that perfect model of philofophical enquiry, which Newton has exhibited in his optics.-The experiments which he defcribes, are numerous, and feem to have been made with uncommon induftry, and with a proper attention to every circumstance that could be fuppofed to affect their refults. At the fame time, the plan on which he has proceeded, and the inferences which he has deduced from his facts, bear marks of that fagacity and ingenuity, which diftinguish the enlightened and philofophical enquirer from the empirical experimenter. How far his theory is juft, it is impoffible to determine, without a careful repetition of his experiments. In juftice, however, to the Author, we must acknowledge, that he relates them with the appearance of accuracy and candour, and that the reafoning which he founds upon them, feems to be ingenious and confequential.

The firft fection of this Treatife contains fome facts and obfervations on heat in general.-For the greater part of thefe, Mr. Crawford acknowledges himself indebted to the lectures read in the Universities of Edinburgh and Glasgow, by Dr. Black and Dr. Irvine.-The following abstract will give an idea of their refult.

If equal quantities of the fame fluid, at different temperatures, be mixed intimately together, the temperature of the mixture will be half the excess of the hotter above the colder. -If, for example, a pint of boiling water, at 212, be mixed with a pint of the fame fluid at 32, the temperature of the mixture will be 122, the warm water will be cooled 90 degrees, and the cold water heated 90 [A].

But if this experiment be made with water and mercury in the fame circumstances, the refult will be different. Thus, if you take equal bulks of mercury and water, and give the water a greater degree of heat than the mercury; the heat of the mixture will always be greater, than half the excess of the heat of the water above that of the mercury. If, on the other hand, the mercury be hotter than the water, the temperature of the mixture will always be lefs than half the excefs of the heat of the mercury above that of the water. In general, if equal bulks of water and mercury, at different temperatures, be mixed together, the change pioduced on the temperature of the mercury, will be to that produced on the temperature of the water, in the proportion of 3 to 2.

In thefe experiments with water and mercury, a quantity of heat is communicated from one fluid to the other: the one gains whatever the other lofes. But the changes produced on their temperatures are different. The fame quantity of heat, which, when communicated to water, produces a change as two, when communicated to mercury produces a change as

three.

three. It appears therefore, that in the cafe of heterogeneous bodies, we can draw no conclufion concerning the comparative quantities of heat taken from them, or added to them, from the changes which take place in their temperatures, as indicated by the thermometer.

This reafoning may be pushed farther. Since the fame quantity of heat produces unequal changes on the temperatures of different bodies, it follows, that two bodies which are at the fame temperature, may contain unequal quantities of abfolute heat. Thus, when water and mercury are at the fame temperature, the water contains a greater degree of heat than the mercury. For, fuppofe a pint of mercury and a pint of water to be both deprived wholly of their heat lince the fame quantity of heat, which, when communicated to water, produces a change in its temperature as two, when communicated to mercury, produces a change in its temperature as three; it follows, that in order to produce equal changes on the temperature of mercury and water, we muft communicate to them quantities of heat in the proportion of two to three; and therefore, if they are wholly deprived of heat, we muft, in order to bring them to the fame temperature, communicate to them quantities of heat in the proportion of thefe numbers. Hence it follows, that when a pint of mercury and an equal bulk of water are at the fame temperature, we may conclude, that the quantity of heat in the water is to that in the mercury, in the proportion of three to two. It is proper to obferve, that in the foregoing reafoning, the water and mercury are fuppofed to continue in their fluid form, when deprived of their whole heat.

In order to prevent ambiguity, Mr. Crawford makes a diftinction between the abfolute and fenfible heat of a body [B]. By abfolute heat, he means the whole quantity of heat (or of the element of fire, as fome philofophers have expreffed themselves) which is contained in a body. By fenfible heat, that part of the abfolute heat, which produces the expanfion of the mercury in the thermometer; and which is found partly to depend on the quantity of the abfolute heat, and partly on the nature of the body, in which this heat is contained.

The conclufions deduced from the reafonings contained in this part of the work concerning heat in general, may be comprehended in the following propofitions.

1. Equal weights of heterogeneous fubftances, having the fame temperature, may contain unequal quantities of abfolute

heat.

2. There are, therefore, certain differences in the nature of bodies, in confequence of which, fome have a greater capacity for collecting and containing heat than others.

3. The comparative quantities of abfolute heat in bodies, are reciprocally proportional to the changes which are produced on their fenfible heats, when they are mixed together at different temperatures. It muft, however, be obferved, that this rule. does not apply to thofe fubftances, which, in mixture, produce fenfible heat or cold by chymical action.

We thought it neceffary, before giving any account of Mr. Crawford's doctrines concerning animal heat, and the inflammation of combuftible bodies, to take notice of the doctrines contained in his introductory fection, as it is from, these that he deduces his method of eftimating the abfolute heat of the different bodies, which are the fubjects of his experiments.

It has been already obferved, that bodies differ from each other, in their capacities for absorbing and retaining heat. From a great variety of experiments, related by Mr. Crawford, it appears, that the capacities of bodies for containing heat are diminished by the addition of phlogifton, and increafed by the feparation of this principle. Thus metals contain lefs abfolute heat than their calces, and fulphur lefs than the vitriolic acid. The calx of antimony, for example, contains nearly three times. as much abfolute heat as the regulus. Hence it follows, that if phlogifton be added to a body, a quantity of the abfolute heat of that body will be extricated; and if the phlogifton be feparated again, an equal quantity of heat will be abforbed.

