Tables of chemical affinities

The dawn of the nineteenth century saw a drastic shift from the dominance of French chemistry to first English-, and, later, German-influenced chemistry. Lavoisier s dualistic views of chemical composition and his explanation of combustion and acidity were landmarks but hardly made chemistry an exact science. Chemistry remained in the nineteenth century basically qualitative in its nature. Despite the Newtonian dream of quantifying the forces of attraction between chemical substances and compiling a table of chemical affinity, no quantitative generalization emerged. It was Dalton s chemical atomic theory and the laws of chemical combination explained by it that made chemistry an exact science. 

The creation of tables of chemical affinities was an attempt to encapsulate all possible reactions between the constituents of chemical compounds. The goal was not only to provide a summary and key to known reactions but also to predict reactions that had not yet been observed. Tables of affinities thus had both a descriptive and a predictive role they could be used as a shorthand for a description and classification of observed reactions, and they could function as instruments of discovery. It was also possible, although not necessary, to use affinity tables as a clue to the mechanism of chemical reactions. It was along... 

FIGURE 218. This is a table of chemical affinities from Torbern Bergman s A Dissertation on Elective Attractions (London, 1785). 

The observation that certain elements prefer to combine with specific kinds of elements prompted early chemists to classify the elements in tables of chemical affinity. Later these tables would lead, somewhat indirectly, to the discovery of the periodic system, perhaps the biggest idea in the whole of chemistry. Indeed, periodic tables arose partly through the attempts by Dimitri Mendeleev and numerous others to make sense of the way in which particular elements enter into chemical bonding. 

We will study the eighteenth-century tables of chemical affinity and their historical context in more detail in part II. At this point we wish to further illuminate the distinctive type of experiments they referred to, and the systematization to which these experiments lent themselves. The core of eighteenth-century affinity tables was built by the salts, acids, alkalis, earths, metals, and alloys. Under largely the same physical conditions, especially at ordinary temperatures, salts, acids, alkalis, earths, metals, and alloys displayed a stable, reproducible pattern of chemical transformation. For example, when a salt, such as copper vitriol, common salt, or saltpeter, was mixed with certain ingredients and heated, it yielded a mineral acid. When the mineral acid obtained in this way was again mixed with calcareous earth, or another salifiable base, the original salt could be restored. Such kinds of reversible chemical transfor-... 

It is therefore obvious that the chemistry of pure substances can be defined only on the basis of its objects of inquiry. But it should be noted that in the eighteenth century it were the chemists themselves who distinguished these objects of inquiry from other ones in their practices of classification. We argue that their distinction corresponds exactly with the boundaries of objects of inquiry in the tableau of the Meth-ode. The question raised above was whether the authors of the Methode were the first to see an inner bond among the many different activities with pure chemical substances scattered through all domains of chemistry, or whether their distinction of the particular sphere of the chemistry of pure substances followed a tradition established earlier. Fortunately, there exists unmistakable evidence for such a tradition. The famous tables of chemical affinities testify unambiguously to the existence of this particular chemical practice. The first of these tables was the Table des differents rapports constmctedby Etienne Francois Geoffroy (1672-1731), published in 1718. We can thus even determine when the distinction of operations with pure chemical substances first became manifest, namely approximately seventy years before the Tableau of 1787. 

Geoffroy the Elder made many contributions to the development of chemistry in the 18 century. We meet him in connection with arsenic, antimony, zinc, manganese and ammonia. He is also well known for a table of chemical affinities. 

Newtonian ideas about chemical combination made inroads in France in the second half of the century, and affinity tables proliferated. By 1778, Mac-quer (1718—84)had decided that there were no separate laws of chemical affinity and that the law of universal attraction would suffice to explain the whole of chemistry, if only we could learn about the shape of the particles of bodies. In the same year, the second edition of the Encyclopaedia Britannica asserted that all theories of affinity were conjectural, neither is it a matter of any consequence to a chemist whether they are right or wrong. Here was a recognition that the utility of a scientific theory need not depend upon its truth. Affinity tables were above all useful, in providing a summary of existing knowledge about chemical reactions as well as a tool for predicting new reactions. 

Not only was elective affinity inadequate as a means of comparing the affinities of different substances it also led to critical and pervasive errors in chemical analysis by compromising the purity of chemical substances obtained in this manner. In other words, the supposition of complete displacement reactions encouraged the use of impure substances as pure ones in chemical analysis, which seriously compromised the accuracy of chemical analysis. Chemists were deceived, for example, in believing that they could obtain pure magnesia from displacement reactions. Berthollet s pointed attack on the notion of elective affinity is understandable in light of the fact that it caused a serious problem for the validity of basic chemical analysis. His attack was quite successful in undermining chemists naive confidence in the absolute order of chemical affinities obtained from displacement reactions and in the analytic purity of the substances thus obtained. He discredited affinity tables as mere memorandums of barren facts ... 

Side by side, theories of chemical affinity -the "force" causing reactions - developed-Affinity was considered in terms of Newton s ideas (which were gaining wide acceptance at the time for explaining physical phenomena) that every particle of matter was endowed with a certain attractive force that uniguely caused all its chemical and physical reactions. To make the concept generally useful, chemists felt the need to draw up tables of affinity that would express the reactivity of individual compounds toward each other. They also hoped that such tables could be used to predict the reactivity of other compounds in similar reactions. 

Up to this point, the affinity tables of the eighteenth century attracted the attention of historians of science mainly because of the conception of chemical affinity embodied in them. They were rarely studied as documents giving testimony of cheuucal classification in this period. Among the many merits of Alistair Duncan s profound book on these tables is that it dedicates a section of its fourth chapter to the question of the classification of chenucal substances in this age—see Duncan [1996] pp. 159-168. In the following we further extend and elaborate Duncan s approach. 

Geoffrey, E.F. (1718). Table des differents rapports observes en chimie entre differentes substances, Memoires de VAcademie Royale des Sciences, p>p. 202-212, Available from http // gallica.bnf.fr/ark/12148/bpt6k3519v/ f330 Gladstone, J.H. (1855). On Circumstances Modifying the Action of Chemical Affinity, Phil. Trans. Roy. Soc. London 175, pp. 179-223, Available from http //www.jstor.org/ stable/108516... 

Many dyes that have no chemical affinity to fibrous substrates can be attached to such substrates by intermediary (go-between) substances known as mordants. These are either inorganic or organic substances that react chemically with the fibers as well as with the dyes and thus link the dyes to the fibers. Mordants are traditionally classified into two main classes, acid and metallic mordants. The acid mordants are organic substances that contain tannins (see Textbox 64) as for example, gall nuts and sumac. The metallic mordants are inorganic substances, mostly mineral oxides and salts that include metal atoms in their composition. Table 94 lists mordants of both these types, which have been used since antiquity. 

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