A system of chemistry : comprehending the history, theory, and practice of the science, according to the latest discoveries and improvements ; illustrated with copper plates.

Date:
MDCCXCI [1791]
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Licence: Public Domain Mark

Credit: A system of chemistry : comprehending the history, theory, and practice of the science, according to the latest discoveries and improvements ; illustrated with copper plates. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    Element of Fire. 84 Difficulty arifing from the fuppo- fition that heat diffu- fes itfelf equally. Another from the fceming dilappear- ance of the heat. 86 Dr Craw- ford's folu- tion. vity ; and we might reafonably fuppofe, that fubftan- ces of the greateib fpecific gravity would contain the ' fmalleft quantity of water, though Hill we could by no means determine what quantity they did contain, unlefs we could lay hold of the element itfelf. This feems to be very much the cafe with elemen- tary fire, if we fuppole it to be a fluid per ft. We judge of its prefence by the degree of expanfion which one heated body communicates to another : but this is only ftmilar to the calculation of the quantity of moi- fture a fponge or any other body contains, by what it communicates to wood when it comes into contact with it ; which never could be fuppofed to carry the leaft pretenfions to accuracy, though we fhould afcer- tain it with all imaginable exactnefs. It is likewife probable, that the molt denfe bodies contain the fmalleft quantity of fire, as they generally communicate lefs when heated to an equal temperature than thofe which are more rare, though we are far from, having any per- fect knowledge in this refpect. But the greateft difficulty of all will be, on the fup- pofition that heat is a fluid, and an omniprefent one (which it mull be, or there would be fome places where bodies could not be heated), to anfwer the queftion, Why are not all bodies of an equal tempera- ture, excepting only the differences arifing from their fpecific denfities, which render fome capable of containing a greater quantity than others \—The dif- ficulty will not be lefTened, though the omniprefence of the fluid fhould he given up, if we fuppofe, as is generally done, that heat has a tendency to diffufe it- felf equably every way. If it has this tendency, what hinders it from doing fo ? Why doth not the heat from the burning regions of the torrid zone diffufe it- felf equally all over the globe, and reduce the earth to one common temperature ? This indeed might require time ; but the experience of all ages has Ihown that there is not the leaft advance towards an equality of temperature. The middle regions of the earth con- tinue as hot, and the polar ones as cold, as we have any reafon to believe they were at the creation of the world, or as we have any reafon to believe they will hz while the world remains. This indeed is one of the many inftances of the impropriety of eftablifhing gene- ral laws from the trifling experiments we are capable of making, and which hold good only on the narrow fcales on which we can make them, but are utterly in- fufficient to folve the phenomena of the great fyflem of nature, and which can be folved only by obferving other phenomena of the fame fyftem undifturbed by any manoeuvres of our own. Again, fuppofing the objection already made could be got over, and fatisfactory reafons fhould be given why an equilibrium of temperature in the earth and its atmofphere fhould never be obtained, it will by no means be eafy to tell what becomes of the heat which is communicated to the earth at certain times of the year. This difficulty, or fomethingiimilar, Dr Craw- ford feems to have had in view when treating of the effects of the evolution and abforption of heat. Thus, fays he, the Deity has guarded againft hidden vicilli- twdes of heat and cold upon the furiacc of the earth. For if heat, were not evolved by the procefs of congelation, all the waters which were expofed to the influence of the external air, when its temperature was reduced below 32°, would fpeedily become folic!; and, Element at the moment of congelation, the progrefs of cooling ofFire. would be as rapid at it was before the air had arri- ' y ved at its freezing point. This is manifeft from what was formerly obferved reflecting the congelation of different fluids. It was fliown, that if the velocities of the feparation of heat were equal, the times of the congelatiou would be in proportion to the quantities of heat which the fluids gave off from an internal fource in the freezing pro- cefs. Whence it follows, that if no heat were evol- ved, the congelation fhould be inflantaneous. In the prefent ftate of things, as foon as the at- mofphere is cooled below 320, the waters begin to freeze, and at the fame time to evolve heat ; in con- fequence of which, whatever may be the degree of cold in the external air, the freezing mafs remains at 320, until the whole is congealed ; and as the quantity of heat extricated in the freezing of water is confiderable, the pregrefs of congelation in large maffes is very flow.—That the abforption and extrication of heat in the melting and freezing of bodies has a tendency to retard the progrefs of thefe proceffes, is remarked by Mr Wilkie in his effay on latent Heat.—The fame doctrine is likewife taught by Dr Black in his lec- tures, g In the northern and fouthern regions, therefore, Severity of upon the approach of winter, a quantity of elementary the cold m fire is extricated from the waters, proportional to the the north- degree of cold that prevails in the atmofphere. Thus e™ re . . the feverity of the frofl is mitigated, and its progrefs SU3X1S ™,tl~ retarded ; and it would feem that, during this retarda- fj^ r^' tion of the cooling procefs, the various tribes of animals di&ion of and vegetables which inhabit the circumpolar regions ice. gradually acquire power of refifling its influence. gg On the contrary, if, in the melting of ice, aquan- inuudati- tity of heat were not abforbed, and rendered infen- ons pre- fible, that fubflance, when it was expofed to a medium vented by warmer than 320, would fpeedily become fluid, and the the fl?w procefs of heating would be as rapid as if no alteration ne.f* Jlth in its form had taken place. If things were thus con- Sealed wa- ftituted, the vafl maffes of ice and fnow which are col- f^meUs.* lectedin the frigid zones would, upon the approach of fummer, fuddenly dillblve, and great inundations would annually overflow the regions near to the poles. But by the operation of the law of the abforp- tion of heat, when the ice and fnow upon the return of fpringhave arrived at 320, they begin to melt, and at the fame time to imbibe heat : during this procefs, a large quantity of elementary fire becomes infeniible ; in confequence of which the earth is flowly heated, and thofe gradual changes are produced which are effential to the prefervationof the animal and vegetable kingdoms. o We may remark, in the laft place, that this kwEquaidi- not only relifls hidden changes of temperature, but ftrilmtion that it likewife contributes to a more equal diftribu- of heat pro-. tion of the principle of heat throughout the various ™oted hJ parts of the earth, in different feafons and climates. s bfo^ Thus the diurnal heats, are moderated by the evapora- solution. tion of the waters on the earth's furface, a portion of the fire derived from the fun being abforbed and ex- tiiiguiihed by the vapours at the moment of their af- cent. On the approach of night the vapours are again condenfed, and falling in the form of dew, communicate C to