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4 53

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by Fahrenheit, and afterwards often repeated by Dr. Black andothers, aflbrds a palpable evidence, that such an evolution of ca-loric actually takes place during congelation.

Third. If, when the air is at 22°, we expose to it a quantity ofwater in a tall beer-glass, with a thermometer in it and covered,the water gradually cools down to 22° without freezing. It istherefore 10° below the freezing point. Things being in this situ-ation, if the water is shaken, putt of it -instantly freezes into aspongy mass, and the temperature of the whole instantly rises tothe freezing point, so that the water has acquired ten degrees ofcaloric in an instant. Now' whpnce came these ten degrees? I itnot evident that it must have come from that part of the water■which was frozen, and consequently that water in the act of freez-ing gives out caloric? From many experiments made on waterin these circumstances, it is found that the quantity of ice whichforms suddenly on lire agitation of water, cooled down below thefreezing point, bears always a constant ratio to the coldness of theliquid before agitation. '1 bus when water is cooled down to 22°Very nearly of the whole freezes; when the previous tempera-ture is 27°, about of the whole freezes. In all cases when wa-ter is cooled down below 32°, it loses a portion of the caloricwhich is necessary to constitute its liquidity. 'Flic instant thatsuch water is agitated, one portion of the liquid seizes upon thequantity of caloric in which it is deficient, at the expence of an-other portion, which of course becomes ice. Thus when water iscooled down to 22°, every particle of it wants 10° of the caloricnecessary to keep it in a state of liquidity. Thirteen parts of itseize 10 degrees each from the fourteenth part. These thirteen ofcourse acquire the temperature of 32°; and the other part, beingdeprived of 10 x 13 = 130, which with the ten degrees that ithad lost before constitute 140°, or the whole of the caloric neces-sary to keep it fluid, assumes of consequence the form of ice.

fourth, ft these experiments should not be considered as suffi-cient to warrant Dr. Black’s conclusion, the follow ing, for whichwe are indebted to the same philosopher, puts the truth of his opi-nion beyond the reach of dispute. He mixed together givenweights of ice at 32°, and water at 190° of temperature. The icev^as melted in a few seconds, and the temperature produced was

The weight of the ice was 119 half-drachms;

That ot the hot water 135of the mixture 254of the glass vessel 16

Sixteen parts of glass have the same effect in heating cold bodies,as eight parts of equally hot water. Therefore, instead of the 16halt-drachms of glass, eight of water may be substituted, whichniakes the hot water amount to 143 halt-drachms. In this experi-htent there were 158 degrees of heat contained in the hot water,to be divided between the ice and the water. Had they been di-vided equally, and had the whole been afterwards sensible to thehermometer, the water would have retained parts of thisheat, and the ice would have received {%% parts: tint is to say,the water would have retained 86°, and tiie ice would have re-«'ved72°; and the temperature after mixture would have beenU4 . But the temperature by experiment is found to be only^ ; the hot water lost 137°, and the ice only received an addition. temperature equal to 21°. But the loss of 1S° of temperatureVr o wa * cr ' s equivalent to the gain of 21° in the ice. Thereforej8 18° = 140° ot heat have disappeared altogether from thewt water. These 140° must have entered into the ice, andcoif-erted it into water without raising its temperature. In the samemanner, if we take any quantity of ice, or, which is the same thing,s novv , at 32°, and mix it with an equal weight of water at 172”, the■ioo W melts, and the temperature of the mixture is onlysi . ere 'de water is cooled 140°, while the temperature of the'ow is not increased at all; so that 140° degrees of caloric have^P-ed. They must have combined with the snow; hutj, e y have only melted it, without increasing its temporalure.

ence it follows irresistibly, that ice, when it is converted intoaft i’ absorbs al '^ combines with 1 40° of calorie. Water, then,14n° f c °°d‘d down to 32°, cannot freeze till it lias parted withtill Ca ' or ' c: ar| d ice, after being floated to 32°, cannot melttre i abs01 'ded !40°of caloric. This is the cause of the ex-tl '" e slowness of these operations. With regard to water, then,e can be no doubt that it owes its fluidity to the caloric whichvot -n— no. 85.

