mediate object of invefligation, proves at laft to be a component part of every thing which we fee or handle.

But this reafoning, which holds good in mechanical philofophy, is totally inapplicable to the fcience of chemistry, which profeffes to treat only of fuch changes in natural bodies as are the refult of infenfible motions. It proves, indeed, with the force of demonftration, that before a man enter on the study of this fcience, he ought to have at leaft fome general knowledge. of natural hiflory and mechanical philofophy; but it proves nothing more. It fuppofes the fludent to open his eyes on the volume of nature for the firft time, when it would certainly be abfurd to pafs over the introduction, and plunge at once unprepared into the moft abftrufe parts of that volume; but the author of a Syftem of Chemiflry reafonably fuppofes his readers to enter on the ftudy of his work with minds flored with preparatory knowledge. Even then it requires fome addrefs to introduce them properly into the arcana of nature.

Dr. Thomson performs this office in the most elegant manner. He confiders as fimple all thofe fubftances, which no phænomenon hitherto obferved indicates to be compound. Very poffibly," fays he, "the bodies which we reckon. fimple, may be real compounds; but till this has actually been proved, we have no right to fuppofe it." The fimple fubstances at prefent known amount to about thirty, which he arranges under five claffes: namely, 1. Oxygen; 2. Simple Combustibles; 3. Metals; 4. Light; 5. Caloric. Of thefe fubftances he treats, in the order in which they are here enumerated, in the first Book of the firft Part of this work; and as the manner in which he introduces Oxygen to the reader's acquaintance, is a model of neatnefs and perfpicuity, we will endeavour to give the reader as diftinct an account of it as poffible, without the aid of a plate.

up

Take a quantity of nitre or faltpetre, and put it into a gun-barrel, of which the touch-hole has been previously closed with metal. This barrel is to be bent in fuch a manner, that while the close end, in which the nitre lies, is put into the fire, the open end may be plunged below the furface of the water, with which a contiguous veffel of confiderable diameter is filled. On a fupport lying at the bottom of this veffel, is placed a glafs jar previously filled with water and then inverted, fo as to be exactly over the open end of the gun-barrel. As foon as the nitre becomes red-hot, it emits a quantity of air, which, illuing from the end of the gun-barrel, afcends to the top of the glass jar, and gradually difplaces all the water; and this air is oxygen. It was difcovered by Dr. Priestley on the ft of Auguft, 1774, and by him called dephlogisticated

air. Mr. Scheele, of Sweden, difcovered it in 1775, without any previous knowledge of what Dr. Prieftley had done; he gave it the name of empyreal air. Condorcet gave it firft the name of vital air; and Lavoifier afterwards called it oxygen gas*, a name which is now generally received.

This method of obtaining and confining air was invented by Dr. Mayhow, and afterwards much improved by Dr. Hales. A more commodious apparatus for obtaining oxygen gas was invented by Dr. Prieffley, and is defcribed by this author; but it is known to every chemift by the name of the pneumatic apparatus; and we have mentioned the original procefs, only to fhow how much more perfectly than by definitions, Dr.. Thomson brings his readers acquainted with the fimple fubftances of chemistry.

Having inftructed them how to obtain oxygen, he details its peculiar properties of fupporting flame and life; fhows in what proportion it exifts in the atmosphere; afcertains its fpecific gravity; and having briefly explained the term affinity, concludes the chapter with a table of the affinities of oxygen for various fubftances.

The second chapter treats, in five sections, of Simple Combuftibles.

"By combustibles," fays the author, "I mean fubftances capable of combuftion, and by fimple combustible bodies of that nature which have not hitherto been decompounded. These bodies are only five in number; namely, Sulphur, Phosphorus, Carbon, Hydrogen, and Azor. The metals might indeed be claffed among combustible bodies; but the greater number of their properties are fo different from thofe of the five bodies juft mentioned, that it is proper to confider them by themselves as a diftinct class of bodies. All our claffifications are in fact artificial; Nature does not know them, and will not fubmit to them. They are useful, however, as they enable us to learn a science fooner, and to remember it better; but if we mean to derive these advantages from them, we must renounce a rigid adherence to arbitrary definitions, which Nature difclaims." P. 26.

