Affinity. thefe elements, will be the fubject of difcuffion in the 18th and 19th chapters. The 20th chapter, in which chemical fcience is applied to the improvement of arts and manufactures, is not one of the least important and interefting, and a full view of this part of the fubject would exhauft the whole of the ufeful detail of chemical knowledge. But, in the following treatise, it is not proposed to enter at full length into the different branches of the arts and manufactures, but only to give a flight view of their general principles, fo far as they depend on chemistry, referring for the particular difcuffion of each to the different heads under which they will be found arranged in the courfe of the work.

49 fiftory.

Elem. hem.

art 2.

CHAP. I. OF AFFINITY.

BEFORE we enter into the detail of thofe changes which take place by the action of bodies upon each other, producing compounds which are poffeffed of totally different properties, and thus exhibiting the characters of chemical action, it is neceflary to take a view of the circumstances in which thefe changes are effected, or in other words, the laws of combination or chemical affinity.

The term affinity, which is the expreffion of a force by which substances of different natures combine with each other, feems to have been pretty early employed by chemical writers. Barchufen, it would appear, is among the first who employed it, and thus characterizes it. "Arctam enim atque reciprocam inter fe habent affinitatem." It was afterwards brought into more general ufe, and its application more precifely defined by Boerhaave*. His words are remarkable. "Particulæ folventes et folutæ, se affinitate fuæ naturæ colligunt in corpora homogenea." And to explain his meaning ftill more clearly, he adds, "non igitur hic etiam actiones mechanicæ, non propulfiones violentæ, non inimicitiæ cogitandæ, fed amicitia." To avoid the metaphorical expreffion affinity, Bergman propofed the term attraction; and to diftinguish chemical attraction, which exifts only between particular fubftances, from that attraction which exifts between all the bodies in nature, he prefixed the word elective. The word affinity, however, is now generally adopted, and employed by all chemifts.

The different tendency of bodies to combine with each other, or the relative degree of affinity which exifts between them, could not long be overlooked by thofe whofe attention was occupied in obferving chemical changes. And to explain this difference of action, a maxim of the schoolmen was adopted; fimile action venit ad fimile. The fame doctrine was held by Becplained cher, that fubftances which were capable of chemical combination, poffeffed a fimilarity of particles. Other attempts were made to explain chemical action, by confidering folvents as confifting of points, finer or coarfer, which were mechanically difpofed to enter into the pores of certain fubftances which they were capable of holding in folution. But Stahl, as appears from his works, rejected the notion of mechanical force, and afcribes the power of folvents to contact, or Stahl. to the attraction of cohesion. "Combinationes quaf cunque non aliter fieri, quam per arctam appofitionem." And afterwards, he speaks ftil! more precisely when he fays, "non per modum cunei, neque per modum in curfus, in unam particulam feparandam, fed potius per

Having made this important step in the confidera- Beccher. tion of chemical action, the experiments and obfervations of the fagacious chemift led him to conclude, that a combination between two fubftances once formed, could not be destroyed, without effecting a more intimate union of one of the conftituent parts with some other fubstance.

His

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The next step in the method of obferving and Tables inftudying chemical affinity was made by Geoffroy the vented, elder. He collected the fcattered facts, to determine the force or meafure of their degrees of union, and to establish rules of analysis and compofition. first table of affinity was prefented to the Royal Academy of sciences at Paris in the year 1718. This confifted only of 17 columns, which were but imperfectly filled up, and exhibited rules which have been mostly changed; but with all its errors, it ought to be confidered as one of the first guides to chemical knowledge.

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The first material improvement on Geoffroy's table enlarged, was made by Gellert, profeffor at Freyburg. In his Chemia Metallurgica, published in 1750, there is a new table of affinity, which extends to 28 columns. At the bottom of each column there is a lift of fubftances with which the body at the head of the column had no action. Rudiger, in the year 1756, inferted a table of affinity in his fyftem of chemistry, in which he reduced the number of columns to 15. In this table and improhe placed the fixed alkalies and lime parallel with each ved other, and before ammonia, the column of acids. He pointed out also with a good deal of accuracy, in a fmall feparate table, thofe fubftances which refuse to combine without fome intermediate fubftance.

