=Stephen Hales= (1677–1761), an ingenious divine—he held the perpetual curacy of Teddington, and lived practically the greater part of his life there—distinguished as a physiologist and inventor, occupied himself in chemical pursuits, and made a number of observations on the production of gaseous substances. His results were communicated to the Royal Society and subsequently republished, in a collected form, under the title of _Statical Essays_. In these experiments he used methods very similar in principle to those subsequently employed by Priestley. It is evident from his description of his experiments that he must have prepared a considerable number of gaseous substances—hydrogen, carbonic acid, carbonic oxide, sulphur dioxide, marsh gas, etc.—but he seems to have made no systematic attempt to study their properties, as he considered that they were simply air, modified or “tinctured” by the presence of substances which he regarded as more or less fortuitous. Prior to the time of Black all forms of gaseous substance were regarded as substantially identical—in fact, as being _air_, as understood by the Ancients—a simple elementary substance. It was Black’s study of carbonic acid which first clearly established that there were essentially distinct varieties of gaseous matter. CHAPTER VII PHLOGISTONISM Even before the appearance of _The Sceptical Chemist_ there was a growing conviction that the old hypotheses as to the essential nature of matter were inadequate and misleading. We have seen how the four “elements” of the Peripatetics had become merged into the _tria prima_—the “salt,” “sulphur,” and “mercury”—of the Paracelsians. As the phenomena of chemical action became better known, the latter iatro-chemists—or, rather, that section of them which recognised that chemistry had wider aims than to minister merely to medicine—felt that the conception of the _tria prima_, as understood by Paracelsus and his followers, was incapable of being generalised into a theory of chemistry. Becher, while clinging to the conception of three primordial substances as making up all forms of matter, changed the qualities hitherto associated with them. According to the new theory, all matter was composed of a mercurial, a vitreous, and a combustible substance or principle, in varying proportions, depending upon the nature of the particular form of matter. When a body was burnt or a metal calcined, the combustible substance—the _terra pinguis_ of Becher—escaped. This attempt to connect the phenomena of combustion and calcination with the general phenomena of chemistry was still further developed by Stahl, and was eventually extended into a comprehensive theory of chemistry, which was fairly satisfactory so long as no effort was made to test its sufficiency by an appeal to the balance.