulled down. If my ideas on the subject of ice-caves are correct, it would be absolutely fatal to shut out the heavy cold air of winter from the grotto.
In 1822, M.A. Pictet, of Geneva, took up the question of natural glacieres, and read a paper before the Helvetic Society of Natural Sciences,[189] describing his visits to the caves of the Brezon and the Valley of Reposoir. In order to explain the phenomena presented by those caves, M. Pictet adopted De Saussure"s theory of the principle of _caves-froides_, rendering it somewhat more precise, and extending it to meet the case of ice-caves. It is well known that, in many parts of the world, cold currents are found to blow from the interstices of rocks; and these are utilised by neighbouring proprietors, who build sheds over the fissures, and so secure a cool place for keeping meat, &c. Examples of such currents are met with near Rome (in the _Monte Testaceo_), at Lugano, Lucerne (the caves of Hergiswyl), and in various other districts. It is found that the hotter the day, the stronger is the current of cold air; in winter the direction of the current is changed, and it blows into the rock instead of out from it.[190] De Saussure"s theory, as developed by M. Pictet, was no doubt satisfactory, so far as it was used to account for the phenomenon of "cold-caves," but it seems to be insufficient as an explanation of the existence of large ma.s.ses of subterranean ice; of which, by the way, De Saussure must have been entirely ignorant, for he makes no allusion to such ice, and the temperatures of the coldest of his caves were considerably above the freezing point.
Pictet represents the case of a cave with cold currents of air to be much the same as that of a mine with a vertical shaft, ending in a horizontal gallery of which one extremity is in communication with the open air, at a point much lower, of course, than the upper extremity of the shaft. The cave corresponds to the horizontal gallery, and the various fissures in the rock take the place of the vertical shaft, and communicate freely with the external air. In summer, the columns of air contained in these fissures a.s.sume nearly the temperature of the rock in which they rest, that is to say, the mean temperature of the district, and therefore they are heavier than the corresponding external columns of air which terminate at the mouth of the cave; for the atmosphere in summer is very much above the mean temperature of the soil, or of the interior of the earth at moderate depths. The consequence is, that the heavy cool air descends from the fissures, and streams out into the cave, appearing as a cold current; and the hotter the day is--that is, the lighter the columns of external air--the more violent will be the disturbance of equilibrium, and therefore the more palpable the cold current. Naturally, in this last case, the air which enters by the upper orifices of the fissures is more heated, to begin with, than on cooler days; but external heat so very slightly affects the deeper parts of the fissures, that the columns of air thus introduced are speedily impressed with the mean temperature of the district. In winter, the external columns of air are as much heavier than the columns in the fissures as they are lighter in summer; and so cold currents of air blow from the cave into the fissures, though such currents are not of course colder than the external air. Thus the mean temperature of the cave is much lower than that of the rock in which it occurs; for the temperature of the currents varies from the mean temperature of the rock to the winter temperature of the external atmosphere.
The descending columns of warmer air, in summer, must to some extent raise the temperature of the fissures above that which they would otherwise possess, that is, above the mean temperature of the place; but that may be considered to be counteracted by the corresponding lowering of the temperature of the fissures by the introduction of cold air from the cave in winter. By a similar reasoning, it will be seen that for some time after the spring change of direction in the currents takes place, the temperature of the cave will be less than would have been expected from a calculation founded on the true mean temperature of the rock through which the fissures pa.s.s. This, together with the fact of the porous nature of the rock in which most of the curious caves in the world occur, which allows a considerable amount of moisture to collect on all surfaces, and thereby induces a depression of temperature by evaporation, may be held to explain the presence of a greater amount of cold than might otherwise have been fairly reckoned upon in ice-caves.
The idea of cold produced by evaporation Pictet took up warmly, believing that when promoted by rapid currents of air it would produce ice in the summer months; and he thus explained what he understood to be the phenomena of glacieres. But it will have been seen, from the account of the caves I have visited, that the glacieres are more or less in a state of thaw in the summer; and M. Thury"s observations in the winter prove conclusively that they are then in a state of utter frost, so that the old belief with respect to the season at which the ice is formed may be supposed to have been exploded. The facts recorded by Mr. Scrope[191]
would appear to depend upon the peculiar nature of rocks of volcanic formation; and I am inclined to think there is very little in common between such instances as he mentions and the large caves filled with ice which are to be found in the primary or secondary limestone.
One of De Saussure"s experiments, in the course of his investigation of the phenomena and causes of cold currents in caves, is worth recalling.
