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it on a strainer covering a cylindrical vessel which is placed in a cauldron of boiling water; the wax is received into the former vessel, and on congealing is ready for the market. The Pe-la or white wax in its chemical properties is analogous to purified beeswax, and also spermaceti, but differs from both, being in my opinion an article perfectly sui generis. It is purely white, transparent, shining, not unctuons to the touch, inodorous, insipid, crumbles into a dry inadhesive powder between the teeth, with a fibrous texture, resembling fibrous calc-spar; it melts at 100° Fah., is insoluble in water, dissolves in heated essential oils, and is scarcely affected by boiling alcohol, the acids, or alkalies.

The aid of analytical chemistry is needed for the proper eluci dation of this most beautiful material. There can be no doubt it would prove altogether superior in the arts to purified beeswax. On extraordinary occasions the Chinese employ it for candles and tapers. It has been supposed to be identical with the white lac of Madras; but as the Indian article has been found useless in the manufacture of candles,* it cannot be the same; it far excels also the vegetable wax (Myrica cerifera) of the United States.

Is this substance a secretion? There are Chinese who regard it as such, some representing it to be the saliva and others the excrement of the insect. European writers take nearly the same view, but the best authorities expressly say that this opinion is incorrect, and that the animal is changed into wax. I am inclined to believe that the insect undergoes what may be styled a ceraceous degeneration, its whole body being permeated by the peculiar produce in the same manner as the Coccus cacti is by carmine. Its cost at Ningpo varies from 22 to 33 cents per pound. The annual produce of this humble creature in China cannot be far from 400,000 pounds, worth more than $100,000. Ningpo, August, 1850.

ART. V. On the sudden disappearance of the Ice on Lake
Champlain, at the breaking up of Winter; by Rev. ZADOCK
THOMPSON, of Burlington, Vermout.

THE vanishing of the ice on Lake Champlain in spring is at times so sudden, as to strike general observers with much surprise. But, for myself, although I have lived thirty years upon its shores, where I have had a full view of its broadest part, and where I have watched its closing in winter and its opening in spring, with no inconsiderable interest, I have not in that time witnessed anything, in relation to the phenomenon above mentioned, which has appeared to me either mysterious or very wonderful. In the last fifty years there have been a few cases (cer

* Dr. Pearson's Philosophical Transactions, vol. xxi.

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tainly not more than three or four), in which the lake has been entirely covered with ice on one day, and entirely clear of ice on the next. One case only of this kind has occurred within the period of my own observations, and that took place about twenty years. since. The ice at that time, though of considerable thickness, had become exceedingly porous and rotten, and for some days had been considered unsafe; but, having been stiffened at its surface by a sharp frost in the night, some persons ventured upon it in the morning, and passed safely over the lake on foot, where the width six or seven miles; but during the day a storm set in, with a very strong wind, and, on the following morning, the ice had entirely disappeared; there was none to be seen on the lake.

Now an occurrence of this kind is certainly calculated to excite wonder in the mind of the general observer, and make the sudden clearing of the lake a common topic of conversation; but it appears to me that all the mystery about the matter vanishes at once whenever the circumstances are carefully considered. These circumstances I have stated in general terms in Part I, p. 14 of my Nat. and Civil Hist. of Vermont, published in 1842. But experiments and observations made since that time require me to modify some of the statements there presented. It was then supposed that, when the ice commenced forming upon the surface of the lake, the great body of water below was at a temperature of about 39° or 40°. This, according to the researches of Count Rumford, would doubtless be true were the waters cooled down without agitation; but I find it is not true in fact, and from the observations I have made, I am now inclined to the opinion that, in consequence of their violent agitation by the cold winds which prevail in the early part of winter, the whole mass of waters is usually cooled down very nearly to the freezing point before any ice forms at the top, and that, after the waters are protected from the winds by a covering of ice, their temperature is gradually raised by the reception of heat from the earth beneath.

Since the publication of my history of Vermont I have made some experiments for the purpose of ascertaining the temperature of the water of the lake at various depths, after it had been for some time entirely covered with ice. In all these I have found the temperature some degrees above freezing, but not quite so much above as I had previously supposed. As an example, I copy the following record from my meteorological journal for 1844: "March 27. Temperature of the water of the lake in contact. with the ice, 32°; six feet below the surface, 32°; twelve feet below, 3430; and twenty-five feet below, being the whole depth of the lake at that place, 350." These observations were made nearly one-fourth of a mile from the shore, and after the lake had been covered with ice about eight weeks.

It has long been my intention to ascertain the temperature at the bottom of the deeper parts of our lake when covered with ice,

but I have not hitherto examined where the depth has exceeded 35 feet; I should have attempted it this spring but for the unexpected disappearance of the ice, the last of March.*

A few days after the disappearance of the ice this year, I found the temperature of the water to be 3610, and, as the lake was at the time much agitated by the wind, this was, doubtless, very nearly the mean temperature of the whole mass of water.

There are three well attested facts connected with this subject, which, taken together, appear to me to afford a full and satisfactory explanation of the phenomenon.

1. The great body of ice is previously always reduced to the honey-comb structure, in which condition any considerable agitation of the water causes it to separate into minute divisions.

2. The temperature of the great mass of water is always several degrees above the freezing point, and in a condition to dissolve. the minute divisions of ice with great facility when agitated in it. 3. The phenomenon is always attended by a very high wind, producing the agitation required.