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55

The next important addition to the knowledge of by Limaffinities, was made by M. Limbourg. In his table bourg. the number of columns was extended to 33. This table was the fulleft and most accurate of any that had yet appeared. He had justly obferved that zinc, of all metallic fubftances, fhould be placed at the head in the column of acids, and that even in the dry way it precipitated them all. He afferted that lime and the cauftic alkalies acted by affinity on animal matters; and befides, he stated fome cafes in which a change took place in the order of affinities, by a change of temperature, or by the volatility of one of the fubftances.

man.

This fubject, the importance of which was fufficient- By Bergly obvious, was now affiduously investigated by many chemists. The number of tables was multiplied, and the fyftem of affinity more fully established. But the greatest improvement which it had hitherto received, was made by the celebrated Bergman, in his differtation on elective attractions, published in the tranfactions of the Royal Society of Upfal, in the year 1775. Thefe tables, editions of which appeared in 1779 and 1783, have been justly regarded as striking inftances of the fagacity and induftry of the author. The affinities of 59 fubftances are afcertained with great accuracy; and the distinction between thofe that take place in the moist and dry way, is particularly stated, as well as the diftinction between fimple and compound affinities,

Affinity, affinities, which has led to the explanation of a great number of apparent anomalies. Since the time of Bergman, this fubject has been profecuted by many of the most diftinguished philofophical chemifts. Among thefe we may mention the induftrious and indefatigable Kirwan of our own country; and among the French philofophers, Morveau, and more especially Berthollet, diftinguished for his fkill and fagacity, who has lately in his refearches concerning the laws of affinity, opened a new field of inquiry, corrected many former errors, and pointed out fome new laws in this interefting and important fubject.

57

All bodies

attract.

58

Different

names of affinity.

All bodies with which we are acquainted, are influenced by a certain force, by which they are attracted, or drawn towards each other. A ftone, when it is unfupported, falls to the ground; the planets are attracted by the fun; two polifhed plates of metal, of glafs, or of marble, when brought into close contact, adhere with a certain force; a piece of wood or stone requires a confiderable degree of force to feparate the particles, or to draw it afunder; and lime and fulphuric acid enter into fuch clofe combination, that it requires an equal degree of force to overcome that combination, or to feparate the particles from each other. Whatever may be the nature of thefe attractions, or the caufe of thefe different combinations, or whether they are to be ascribed to the fame univerfal law pervading matter, as fome have supposed, they have been defcribed by philofophers under different names. The attraction which exifts between all bodies in the folar fyftem, was denominated by Newton, by the general term attraction; and he demonstrated that this uniform and univerfal law was precifely the fame as the law of gravitation, or the defcent of heavy bodies towards the earth; and that this attraction was an effential property of all matter; that the minutest particles, in proportion to their bulk, were equally influenced with the largeft maffes; that the fame power which retained the planets in their orbits, gave form to the drops of rain.

We have faid, that thefe different forces er attractions have been diftinguished by different names. That attraction which is exerted between two polifhed furfaces brought into contact, has been called adhesion. When particles of the fame nature are attracted or held together, the expreflion of the force by which this is effected, has received the name of cohesion, homogeneous affinity, or the attraction of aggregation; but when diffimilar particles, or the particles of two fubftances of different qualities combine together, the force or attraction which is here exerted has been called heterogeneous affinity, the attraction of compofition; or, ftrictly fpeaking, chemical affinity. In the three following fections, we propose to give an account of the opinions and doctrines which have been held by philofophers with regard to the nature and force of these attractions. Of the two firft we fhall only take a fhort view; but fhall enter more fully into the detail of the latter, namely, chemical affinity, which more strictly belongs to our prefent fubje&t.

SECT. I. Of ADHESION.