* The word gas, which is the name now given to every kind of air, which differs from the air of the atmosphere, was first introduced into chemistry by Van Helmont. He feems to have intended to denote by it every thing which is driven off from bodies in the state of vapour by heat. He divides gafes into five claffes: "Nefcivit inquam fchola Galenica hactenus differentiam inter gas ventofum (quod mere aer eft, id eft, ventus, per fyderum blas commotus), gas pingue, gas ficcum, quod fublimatum dicitur, gas fuliginofum, five endimicum, et gas fylveftre, five, incoercibile, quod in corpus cogi non poteft vifibile." Van Helmont De Flatibus, § 4. The

The third chapter, in twenty-two fections, treats of the METALS. It is remarkable for perfpicuity and precifion; but our limits admit not of fuch an abridgment as would even give the reader a diftin&t notion of the author's method of treating this interefting part of his fubject. With Bergman, and other eminent chemifts, he explodes the idea of femimetals, as founded on a falfe hypothefis, and giving rife to various errors; and arranges the metals which are now known under three claffes; namely, 1. Malleable Metals; 2. Metals brittle and eafily fufed; and, 3. Metals brittle and with difficulty fufed.

The fourth chapter is employed on LIGHT, which the author, with great judgment, confiders firft as a mechanical philofopher, and then as a chemift. For reafons, which will ap pear prefently, we extract that part of the chapter which mentions the fources of light. These are four: 1. The Sun and Stars; 2. Combuftion; 3. Heat; and, 4. Percuffion.

"The light emitted by the fun is familiarly known by the names of funfbine and light of day. The light of the ftars, as has been afcertained. poffeffes precifely the fame properties. With respect to the caufe why the fun and ftars are conftantly emitting light, the question will probably for ever baffle the human understanding; at any rate, it is not confidered as connected with the fcience of chemistry.

"Light is emitted in every cafe of combuftion. Now combuftion, as far at leait as regards fimple combuftibles and metals, is merely the act of combination of the combuftible with oxygen. Confequently the light which is emitted during combuftion must have exifted previously combined either with the combuftible or with the oxygen; but with which of the two, the prefent ftate of chemistry is infufficient to determine. But this fubject will be refumed in the next Chapter, where the nature of combuftion will be particularly confidered.

"If heat be applied to bodies, and continually increased, there is a certain temperature at which, when they arrive, they become luminous. No fact is more familiar than this; fo well known indeed is it, that little attention has been paid to it. When a body becomes Juminous by being heated in a fire, it is faid in common language to be red hot. As far as experiments have been made upon this fubject, it appears, that all bodies which are capable of enduring the requifite degree of heat without decompofition or volatilization begin to emit light at precifely the fame temperature. The first perfon who examined this fubject with attention was Sir Ifaac Newton. He afcertained, by a very ingenious fet of experiments, firit published in 1701, that iron is juft vifible in the dark when heated to 635°; that it fhines ftrongly in the dark when railed to the remperature of 752°; that it is luminous in the twilight, juft after funfet when heated to 884°; and that when it fhines, even in broad day-light, its temperature is above 1000°. From the experiments of Mufchenbroeck and others, it appears, that what in common language is called a red heat, commences about the temperature of 800°. "A

"A red hot body continues to fhine for fome time after it has been taken from the fire and put into a dark place. The conftant acceffion, then, either of light or heat, is not neceffary for the fhining of bodies;' but if a red hot body be blown upon by a strong current of air, it immediately ceases to shine. Confequently the moment the temperature of a body is diminished by a certain number of degrees, it ceafes to be luminous.

"Whenever a body reaches the proper temperature, it becomes luminous, independent of any contact of air; for a piece of iron wire becomes red hot while immerfed in melted lead." P. 253.

