Imágenes de páginas

That two heierogeneous bodies (such as mercury and water, for instance), of different temperatures, and which are not known to have any cheatical action upon each other, should, on their admixture, indicate a temperature, in many cases, very different from that which would refult from a mixture of two homogeneous bodies, the temperatures of which differed in the same ratio as the former - is certainly a fast of no small importance towards the acquiring some knowledge of the nature of that mysterious element--if it be an element-called Fire. Dr. Crawford accordingly, availing himself of this fact, has deduced certain important consequences from it; the justice of which however is arraigned by the very acute and ingenious Author of the present performance.

According to Mr. Morgan's exposition of Dr. Crawford's system, fire, or heat, 'is contained in a great quantity in most bodies, when at the common temperature of the atmosphere; but being in this case latent, and producing no effects which render it the object of our senses, he distinguishes it by the appellation of absolute heat: and if this same principle be so increased that it shall exceed the heat of the surrounding medium, and become obvious both to fight and touch, or to the latter separately, he denominates it sensible heat.' In other words, if fire, considered as a component part of bodies, exists in them only in fuch a quantity, as not to render them warmer than the atmosphere, it is called ab/olute heaz: but if its quantity be so large as to produce what is vulgariy called heat, it is then denominated sensible heat: so that Dr. Crawford's distribution of fire into two claffes regards only the quantity, and not any difference in the quality of this principle. According to this idea, equal weights of heterogenevus substances may contain unequal quantities of abfolute heat; and the powers which certain bodies poffefs, of collecting and retaining the element of fire in greater quantity than others, Dr. Crawford calis the inpacities of bodies for containing heat.

This capacity is greater, for instance, in atmospherical, and dephlogisticated, that is, pure air, than in fixed and phlogisticated air. Dr. Crawford's experiments lead likewise him to maintain, that the earths or calces of metals poffeís a capacity of containing, and that they actually contain, more heat than the metals themselves : and combining these two general facts, he deduces from them this consequence;--- that bodies when joined to phlogision contain less absolute heat than when they are separated from ic;" and therefore, “ that if phlogiston be added to a body, a quantity of the absolute beat of that body will be extricated; and if phlogiston be separated again, an equal quantity of heat will be abforbed.” From these principles he infers, “that heat and phlogiston appear to be two oppolite principles in nature ;"


and from thence he deduces the whole mystery of como bustion,

Such is the substance of part of Mr. Morgan's explanation of Dr. Crawford's theory of heat, which the latter afterwards applies to account for the production of animal heat. This part of Dr. Crawford's hypothesis, says the Author, depends fo much on the repugnancy of phlogiston to fire, that if these two principles can be thewn not to be averse to one another, a theory founded on tnis mutual averfion, and supported likewise by lo few experiments, will hardly be thought to deserve much attention.'

The Author then inquires into the principles on which Dr. Crawford has founded his theory; and particularly examines the experiment, which was, we believe, first made by Fahrenheit, in which a pint of water at 100° F. is mixed with a pint of quicksilver at 50 degrees. In this case, we are told that the common temperature of the whole will be nearly 80 degrees; so that the water will have lost 20 degrees of its heat, and the quicksilver will have gained 30 degrees. Hence it is inferred, that the comparative quantities of absolute heat in water and quicksilver are as 30 to 20; or, that a pint of water has more heat than a pint of quicksilver in the proportion of 3 to 2; and therefore, the Author adds, the same quantity of fire, according to Dr. Crawford's reasoning, which will require only two parts of water to contain it, will not be contained in less than three parts of quicksilver.

Mr. Morgan endeavours to fhew, that the conclusions which have been drawn from experiments of this kind, allowing the truth of the results, are not well founded; and illustrates his objection by alluding to some cases respecting saline solutions. He does not see that the experiment in question proves any thing more than that some bodies, as Dr. Franklin has observed, are better conductors of fire than others; or are more or less capable of being acted upon by heat, whatever it is. In short, he does not perceive that facts of this kind are adapted to prove any particular theory of heat; and on the whole confiders the matter as

a subject which is much more easily controverted than understood.'

The Author afterwards relates various experiments, which seem to render very questionable the accuracy of the manner in which Dr. Crawforu's experiments were made; and to invalidate the rule on which they are founded. Thus-to give only a few instances-ufing always equal weights (not bulks) of mercury and water, of different temperatures; in one case, the allsolute heats of mercury and water are found to be to each other in the ratio of 1 to 15.9 ; in a second experiment, of 1 to 34.6; in a third, the ratio turns out to be I to 32.8. In this experi


ment, the mercury (at 1810) loft 112 degrees; and the water (at 650) gained only 3 į degrees. In a fourth trial, the absolute heat of the mercury to that of the water, was in the ratio of I to 21.6. - In fhort, the Author produces many experiments, and offers various other observations, to thew that the conclu. fions, deduced by Dr. Crawford from his experiments, must be rendered precarious, at least, by the inaccuracies, and the various fources of inaccuracy, which he has pointed out.

