relative to Black, Watt, and Cavendish 515 



ous substances, by the result of those experiments. In every 

 such course of research, whatever offers itself fortuitously is 

 observed by an eye which is on the watch for the appearance 

 of the laws, known or assumed, that fill its meditations; and 

 the whole design with which each experiment is instituted, is 

 to test the applicability of those laws, and to try the validity, 

 or the accuracy, of principles which have more or less the 

 character of foregone conclusions. 



This is experimental philosophy : this is the science of ob- 

 serving, interrogating, and interpreting, nature — apart from 

 that faculty of catchingyar analogies on the wings of a lively 

 and just imagination, which constitutes perhaps the highest 

 part of the genius of a philosopher, though we should be much 

 in error, if we regarded even this high gift of Heaven as inca- 

 pable of being improved by rule, example, and use. 



Thus it was that Black, under the guidance of the light 

 which a clear conception of the laws of affinity shed over his 

 mind, proved by a short series of experiments so devised as to 

 eliminate, one by one, all alternative suppositions, the follow- 

 ing points : — 1. That magnesia is a distinct substance, having 

 its own laws of combination to distinguish it from other earths 

 — 2. that that substance, which is sometimes found in air, and 

 sometimes fixed both in this and other absorbent earths and 

 alkalies, is subject to the laws of chemical composition, decom- 

 position, and transfer — 3. that common air does not enter into 

 the same combinations as fixed air; — and lastly, he inferred from 

 the general analogy of the effects of chemical attraction, that 

 unsaturated affinity is the form, as Bacon would have termed 

 it, of causticity. This brief, simple, and choice specimen of 

 synthetico-analytical research, to that time unexampled in 

 chemistry, he completed and crowned, by denoting the law of 

 double decomposition as dependent on " the sum of the forces," 

 and fixing the place, not of magnesia only, which was as much 

 as he at first contemplated, but of fixed air, side by side with 

 the acids, in its own place in the table of relative affinities*. 



* Essays and Obs. Phys. and Lit., vol. ii. pp. 221-24. The following de- 

 scription, by the French chemist De Lasone, in 1753, of the manner in which 

 an aerial spirit is combined with lime and iron in the waters of Vichy, is 

 worthy of notice, as a curious anticipation of truth since more exactly de- 

 veloped : — 



" Toutes ces experiences prouvent evidemment que ces eaux sont alka- 

 lines, par un principe salin et par une terre absorbante ; qu'elles contien- 

 nent une maticre ferrugineusej qu'elles contiennent un principe spiritueux, 

 compose non seulement d'mi air sur-abondant, comme il s'en trouve dans 

 quelques eaux, mais encore d'une portion de cette terre subtile dont nous 

 venons de parler, jointe au principe huileux du bitume, et volatilisee par 

 cet air, qui vrai-semblablement est le principal agent qui tient cette terre sus- 