In addition to theoretic objections to the opinion that the ice sinks when it disappears suddenly, we, who live on the shores of the lake, think we have ocular proof that it does not take place. We see the ice, while yet spreading over the whole surface of the lake, gradually wasting as the spring advances, and becoming less firm, till at length it is so far disintegrated that a stick may be easily thrust through it, while it is still from six to twelve inches thick. This disintegration is often carried so far, before the general icy covering is disturbed, that the ice has little more solidity or tenacity than loose snow saturated with water. In this state of things a strong wind soon produces rents in the ice, the waters, before pent up and quiet, are thrown into violent agitation, and we actually see the slightly cohering masses falling to pieces and dissolving at the surface of the lake; but we never see the ice sinking, nor can I learn that any evidence of this has ever been observed in masses lying at the bottom of the lake.

The objection to this hypothesis-"that so sudden and extensive a conversion of a solid into a fluid, as it supposes, would produce a sudden and violent frost through the neighboring country," ,"-would, undoubtedly, have weight were the caloric required

* Although the lake this year began to open, in some places, before the middle of March, it remained mostly covered with ice of considerable thickness and firmness till the 29th of that month, when, in the afternoon, a brisk wind sprang up from the south, which increased to a strong gale during the night, and continued to blow with great violence in the same direction during the 30th and up to noon on the 31st, when it changed to the west and blew a strong gale in that direction through the remainder of the day. Before evening on the 30th, the ice had all vanished from the broader parts of the lake, and on the 1st day of April, the lake and bays, so far as visible from this place, were entirely cleared of ice. This, considering the thickness and firmness of the ice at the time, is one of the most remarkable clearances of the lake we have witnessed for several years.

to liquefy the ice supposed to be derived wholly or principally from the atmosphere. But this I do not believe to be the fact. The process of liquefaction is aided, more or less, according to the temperature, by the warmth of the atmosphere, and, to the same extent, the air is cooled; but the great supply of caloric for the liquefaction of the ice is derived from the waters beneath. This absorption of heat from the waters below has no effect in diminishing the temperature of the air above. By it, the great mass of water may be cooled down a few degrees, still leaving the surface of the lake, in contact with the atmosphere, warmer by some degrees than the previous covering of ice. If we assume, for example, the average depth of Lake Champlain to be fifty feet, and the thickness of the ice to be eight inches, and the temperature of the water below to be 36°, immediately before the sudden disappearance of the ice, (and these do not, probably, differ very much from the usual conditions,) then, since the heat of fluidity in water is 140°, it is easy to calculate that the sudden liquefaction of the ice, by heat derived wholly from the water, would diminish the temperature of the whole mass of water less than two degrees, leaving the surface in contact with the atmosphere more than two degrees warmer than the ice.

There are some other phenomena connected with this lake, which I regard as interesting, but I defer touching upon them to another time.

Burlington, Vermont, April, 1851.

ART. VI.-On Coral Reefs and Islands; by JAMES D. Dana.— Part II.

From the Report on Geology of the Exploring Expedition under Capt. Wilkes, U.S. N. 3. CORAL ISLANDS.

A. Forms and general features of Coral Islands.

A barrier reef, and a lagoon enclosed by it, are the prominent features of a coral island; yet there are a few of small size in which the lagoon is wanting. In the larger islands, the waters within look like the ocean, and are similarly roughened by the wind, though not to the same extent. Standing on the north shore of the Raraka lagoon, (in the Paumotus,) and looking southwest, nothing is descried but blue waters;-far in the distance, to the right or left, a few faint dots are distinguished; and as the eye sweeps around, these gradually enlarge into lines of palms and other verdure, which finally become distinct groves on nearing the observer. At Dean's Island, another of the Paumotus, and at many of the Carolines, the resemblance to the ocean is still more striking. The lagoon is in fact but a fragment of the ocean cut SECOND SERIES, Vol. XII, No. 34.-July, 1851.

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off by more or less perfect walls of coral reef-rock; and the reef is here and there surmounted by verdure, forming a series of islets.

In many of the smaller coral islands, the lagoon has lost its ocean character, and become a shallow lake, and the green islets of the margin have coalesced in some instances into a continuous line of foliage. Traces may perhaps be still detected of the passage or passages over which the sea once communicated with the internal waters, though mostly concealed by the trees and shrubbery which have spread around and completed the belt of verdure. The coral island is now in its most finished state: the lake rests quietly in its bed of palms, hardly ruffled by the storms that madden the surrounding ocean.

From the islands with small lagoons, there is every variety in gradation down to those in which there is no trace of a lagoon. These simple banks of coral are the smallest of coral islands.

These remarks, in connection with the general view given on a preceding page, will prepare the reader to appreciate the following descriptions of various coral islands, illustrating their forms, actual size, and condition.

A single group of islands, the Tarawan or Kingsmills, (see Plate,) affords good examples of the principal varieties. The irregularity of shape and size is at once apparent to the eye. In the southernmost, Taputeouea, the form is very narrow, the length being thirty-three miles, with the width of the southern portion scarcely exceeding six miles, and that of the northern more than one-half less. The emerged land is confined to one side, and consists of a series of islets upon the eastern line of coral reef. The western side is for the most part some feet under water, and there is hardly a proper lagoon. Sailing by the island, to windward, the patches of verdure thus strung together seem to rise out of a long white line of breakers, the sea surging violently against the unseen coral reef upon which they rest.

Namouti, the next island north, is about twenty miles long by eight broad. The rim of land, though in fewer islets, is similar to that of Taputeouea in being confind to the reef frouting northeast. The reef of the opposite side, though bare of vegetation, stands near low tide level, and the whole encloses a large lagoon.

Nanouki and Apamama, though smaller than Namonti, have the same general character. Nanouki is triangular in shape, and has an islet on the western point or cape, which is quite prominent. Apamama differs from either of the preceding in having two narrow ship entrances to the lagoon, one through the northwestern reef, and another through the southwestern.

* The plate is a reduced copy of the chart of these islands, as surveyed by the Exploring Expedition.

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