Those propositions particularly, on which Dr. Crawford has founded bis theory of Animal heat, are next examined, with the same attention and precision ; as well as the experiments from which Dr. Crawford inferred the very great quantity of absolute heat contained in atmospherical and deplogisticated air: while fixed, or impure air, heated, and contained in bladders, is said to have imparted no more heat to water, than would have been communicated to it by the same bladder empty, or in a collapsed ftate,

In our Review of Dr. Crawford's performance, we queitioned the practicability of deducing any satisfactory results from this class of his experiments, relative to air, on two accounts. [See M. Review, November 1779, pag. 385, 386.] These were, the extreme smallness of the scale; and the miitake which we conceived he had made, and which we still conceive to be a mistake, in his confounding fixed and phlogisticated air; or, in other words, in considering atmospherical air, in certain procefies, such as respiration, combustion, &c. to be changed into fixed air:-an opinion, which, though maintained by inany respectable philosophers, has not, to the best of our recollection, been hitherto supported by any decisive or even plausible experia ment; while there are numerous considerations that militate against it: the appearance of a small quantity of fixed air, in certain phlogistic and other processes, being easily accounted for, by considering it as either previously existing in, and now precipitated from, the atmospherical (or even dephlogisticated) air, in which the experiment was made; or as furnished by fome of the materials employed in the process. With respect to this last particular -- the fame portion of charcoal, for instance, will, for a long time, continue to furniih fixed air in numerous and various processes, and even, when at length its phlogiston is all diffipated, and it is reduced to white athes, the small quantity of alcaline salt contained in them, is still found to be in a mild ftaté, that is, to contain fixed air.-But to return from this seeming digreffion from our immediate subject ;-though it can scarce be properly called a digreflion: as Dr, Crawford, in accounting for the heat produced in combustion, founds his reasoning on the supposition thac fixed air is a combination of common

Rev. May 1781.



air and phlogiston; and that, in the procefs of combustion, common air is immediately changed into fixed air.

His present Examiner, who likewise controverts this last pofition (from which Dr. Crawford had inferred that atmosphe. rical air, by being suddenly converted into fixed air, is capable of raising the latter 13,400 degrees, or to 12 times the heat of red hot iron') relates several experiments, made with the affiftance of a particular and commodious apparatus, here described and delineated. From these it appears, that no one kind of air seems to have a greater power of heating water than another; nor does one species of air contain more absolute heat than another. In short, according to his experiments, to which we cannot avoid giving full credit, the effects of heated air, pure or impure, mixed with cold water, are nearly imperceptible; or, if perceptible, are such as may very easily be accounted for, from the unavoidable inaccuracies in making the experiment.

Dr. Crawford's third propofition~" that the capacities of bodies for containing heat are diminished by the addition of phlogiston, and increased by the separation of this principle.-is contested by the Author ; first, as being founded only on a few experiments, and these too, made on one class of bodies: they cannot therefore jurtify a general conclufion applicable to all bodies. - To say that every fubstance in nature has less heat in proportion as its phlogiston is increased, merely because metals may have this property, is to take a vast deal too much for granted, and to betray a greater fondness for systems than appears to be confiftent with true philosophy.' - In the fecond place, admitting even that tin, lead, iron, and antimony, nay that all metallic fubftances have their heat diminished in proportion as their phlogifton is increased; or that they contain less heat than their calces; the Author proceeds to thew, by a few experiments, that this propotition is not true with respect to some other bodies; and concludes from thence that Dr. Crawford's system is not founded upon an universal law of nature. The objects of the Author's four experiments, respecting this subject, are drawn from the vegetable and mineral kingdom. Saw dust, or wood, containing all its phlogiston, is put to the trial with potashes, or wood de prived of its phlogiston; and pit coal is opposed to its afhes. Both these fets of experiments seem to bear their teftimony against Dr. Crawford's too general proposition. The absolute heat of crude wood to that of calcined wood was found to be in the ratio of 23.6 to 7.8, or of 3 to 1; and the heat of coal to that of calcined coal, in the ratio of 5 to 3.4.

The Author's last section contains many pertinent observations, with respect to several remarkable facts relating to heat and combustion; and which Dr. Crawford had noticed, as re



ceiving an easy solution from his system. Mr. Morgan shews, that the heat which is known to be produced in these cases, and for the production of which Dr. Crawford accounts, by suppofing it to be occasioned by a decomposition of the air, may be produced likewise in vacuo, or in an exhausted receiver, in air containing phlogiston. Thus the heat produced by a paste formed of iron filings, sulphur, and water, takes place even when the materials are exposed to impure, that is nitrous air. Again, the heat and flame produced on the admixture of effential and other oils with spirit of nitre; the heat occafioned by the attrition of a Aint and steel; and that produced by the electric spark, have all been observed to take place in vacủo. In the last mentioned case, the Author drew the sparks from a very large conductor, through a vacuum one inch long, and of an inch in diameter. The


rose in less than a minute two degrees; nor did he find it rise much, if at all, fafter, when he made the sparks pass through common air, exactly in the same circumstances.

One of Dr. Crawford's observations, or rather deductions from some complicated experiments, which the Author particularly notices, is, that the same heat which raises cominon air one degree, will raise fixed air 67 degrees.' This affertion appeared to extraordinary to the Author, and at the same time, if true, so easy to be verified, that he determined to put it to the test of a direct experiment: for if heat produced effects fo immensely greater on fixed; than it does 'on common air ; these effects should be visible in a thermometer suspended in each..

Two thermometers were accordingly suspended in two separate bladders, one of which contained a pint of fixed air, and the other an equal quantity of common airg both at the tempetature of 75 degrees. Plunging both the bladders at the same instant into a pail of water at 150°; the mercury was seen to rise as fast in the fixed as it did in the common air, Infiam. mable air was tried in the same manner; and also phlogisticated and deplogisticated airs; ? in which case, the one ought to have heated 300 times faster than the other :' but the relults of all these experiments were much the fame.

It may be alleged, says Mr, Morgan, that Dr. Crawford's meaning is only that common air, in a given degree of sensible heat, contains, in a latent ftate, 67 times as much heat as fixed air of the same degree of fentible heat; yet, if by the degree of absolute heat he meant, what alone it properly signifies, the different capacities of being beater and cooled, or different susceptibilities of the action of fire; then does this experiment undeniably prove, that there is no difference in the degree of this absolute heat in fixed and common air. Were there any such difference, the thermometer would rise faster in the one than in the orbes;

Аа 2


« AnteriorContinuar »