it contains, and that the caloric necessary to give fluidity to ice iseuual to 140°. To the quantity of caloric which thus occasionsthe fluidity of solid bodies by combining with them, Dr. Blackgave the name of latent heat, because its presence is not indicatedbv the thermometer; a term sufficiently expressive, hut otherphilosophers have rather chosen to call it caloric of fluidity. Dr.Black and his friends ascertained also, by experiment, that thefluidity of melted wax, tallow, spermaceti, and metals, is owing tothe same cause. I.andriani proved, that this is the case with sul-phur, alum, nitre, a^d several of the metals; and it has been foundto be the case with every substance hitherto examined. We mayconsider it therefore as a general law, that whenever a solid is con-verted into a fluid, it combines with caloric, and that this is thecause of its fluidity. From the experiments of Dr. Irvine , it ap-pears that the caloric of fluidity of spermaceti is.145°

Bees-wax.1 ?5

Tin.500

These are the only substances in which the quantity of caloric,absorbed during fusion, has been ascertained, in all of them wesee this rule to hold, that tlie caloric of fluidity increases with thetemperature at which liquidity takes place. Dr. Black has ren-dered it exceedingly probable also, or rather he has proved by hisexperiments and observations, that the softness of such bodies asare rendered plastic by heat, depends upon a quantity of latentheat which combines with them. Metals also owe their malleabi-lity and ductility to the same cause. Hence the reason why theybecome hot ami brittle when hammered.

If. Thus it appears, that the conversion of solids into liquids, isoccasioned by the combination of a dose of caloric with the solid.But there is another change of state still more remarkable, towhich bodies are liable when exposed to the action of heat. Al-most all liquids, when raised to a certain temperature, graduallyassume the form of an elastic fluid, invisible like air, and possessedof the same mechanical properties. Thus water, by boiling, isconverted into steam, an invisible fluid, 1800 times more bulkythan water, and as elastic as air. These fluids retain their elasticform as long as their temperature remains sufficiently high; butwhen cooled down again, tliev lose that form, and are convertedinto liquids. All liquids, and even a considerable number of so-lids, are capable of undergoing this change when sufficiently heat-ed. With respect to the temperatures ai which liquids undergothis change, they may be all arranged under two divisions. Therearc some liquids which are gradually converted into elastic fluidsat every temperature, while others again never begin to a Mimelliat change till their temperature reaches a certain point. Wateris a well-known example of the first class of bodies. If an openvessel, filled with water, is carefully examined, we find that thewater diminishes in bulk day after day, and at last disappears alto-gether. It the experiment is made in a vessel sufficiently large,and previously exhausted of air, we shall find that the writer willfill the vessel in the state of invisible vapour, in w hatever tempera- 'ture it is placed. Alcohol likewise, and ether, and volatile this,gradually assume the form of an clastic fluid in all temperatures.But sulphuric acid and the fixed oils never begin to assume theform ol vapour till they are raised to a certain temperature.'Though left in open vessels they lose no perceptible weight; nei-ther does sulphuric acid lose any weight, though kept ever so Ioutin the temperature of boiling water. When liquids gradually as-sume the lorm of elastic fluids in all temperatures, they are said toevaporate spontaneously. The second class of liquids want thatproperty altogether. When all oilier circumstances are the same,the evaporation of liquids increases with their temperature; andafter they arc heated to a certain temperature, they assume theform of elastic fluids with great rapidity. If the heal is applied totiie bottom of the vessel containing the liquids, as is usually thecase, after the w hole liquid has acquired this temperature, thoseparticles of it which are next the bottom become an el istic fin'dfirst: they rise up, as they are formed, through the liquid, likeair-bubbles, and throw tile whole into violent agitation. The li-quid is then said to boil, F.very particular liquid has a fixedpoint at which this boiling commences (other things being theMime), and this is cal led the boiling point of the liquid. ‘Thuswater begins to boil when raised to 213°. It is remarkable, thatafter a liquid has begun to boil, it never becomes any hotterhowever strong the lire,to which it is exposed. A strong" heat m-. ^ deed