It is a confequence of thefe experiments, that heat and phlogifton, fo far from being intimately connected, as moft philofophers have imagined, act in fome measure in oppofition to each other. By the action of heat on bodies, the force of their attraction to phlogifton is diminished, and by the action of phlogifton a part of their abfolute heat is expelled.

It has been demonftrated by Dr. Prieftley, that in refpiration, phlogifton is feparated from the blood, and combined with the air. If therefore it be a general fact, that the separation of the phlogiston from bodies increafes their capacity for containing heat, and that the addition of it has the contrary effect; it will follow, that in refpiration, a quantity of heat must be difengaged from the air by the union of the air with phlogifton; while, at the fame time, the feparation of the phlogifton from the blood increases its capacity for containing heat, and difpofes it to absorb that portion of it which the air has depofited.

This conclufion is perfectly agreeable to Mr. Crawford's experiments, from which it appears, that atmospherical air contains a greater quantity of abfolute heat, than the air which is expired from the lungs of animals; in particular, that the fixed air which is exhaled by expiration, contains only the fixtyfeventh part of the heat which was contained in the atmospherical air, previous to infpiration [C].

From

From Dr. Priestley's experiments, it alfo appears, that arterial blood has a strong attraction to phlogifton. During the circulation, therefore, the blood which had been dephlogisticated by the procefs of refpiration, will imbibe the phlogifton from thofe parts which retain it with the leaft force, that is, from the putrefcent parts of the fyftem. Accordingly, the venous blood, when it returns to the lungs, is found to be highly impregnated with phlogifton. In proportion as the blood becomes again combined with phlogifton in the courfe of the circulation, it will gradually give out that heat which it had received in the lungs, and diffufe it over the whole fyftem.

Mr. Crawford endeavours to confirm this theory, by experiments made in order to determine the abfolute heat of the blood which paffes from the lungs to the heart, by the pulmonary vein, and of that which paffes from the heart to the lungs, by the pulmonary artery. In the fourth fection, a very ingenious application is made of thefe principles, to explain the most remarkable facts relating to animal heat [D].

Before we leave this part of our Author's fubject, it may not be improper to remark the difference between his theory, and that which has been lately published by Dr. Leflie*. According to Dr. Leflie, animal heat is produced by the evolution of the phlogifton of the blood in the courfe of the circulation; according to Mr. Crawford, it is produced by the heat which the blood depofits in confequence of its being gradually impregnated with phlogifton. The former feems to conceive heat and phlogifton as connected together; the latter confiders them as acting in fome meafure in oppofition to each other. With refpect to Dr. Leflie's hypothefis, it may be obferved, that it is altogether inconfiftent with thofe experiments of Dr. Prieftley's, from which it appears, that arterial blood has fo ftrong an attraction to phlogiston, that it is capable of feparating it from fixed and phlogifticated air. If this be the fact, how fhall we account for the evolution of the phlogifton of the blood, during the time of circulation?

Our Author's theory with refpect to the inflammation of combuftible bodies, is founded on the fame principles as his doctrine concerning the heat of animals. According to him, the heat which is produced by combuftion is derived from the air, and not from the inflammable body. In fupport of this doctrine, he reafons thus: Inflammable bodies abound with phlogifton, and contain little abfolute heat; atmospherical air, on the contrary, abounds with abfolute heat, and contains little phlogifton. In the procefs of inflammation, the phlogiston is feparated from the inflammable body, and combined with the

* See Review for May laft, p. 385.

air; the air is converted into fixed and phlogifticated air, and at the fame time gives off a very great proportion of its abfolute heat, which, when extricated fuddenly, burfts forth into flame, and produces an intenfe degree of fenfible heat. We have found by calculation, that the heat which is produced by the converfion of atmospherical into fixed air is fuch, if it were not diffipated, as would be fufficient to raife the air. fo changed to more than twelve times the heat of red hot iron. It appears, therefore, that in the procefs of inflammation, a very great quantity of heat is derived from the air.

It is manifeft, on the contrary, that no part of the heat can be derived from the combuftible body; for the combuftible body, during the inflammation, being deprived of its phlogiston, undergoes a change fimilar to that which is produced in the blood by the process of refpiration; in confequence of which, its capacity for containing heat is increased. It therefore will not give off any part of its abfolute heat, but, like the blood in its paffage through the lungs, it will abforb heat. The calx of iron, for example, is found to contain more than twice as much abfolute heat as the iron in its metallic form; from which it follows, that in the process of inflammation, the former muft neceffarily abforb a quantity of heat, equal to the excess of its heat above that of the latter. Now, from whence does it receive this heat? It cannot receive it from the iron. For the quantity of heat in the calx is more than double of that which was contained in the iron, previous to the calcination. But in the burning of iron, the phlogifton is feparated from the metal, and combined with the air; and it has been proved, that by the combination of phlogifton with air, a very intense heat is produced. From hence it is manifeft, that in the inflammation of iron the atmospherical air is decompofed, a very great proportion of its abfolute heat is feparated, part of which is abforbed by the calx, and the reft appears in the form of fame, or becomes moving and fenfible heat. We may conclude, therefore, that the fenfible heat, which is excited in combustion, depends upon the feparation of abfolute heat from the air, by the action of phlogifton.'

In this manner, our Author attempts to explain the principal phenomena of animal heat, and of the inflammation of combuftible bodies, from the general fact, that the capacities of bodies for containing heat are diminished by the addition of phlogifton, and encreafed by its feparation. Towards the conclufion of the Treatife, he applies the fame principle to account for a variety of natural phenomena. From the obfervations

made in this part of his work, it appears, that many important effects are produced in the univerfe, in confequence of the mutual oppofition of phlogifton and fire.

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