19. The last of the labours of Bergman which I shall notice here is his Essay on Elective Attractions, which was originally published in 1775, but was much augmented and improved in the third volume of his Opuscula, published in 1783. An English translation of this last edition of the Essay was made by Dr. Beddoes, and was long familiar to the British chemical world. The object of this essay was to elucidate and explain the nature of chemical affinity, and to account for all the apparent anomalies that had been observed. He laid it down as a first principle, that all bodies capable of combining chemically with each other, have an attraction for each other, and that this attraction is a definite and fixed force which may be represented by a number. Now the bodies which have the property of uniting together are chiefly the acids and the alkalies, or bases. Every acid has an attraction for each of the alkalies or bases; but the force of this attraction differs in each. Some bases have a strong attraction for acids, and others a weak; but the attractive force of each may be expressed by numbers. Now, suppose that an acid _a_ is united with a base _m_ with a certain force, if we mix the compound _a m_ with a certain quantity of the base _n_, which has a stronger attraction for _a_ than _m_ has, the consequence will be, that _a_ will leave _m_ and unite with _n_;--_n_ having a stronger attraction for _a_ than _m_ has, will disengage it and take its place. In consequence of this property, which Bergman considered as the foundation of the whole of the science, the strength of affinity of one body for another is determined by these decompositions and combinations. If _n_ has a stronger affinity for _a_ than _m_ has, then if we mix together _a_, _m_, and _n_ in the requisite proportions, _a_ and _n_ will unite together, leaving _m_ uncombined: or if we mix _n_ with the compound _a m_, _m_ will be disengaged. Tables, therefore, may be drawn up, exhibiting the strength of these affinities. At the top of a column is put the name of an _acid_ or a _base_, and below it are put the names of all the _bases_ or _acids_ in the order of their affinity. The following little table will exhibit a specimen of these columns: _Sulphuric Acid._ Barytes Strontian Potash Soda Lime Magnesia. Here sulphuric acid is the substance placed at the head of the column, and under it are the names of the bases capable of uniting with it in the order of their affinity. Barytes, which is highest up, has the strongest affinity, and magnesia, which is lowest down, has the weakest affinity. If sulphuric acid and magnesia were combined together, all the bases whose names occur in the table above magnesia would be able to separate the sulphuric acid from it. Potash would be disengaged from sulphuric acid by barytes and strontian, but not by soda, lime, and magnesia.