He pa.s.sed a current of air through a gla.s.s tube an inch in diameter, filled with moistened stones, and by that means succeeded in reducing the temperature of the current from 18 C. to 15 C.; and when the refrigerated current was directed against a wet-bulb thermometer, it fell to 14 C., thus showing a loss of 72 F. of heat. No one can see much of limestone caverns without discovering that the surfaces over which any currents there may be are constrained to pa.s.s, present an abundance of moisture to refrigerate the currents; and it is not unreasonable to suppose that the large number of evaporating surfaces, which currents pa.s.sing through heaps of debris--such as the basaltic stones described on page 261--come in contact with, are the main cause of the specially low temperature observed under such circ.u.mstances.
Pictet"s theory, however, did not convince all those into whose hands his paper fell, and M.J. Deluc wrote against it in the _Annales de Chimie et de Physique_ of the same year, 1822.[192] Deluc had not seen any glaciere, but he was enabled to decide against the cold-current theory by a perusal of Pictet"s own details, and of one of the accounts of the cave near Besancon. He objected, that in many cases the ice is found to melt in summer, instead of forming then; and also, that in the Glaciere of S. Georges, which Pictet had described, there was no current whatever. Further, in all the cases of cold currents investigated or mentioned by De Saussure, the presence of summer ice was never even hinted at, and the lowest temperatures observed by him were considerably above the freezing point. I may add, from my own experience, that on the only occasions on which I found a decided current in a glaciere--viz., in the Glaciere of Monthezy, and that of Chappet-sur-Villaz,--there was marked thaw in connection with the current. In the latter case, the channel from which the current came was filled with water; and in the former, water stood on the surface of the ice.
The view which Deluc adopted was one which I have myself independently formed; and he would probably have written with more force if he had been acquainted with various small details relating to the position and surroundings of many of the caves. The heavy cold air of winter sinks down into the glacieres, and the lighter warm air of summer cannot on ordinary principles of gravitation dislodge it, so that heat is very slowly spread in the caves; and even when some amount of heat does reach the ice, the latter melts but slowly, for ice absorbs 60 C. of heat in melting; and thus, when ice is once formed, it becomes a material guarantee for the permanence of cold in the cave.
For this explanation to hold good, it is necessary that the level at which the ice is found should be below the level of the entrance to the cave; otherwise the mere weight of the cold air would cause it to leave its prison as soon as the spring warmth arrived. In every single case that has come under my observation, this condition has been emphatically fulfilled. It is necessary, also, that the cave should be protected from direct radiation, as the gravitation of cold air has nothing to do with resistance to that powerful means of introducing heat. This condition, also, is fulfilled by nature in all the glacieres I have visited, excepting that of S. Georges; and there art has replaced the protection formerly afforded by the thick trees which grew over the hole of entrance. The effect of the second hole in the roof of this glaciere is to destroy all the ice which is within range of the sun. A third and very necessary condition is, that the wind should not be allowed access to the cave; for if it were, it would infallibly bring in heated air, in spite of the specific weight of the cold air stored within. It will be understood from my descriptions of such glacieres as that of the Grand Anu, of Monthezy, and the Lower Glaciere of the Pre de S. Livres, how completely sheltered from all winds the entrances to those caves are.
There can be no doubt, too, that the large surfaces which are available for evaporation have much to do with maintaining a somewhat lower temperature than the mean temperature of the place where the cave occurs. This had been noticed so long ago as Kircher"s time; for among the answers which his questions received from the miners of Herrengrund, we find it stated that, so long as mines are dry, the deeper they are the hotter; but if they have water, they are less warm, however deep.
From the mines of Schemnitz he was informed that, so long as the free pa.s.sage of air was not hindered, the mines remained temperate; in other cases they were very warm. Another great advantage which some glacieres possess must be borne in mind, namely, the collection of snow at the bottom of the pit in which the entrance lies. This snow absorbs, in the course of melting, all heat which strikes down by radiation or is driven down by accidental turns of the wind; and the snow-water thus forced into the cave will, at any rate, not seriously injure the ice. It is worthy of notice that the two caves which possess the greatest depth of ice, so far as I have been able to fathom it, are precisely those which have the greatest deposit of snow; and the ice in a third cave, that of Monthezy, which has likewise a large amount of snow in the entrance-pit, presents the appearance of very considerable depth. The Schafloch, it is true, which contains an immense bulk of ice, has no snow; but its elevation is great, as compared with that of some of the caves, and therefore the mean temperature of the rock in which it occurs is less unfavourable to the existence of ice.