By adhesion, then, is to be understood, that force which retains different fubitances in contact with each other. Thus, water adheres to the finger, which is

59

In what

faid to be wet, and mercury brought into contact with Affinity. gold, adheres with great force. Adhesion takes place, either between two folids, as marble or glass; or be-, tween folids and fluids, as when water rifes in capilla- circum ry tubes; or between two fluids, as water and oil. In ces it hap an experiment made by Dr Defaguliers, he obferved, pens that two plates of glafs, of one-tenth of an inch in diameter, adhered with a force equal to 19 ounces. The adhefion of two fluids has been proved by the experi ment of Lagrange and Cigna, as that of oil and water, between which it was formerly fuppofed there existed a natural repulfion; and the experiments on capillary attraction, and particularly the afcent of water between two panes of glafs, which was afcertained by Dr Brook Taylor, have eftablished the attraction between folids and fluids.

for.

6:

This adhesive force, or the caufe of this attraction, Account has been differently accounted for by philofophers. In a differtation on the weight of the atmosphere, publifhed in 1682 by James Bernouilli, he afcribes the refiftance which two polished pieces of marble oppofed to their feparation to the preffure of the air; and in proof of this, he states as a fact, that the two plates could be eafily feparated in vacuo. But it has been fuppofed that he had either never attempted to verify this fact, or that the experiment had been accompanied by fome fallacy. From the experiments made by Dr Taylor, he concluded that the intensity of the adhesive power of furfaces might be measured by the weight which was required to feparate them. About the fame time Mr Hawkibee proved by experiment, that the adhefion of furfaces and capillary attraction were not to be afcribed to the preffure of the atmosphere, as Bernouilli had fuppofed; but Lagrange and Cigna, after having proved the adhesion between oil and water, thought that it was owing to a different caufe from that of attraction. They fuppofed that it was occafioned by the preffure of the air, and that the opinion of Dr Taylor was not well founded. Such were the opinions held by philofophers on this fubject, when Morveau, in the year 1773, was led to inftitute a series of experi ments on adhefion, which he exhibited at Dijon. thefe experiments he proved, that this attraction was not owing to the preffure of the air, but entirely to the attraction of the two fubftances between themfelves. To prove this, a polifhed plate of glafs was fufpended from the arm of a balance, and placed in contact with a furface of mercury. The weight neceffary to feparate the two surfaces was equal to nine gros and fome grains. The whole apparatus was placed under the receiver of an air-pump, which was exhaufted of the air as much poffible. It required exactly the fame force to feparate the furfaces. The fame difk of glafs brought into contact with pure water, adhered to it with a force equal to 258 grains; but from the furface of a solution of potash, it required only a force of 210 grains. This inequality of effects with equal diameters, and in the inverfe order of the refpective denfities, feemed not only to be decifive in favour of Dr Taylor's method, but appeared to point out the poffibility of applying it to the calculation of chemical affinities. For the force of adhefion being neceffarily proportional to the points of contact, and the fum of the points of contact not varying in the adhesion of a fluid and a folid with equal furfaces, but

By

Achard alfo inftituted a series of experiments with different folid fubftances, formed into difks of equal diameters, and applied to the furface of different fluids. The following table shows the refults of thefe experiments; but from these results it appears, that the force of adhefion does not depend on the fpecific gravity, either of the folid or the fluid; nor does it correfpond

with the order of chemical affinities. But befides, fome of Affinity. the refults cannot be admitted as perfectly legitimate, on account of the chemical action which would neceffarily take place when fome of the fubftances were brought into contact; as fome of the metals would be acted on by the acids, and others by the folutions of metallic falts.

From all these observations, then, we may conclude, that the force of adhesion between different bodies is altogether independent of the preffure of the air; that it varies according to the number of points of contact of the touching furfaces; and that it is probably owing to the fame caufe as the force of affinity. It appears alfo, that the force of adhesion between folids and fluids is in the inverse ratio of the temperature indicated by the thermometer, and the direct ratio of the fquares of their furfaces; that different folids adhere with different degrees of force to the fame fluid; but ftill it must be allowed, that experiments and obferva

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Of diterent folids to different fluids.