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stance of the first change ; for in that substance there is no inter-vening state between solidity and fluidity. The melting of glass,of wax, and of tallow', exhibits instances of the second hind ofchange; for these bo dies pass through every possible degree otsoftness before they terminate in fluidity. In general, these solidbodies which crystallise or assume regular prismatic figures, haveno interval between solidity and fluidity, while those that do notusually assume such shapes have the property of appearing suc-cessively in all the intermediate states. Solid bodies never beginto assume ,a liquid form till they are healed to a certain tempera-ture; this temperature is constant in all. In the first class of bo-dies it is very w ell defined ; but in the second, though it is equallyconstant, the exact temperature of fluidity cannot be pointed outwith such precision on account of the infinite number of shades ofsoftness through which the bodies pass before they acquire theirgreatest possible fluidity. But even in these bodies we can easilyascertain that the same temperature always produces the samedegree of fluidity. The temperature at which this change fromsolidity to liquidity takes place, receives different names accordingto the usual state of the body thus changed. When the bodv isvisually observed in a liquid state, we call the temperature at whichit assumes the form of a solid, its freezing point, or congealingpoint. Thus the temperature in which water becomes ice, is call-ed the freezing point of water; on the other hand, when the bodyis usually in the state of a solid, we call the temperature at whichit liquefies its melting point; thus 212* is the melting point of sul-phur, 442° the melting point of tin. The following table containsa list of the melting points of a considerable number of solid bo-dies :

Substance. Melting point.

Lead. 594°

Bismuth.576

Tin.442

Sulphur. 212

Wax. 142

Spermaceti.133

Phosphorus. ..100

Tallow. 92

Oil of anise. 50

Olive-oil... 36

Ice. 32

Milk. 30

Vinegar. 28

Blood. 25

Oil ofbergamot. 23

Wines.... 20

Oil of turpentine. 14

Sulphuric acid. 36

Mercury. 39

Liquid ammonia. 46

Ether. 46

Nitric acid...-.. 66

Before Dr. Black began to deliver his chemical lectures inGlasgow in 1757, it was universally supposed that solids were con-verted into liquids by a small addition of beat after they had beenonce raised to the melting point, and that they returned again tothe solid state on a very small diminution of the quantity of heatnecessary to keep them at that temperature. An attentive viewof the phenomena of liquefaction and solidification gradually ledthis sagacious philosopher to observe their inconsistence with thethen received opinions, and to form another, whirl) lie verified bvdirect experiments, and drew up an account of his theory, andthe proofs of it, which was read to a literary society in Glasgow onApril 23d, 1762 ; and every year after he gave a detailed accountof the whole doctrine in his lectures. The opinion which heformed was, that when a solid body is converted into a liquid, amuch greater quantity of heat enters into it than is perceptibleimmediately after by the thermometer. This great quantity ofheat does not make the body apparently wanner, but it must bethrown info it in order to convert it into a liquid; ami this greataddition of heat is the principal and most immediate cause of thefluidity induced. On the other hand, when a liquid body assumesthe form of a solid, a very great quantity of heat leaves it withoutsensibly diminishing its temperature; and the slate of solidity can-

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not be induced without the abstraction of this great quantity ©fiieat. Or, in oilier words, whenever a solid is converted into afluid, it combines w.ih a certain dose of calorie, witliout any aug-mentation of its temperature; and it is this dose of caloric whichoccasions the change oi the sol.(I into a fluid. When the fluid isconverted again into a solid, tiie do-e of caloric leaves it, withoutany diminution of its temperature; and it is this abstraction whichoccasions the change. Minis the combination of a certain dose ofcaloric with ice causes it to become water, and the. abstraction of acertain dose of caloric from wafer causes it to become ice. Waterthen is a compound of ice and caloric; and in general all fluidsare combinations of the solid, to which they may he converted bythe application of cold, and a Certain dose of caloric. Such is theopinion concerning the cause oi fluidity taught by Dr. Black asearly as 1762. Its truth was established by the following experi-ments:

First. If a lump of ice, at the temperature of 22°, is broughtinto a warm room, in a very short time it,is heated eo 32°, thefreezing point. Il then begins to melt; but the process goes onvery slowly, and several hours elapse before the whole ice is melt-ed. During the wliol ■ of that time its temperature continues at32°; yet as it is constantly surrounded by warm air, we have rea-son to believe that caloric is constantly entering into it. Now asnone of this caloric is indicated by the thermometer, whit be-comes of it, unless it has combined with that portion of the icewhich is converted into water, and unless it is tlie cause of themelting of the ice? Dr. Black took two thin globular glasses,four inches in diameter, and very neatly of the same weight.Botli were filled with water; the contents ot the one were frozeninto a solid mass of ice, the contents of tin* other were cooleddown to 33°; the two glasses were then suspended in a large roomat a distance from all other bodies, the temperature of the air be-ing 47°. In half an hour the thermometer placed in the water-glass rose from 33° to 40°, or 7 degrees; the ice was at first 4 or 5degrees colder than melting snow, but in a few minutes the ther-mometer applied to it stood at 32". 'l'lie instant of time when itreached that temperature was noted, and the whole left undisturb-ed for ten hours and a half. At the end of that time the wholeice was melted, except a very small spongy mass, which floated atthe top, and disappeared in a few minutes. The temperature ofthe ice water was 40°. Thus 10-J hours were necessary to meltthe ice, and raise the product to the temperature of 40°. Duringall this time it must have been receiving heal with the same cele-rity as the water-glass received it during the first half-hour. Thewhole quantity received th«*n was 21 times 7, or 147°; but its tem-perature was only 40°: therefore 139 or 140 degrees had been ab-sorbed by the melting ice, and remained concealed in the waterinto which it had been converted, its presence not being indicatedby the thermometer. Tint caloric, or beat, is actually enteringinto the ire, is easily ascertained by placing the hand on a thermo-meter under the vessel containing it. A current of cold air maybe perceived descending from it during the whole lime of the pro-cess. But it will be said, perhaps, that the heat winch enters intothe ice does not remain there, but is altogether destroyed, Thisopinion is refuted by the following experiment.

Second. If, when the thermometer is at 22°, we expose a ves-sel full of water at 52° to the open air, and beside it another vesselfull of brine at the same temperature, with' thermometers in each,we shall find that both of them gradually lose calorie, and arecooled down to 32°. After this the brine (which dots not freezetil! cooled down to 0°) continues to cool without interruption, andgradually reaches 22°, the temperature of the air, but the purewater remains stationary at 32". It freezes, indeed, but veryslowly; and during the whole process its temperature is 32 •Now, why should the one liquid refuse ail on a sudden to give outcaloric and not the other? Is it not much move probable that Urnwater, as it freezes, gradually gives out the heat which it had ab-sorbed (hiring its liquefaction; and that this evolution maintainsthe temperature of the water at 3!T, notwithstanding what it partswit!) to the air during the whole process? We may easily satisfyottr-elves that the water while congealing is constantly impartingheat to the surrounding air; for a delicate thermometer suspendedabove it is constantly affected by an ascending stream of air lesscold titan the air around. The’following experiment, first made