Page:Popular Science Monthly Volume 17.djvu/407

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IN a former paper[1] I endeavored shortly to summarize the more important differences between that system of chemistry which was founded on a so-called equivalent notation and the modern, or atomic phase of the science. The general conclusion to which that summary led was, that the old chemistry was empiric, while the new is scientific; but, as was there remarked, empiricism precedes science: science is the natural development of empirical statements, and is not to be regarded as entirely a new departure.

Believing, as I do, that the old and new chemistry are essentially opposed in their methods, I nevertheless am certain that the germs, at least, of many of our modern chemical theories are to be found in the statements, and even in the hypotheses, of the workers of half a century since: and in the present paper I propose to trace, in a little detail, what I believe to be a correct outline of the development of two of the more important theories of modern chemistry.[2]

The chemical views most in vogue before the strictly modern epoch were founded more on considerations of the composition of compounds than on the actions of these compounds. Dumas introduced wider views by recalling the attention of chemists to the fact that, in order to frame even a tolerably complete system of classification, an answer must be given to the question, "What does this substance do?" no less than to the other question, "Of what is this substance composed?"

But, if we go back to the time before Lavoisier and his associates, we find that the system then predominant in chemistry was founded almost entirely on the reactions, and but to a very small extent on the composition, of chemical substances. Chemists then busied themselves continually with studying processes of chemical change; only they contented themselves with qualitative knowledge, and hence their hypotheses were for the most part extremely vague and their facts disconnected. John Joachim Beccher, born about 1630, seems to have been the first to weave together the scattered chemical facts and guesses into a consistent general theory, which was subsequently augmented and defined by Stahl (1660-1734).

Looking at the wonderful changes produced in substances by the action of chemical force, the question arose, What happens when a body undergoes chemical change?—and, as burning or combustion was

  1. "Popular Science Review," January, 1878.
  2. In the paper referred to, I briefly sketched the history of the development of the older doctrine of "Equivalents" into the modern hypothesis of "Valency."