But, in extensive divisions of thought, truth is only a relative thing; of inestimable value for its time, and most of all valuable as a means of getting away from it and attaining more perfect truth. Logic is the art or science of arriving at truth by ratiocination; but science is the field where logic is put to practical application and subjected to the most rigorous tests. The human mind if left to logic alone may go wild in any direction; science holds it steadily to the observed order of Nature as the standard by which it is to be tried. The whole circle of the sciences bears witness to the correctness of scientific thinking; and the history of every science abounds in proofs of the relativity of truth. Certain parts of elementary facts may remain constant, but even the interpretations of these, true only for their time, are changed, age after age. The science of chemistry affords an admirable exemplification of this view.
There was vague and indefinite truth even in the chimeras of the alchemists, mixed indeed with an enormous amount of gross error and preposterous speculation as seen from subsequent points of view; but there was sufficient of verity and correspondence to reality in those mystical times to guide men to important discoveries. The alchemists found out a great number of new and valuable things. They worked under delusions, but these were far from being destitute of plausibility; and were in fact in no small measure consistent and rational. Experimental knowledge at any rate grew in extent, and somewhat in coherency, until the absurdities of the epoch fell away and a definite and rational chemical system ensued.
This was the epoch of Phlogiston which was held to be a kind of subtile matter or energetic essence, present in all combustible bodies and absent in all incombustible bodies, and which caused combustion-changes in its escape. It was a theory of the nature and cause of fire; and, as heat is implicated in nearly all chemical changes, it was a crude theory of chemical action. It served the most important uses. It was a principle of connection and association, and explication, which stimulated investigation, guided inquiry, and enlarged the domain of actual knowledge. A chemical belief that the discoverer of oxygen. Dr. Priestley, held to the day of his death, could certainly not have been an absurdity. Prof. Cooke has the following excellent remarks on this early theory: "That it was not absurd a single consideration will show. Translate the word phlogiston, energy, and in Stahl's work on chemistry and physics, of 1731, put energy where he wrote phlogiston, and you will find there the germs of our great modern doctrine of conservation of energy—one of the noblest products of human thought. It was not a mere fanciful speculation which ruled the scientific thought of Europe for a century and a half. It was a really grand generalization; but the generalization was given to the world clothed in such a material garb that it has required two centuries to unwrap the truth." Nevertheless there was invaluable truth in it, but truth obscured, imperfect, and in relation to the time.
The phlogistic doctrine broke down as the facts accumulated and outgrew it; and chemical science passed into a new phase. That which had long helped at length became an obstruction, and, with the abandonment of the entity, chemical effects began to be referred to inherent attractions among different kinds of matter. But the facts must still be interpreted by principles or theories, and, at the epoch of Lavoisier, affinity, or the energy of chemical change, was viewed simply as a coupling force. Combination and decomposition were supposed to take place directly among bodies in pairs; elements uniting with elements to form binary compounds, and these uniting again by twos to form
