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ftratum did, fending down at the fame time afrefli portion cooled to about 40°. Thus, what-ever be the heat of the water-at top of the jar,the ice itfelf is only acted on by water at about40°, for only at this denfity can it fink througha ftratum at 3 2°; and therefore, as long as anatom of ice remains unmelted, this afcent ofwater at 32 0 and defcent of water at 40° mufttake place.
Hence it is inferred that as much ice will bemelted in a given time by fuperincumbent waterat 40°, as by the fame quantity of boiling waterwhen at reft. In the experiment the undulationcaufed by the pouring in the water muft havelubfided before a fair comparifon can be made,and then it was actually found after allowingfor accidental irregularities* that full as muchice was melted in equal times by the cold as bythe hot water, however paradoxical it mayappear at firft fight. Some powerful objectionshave been made to Count Rumford ’s opinion ofthe non-conducting power of water when atreft, taken abfolutely; but there can be no doubtwhatever that by far the greateft part of thetranfmiffion of caloric through fluids is made bythe mutual currents of the hotter and colderpart; and therefore that it is very flowly andwith extreme difficulty that heat can be madeto pafs downwards through fluids, the fingleinftance of water between 53° and 32 0 beingexcepted, which is onlv a confirmation of thefa£l.
Before we mention the objections to theopinion of the abfolute non-conducting powerof water, we cannot refill inferring the beauti-ful fuggeftion attempted by the fame ingeniousphilofopher, of the final caufe of this Angulardifference in the common law of regular con-denfation by cold, which occurs in water andno other fluid in nature, and only when ap-proaching to within a few degrees of the freezingpoint. The direift operation of this law is toretard congelation, and to preferve a moderatetemperature in all natural frelh waters duringthe fevereft cold, whereby it appears that thelives of a vaft number of its fubaqueous inhabit-ants are preferred, and an opportunity is af-forded to man and other animals to procure inthe coldeft feafons this neceffary element. Theearth being always warmer in fevere froft t-hanthe air, the cooling of the waters on the face oftire earth muft of courfe begin from its upperfurface, which is expofed to the cold atmof-phere. Therefore, when an intenfe cold airfweeps over a piece of water (a frelh-water lakefor example) the furface as it cools becomes of
courfe of greater denfity and finks down, givingplace to the warmer ftratum below to come tothe furface. This in its turn becomes cooledand denfer, and again fubfides to give room tothe next ftratum. Thus the whole mals is firftcooled to the point of extreme denfity, that isabout 42 0 . But if this point were, like otherfluids, the inftant before freezing, the waterwould cool to 3 2 0 uniformly, after which (theexternal cold Hill continuing much below thatpoint) the next ftep would probably be a fuddencongelation of the whole body of ice-cold water.But as the denfity of water diminifhes from 42°to the freezing point, no further finking of thecolder and rife of the warmer ftrata can takeplace, fo that the upper ftratum alone nowreceives the whole of the chilling aftion of theair, and there remains till it is converted into acruft of ice, the water beneath being all thewhile about 42 0 . When ice, it is ftill a worfeconductor of heat than water at reft, and thusit long interpofes between the cold of the air andthe warm water beneath, befides that it prote£lsit from the extreme chilling effect of fweepingwinds ; and it muft be very flowly, ftratum afterftratum, that the procefs of congelation canproceed downwards through a confiderabledepth of water.
On the other hand the fame circumftancecontributes to haften the thawing of ice in awarm atmofphere. If this peculiarity did notoccur, and if water at reft were an abfolutenon-conduftor of heat, it would be impoffiblethat a mafs of ice fhould ever thaw except bythe flow and circuitous paflage of heat upwardsthrough the earth. For the firft effect of thewarm air would be to thaw a thin ftratum onthe furface of the ice, which if it were (as inother liquids) lighter than the cold furface thattouched the ice, would only remain at reft onthe fpot in which it was thawed, and wouldtranfmit no heat whatever to the ice below.Whereas, as it actually happens, the thawedfurface with the firft additional heat that itacquires, becomes heavier inftead of lighter, andof courfe finks to the furface of the yet un-thawed ice, imparts to it its excefs of heat, andagain, becoming lighter thereby, returns to theair, to acquire a frefli portion of heat which itagain carries down. Ihus a conftant circulationof alternately wanner and colder water goes onabove the ice till it is all thawed, and alfo pro-ceeds further downwards till the temperature ofthe whole water riles to about 42 0 ; after whichthe ufual law of expanfion by heat is refumed,for all the higher temperatures.