I believe that the true explanation of the curious phenomena presented by these caves in general, is to be found in Deluc"s theory, fortified by such facts as those which I have now stated. The mean temperature of the rock at Besancon, where the elevation above the sea is comparatively so small, renders the temptation to suggest some chemical cause very strong.
The question of ice in summer where thaw prevails in winter, may fairly be considered to have been eliminated from the discussion of such caves as I have seen, in spite of the persistent a.s.sertions of some of the peasantry. The observations, however, in caverns in volcanic formations, and in basaltic debris, are so circ.u.mstantial that it is impossible to reject them; and in such cases a theory similar to that enunciated by Mr. Scrope[193] seems to be the only one in any way satisfactory, though I have not heard of such marvellous results being produced elsewhere by evaporation. One observer, for instance, of the cavern near the village of Both, in the Eiffel, found a thickness of 3 feet of ice; and in that case it was melting in summer, instead of forming. In some cases it has been suggested that the length of time required for external heat or cold to penetrate through the earth and rock which lie above the caves is sufficient to account for the phenomenon of summer frost and winter thaw. Thus, it is said, the thickness of the superinc.u.mbent bed may be such that the heat of summer only gets through to the cave at Christmas, and then produces thaw, while in like manner the greatest cold will reach the cave in mid-summer. But there is a fatal objection to this idea in the fact that the invariable stratum--i.e., the stratum beyond which the annual changes of external temperature are not felt--is reached about 60 feet below the surface in temperate lat.i.tudes,[194]
while at the tropics such changes are not felt more than a foot below the surface. Humboldt calculated that in the lat.i.tude of central France the whole annual variation in temperature at a depth of 30 feet would not amount to more than one degree.[195]
FOOTNOTES:
[Footnote 174: As Gollut"s phraseology is peculiar, it may be as well to reproduce his account of the cave:--"Je ne veux pas omettre toutefois (puisque je suis en ces eaux) de mettre en memoire la commodite que nature hat done a quelques delicats, puis qu"au fond d"un mntagne de Leugne, la glace (_gla.s.se_ in the index), se treuve en este, pour le plaisir de ceux qui aim[~e]t a boire frais. Neanmoins dans ce t[~e]ps cela se perd, n pour autre raison (ainsi que ie pense) que pour ce que lon hat depouille le dessus de la mtagne d"une epoisse et aulte fustaie de bois, qui ne permettoit pas que les raions du soleil vinsent echauffer la terre et deseicher les distillations, que se couloi[~e]t iusques au plus bas et plus froid de la montagne: ou (par l"antiperistase) le froid s"epoississoit, et se reserroit, contre les chaleurs, entornantes et environnantes le long de l"este, toute la circonference exterieure du mont."--_Histoire_, &c., p. 87.]
[Footnote 175: _Hist. de l"Acad._, t. ii., p. 2.]
[Footnote 176: _Hist. de l"Acad._, an 1712, p. 20.]
[Footnote 177: _C"est a dire_--M. Billerez explains--_a 10 degres au-dessous du tres-grand froid._ What the 60 may be worth, I cannot say.]
[Footnote 178: Tournefort (_Voyage du Levant_, iii. 17) believed that the ammoniac salt, of which the earth was full in some districts near Erzeroum, had something to do with the persistence of snow on the ground there.]
[Footnote 179: _Hist, de l"Acad.,_ an 1726, p. 16.]
[Footnote 180: But see on this point the experience of M. Thury, in the Glaciere of S. Georges (Appendix).]
[Footnote 181: Sir Roderick Murchison"s suggestion of the possible influence of salt in producing the phenomena of his ice-cave in Russia, did not, of course, proceed upon the supposition of salt actually mingling with water, but only of its increasing the evaporation of the air which came in contact with it.]
[Footnote 182: _Mem. presentes a l"Academie par divers Scavans_, i, 195.]
[Footnote 183: A long account was published in a history of Burgundy, printed at Dijon, in quarto, in 1737, which I have not been able to find. It was from the same source as the account in the Hist. of the Academy, in 1726.]
[Footnote 184: I took this earth to be a collection of the particles carried down the slope of ice by the heavy rains of the month preceding my visit. M. de Cossigny speaks of the abundant rains of July, his visit being in August.]
[Footnote 185: _Recherches sur la Chaleur_; Geneva and Paris, 1792.]
[Footnote 186: P. 65. Now called _Annales des Mines_.]
[Footnote 187: T. xlv. p. 160.]
[Footnote 188: _Bibliotheque Universelle de Geneve_, Premiere Serie, t.
xx.]
[Footnote 189: See De Saussure"s account of his numerous observations of such caves in the _Voyage dans les Alpes_, sections 1404-1415.]