"To this general law there is one remarkable exception. It does not appear that the gafes become luminous even at a much higher temperature;" and Mr. T. Wedgewood proved, by a decifive experiment, that atmospheric air is not luminous when hot enough to raise other bodies to the shining tempe,

rature,

"The laft of the fources of light is percuffion. It is well known. that when flint and fteel are fmartly ftruck against each other, a fpark always makes its appearance, which is capable of fetting fire to tinder or to gunpowder. The fpark in this cafe, as was long ago afcertained by Dr. Hooke, ie a fmall particle of the iron, which is driven off, and catches fire during its paffage through the air. This, therefore, and all fimilar cafes, belong to the clafs of combuftion. But light often makes its appearance when two bodies are ftruck against each other, when we are certain that no fuch thing as combuftion can happen, because both the bodies are incombuftible. Thus, for instance, fparks are emitted, when two quartz ftones are ftruck fmartly against each other, and light is emitted when they are rubbed against each other. The experiment fucceeds equally well under water. Many other hard ftones alfo emit sparks in the fame circumftances." P. 256.

The fifth chapter, in twelve fections, treats of CALORIC; 1. Of the Nature of Caloric; 2. Of Expanfion by Caloric: 3. Of equal Diftribution of Temperature; 4. Of the Motion of Caloric; 5. Of fpecific Caloric, or that quantity of caloric which a body requires in order to be heated to a certain temperature; 6. Of the Caloric of Fluidity; 7. Of the Quantity of Caloric in Bodies; 8. Of Cold; 9. Of Combuftion; 10. Of Percuffion; 11. Of Friction; 12. Of Light as a fource of Caloric.

This, as the reader cannot but perceive, is an exceedingly interefting chapter; but it is not fufceptible of a perfpicuous abridgment, within the limits allotted to a Review. The following extracts, however, will be acceptable to all who are in any degree converfant with the subject.

It is well known that Count Rumford, from fome ingenioufly devised experiments, inferred, that fluids are carriers but not conductors of caloric. Dr. Thomfon repeated the

experiment;

experiment; and, after paying fome well deferved compli ments to the Count, fhows, in a very fatisfactory manner, that his inference was haftily drawn.

"Count Run ford's experiments then do not prove his pofition, that fluids are non-conductors, but rather the contrary. That they are all in fact conductors of caloric, I afcertained in the following manner: The liquid whofe conducting power was to be examined was poured into a glafs veffel till it filled it about half way; then a hot liquid of a lefs fpecific gravity was poured over it. Thermometers were placed at the furface, in the centre, and at the bottom of the cold liquid; if these rofe, it followed that the liquid was a conductor, because the caloric made its way downwards. For instance, to examine the conducting power of mercury, a glass jar was half filled with that liquid metal, and boiling water then poured over it. The ther mometer at the furface began immediately to rife, then the thermome er at the centre, and lattly that at the bottom. The firft rose to 118°, the fecond to 90°, the third to 86°: the first reached its maximum in 1, the fecond in 15', the the third in 25'. The conducting power of water was tried in the fame manner, only hot oil was poured over it. A variety of precautions were neceffary to enfure accuracy; but for thefe I refer to the experiments themfelves, which are detailed in Nh len's Journal.

Fluids, then, as far as experiments have been made, are conductors of caloric as well as folids. Hence it follows, that all bodies with which we are acquainted are capable of conducting caloric.

"If we take a bar of iron and a piece of stone of equal dimenfions, and putting one end of each into the fire, apply either thermometers or our hands to the other, we fhall find the extremity of the iron fenfibly hot long before that of the ftone. Caloric therefore is not conducted through all bodies with the fame celerity and cafe. Thofe that alJow it to pafs with facility, are called good conductors; thofe through which it paffes with difficulty, are called bad conductors." P. 302.

The author having flated the various theories of combustion, which have at different times been adopted, and given due praife to Lavoifier for the important ftep which he has eftablifhed in this interefting procefs of nature, thus ftates his theory, with the feveral objections to which it is liable.

"According to the heory of Lavoisier, which is now almoft generally received, and confidered by chemifts as a full explanation of the phenomenon, combuftion confifts in two things: first, a decompofition; fecond, a combination. The oxygen of the atmosphere being in the flate of gas, is combined with caloric and light. During combution this gas is decompofed, its caloric and light efcape, while its bafe combines with the combuftible and forms the product. This product is incombustible; because its base, being already faturated with oxygen, cannot combine with any more. This theory is evidently liable to feveral objections, which require to be examined before it can be admitted.

" 1. If