[Reasoning by analogy in chemical history: fallacies and guidelines]

In chemistry (and natural science in general) reasoning in terms of analogy may be of great practical value as well as of theoretical significance. At the same, however, the use of analogy is risky and the results are uncertain. In alchemy and iatrochemistry analogies between the living and the non-living nature have been carried too far. At the end of the 19th century an analogy was drawn between chemical phenomena and the Darwinian theory of evolution. Another example of an overexaggerated analogy can be found in Lavoisier's theory of acids, where oxygen was considered to be the general principle of acidity. Reasoning by analogy is mostly based on an inner conviction. It leads to provisional predictions in such cases where the required facts are not, or incompletely, at our disposal.