Before relating the history of the discoveries of Black, Rutherford, and Priestley, it will be appropriate to give an account of a theory which professed to explain the phenomena of combustion, and with it the conversion of metals into calces, and the reduction of these calces to the reguline or metallic state. Like other theories, it was slow in developing. Its germ is to be traced to the writings of Johann Baptist van Helmont of Brabant, Seigneur of Merode, Royenboch, Oorshot, and Pellines, who was born in Brussels in 1577. He adopted a fantastical creation of Paracelsus, the _archaeus_, a kind of demon which, by means of fermentation, draws together all the particles of matter. Believing that water was the true principle and origin of everything (for he had succeeded in producing a willow tree, weighing 164 lbs., from water alone, the earth in which it grew having neither gained nor lost appreciably in weight), he conceived that it was acted on by a _ferment_ or principle pre-existing in the seed developed by it, and exhaling an odour by which the archaeus was attracted. Water undergoing the action of this ferment developed a vapour, to which van Helmont gave the name of “gas.” A “gas” was a substance intermediate between spirit and matter, and the word was probably derived from _Geist_, the common German word for spirit. Another word introduced by him to denote the life-principle of the stars was _Blas_, connected probably with _blasen_, to blow, and our English word _blast_. It is curious to notice how the idea of an _archaeus_ survived down to later times under the name of a “life-principle”--a conception that all organic substances must necessarily owe their origin to life itself, and not to the usual chemical and physical transformations. Van Helmont was acquainted with various kinds of gases, as appears from his treatise “De Flatibus.” His _gas sylvestre_ was evolved from fermenting liquors, and he knew that it was formed during the combustion of charcoal, and also that it was present in the Grotto del Cane near Naples. He was likewise acquainted with combustible gases, which he named _gas pingue_, _gas siccum_, or _gas fuliginosum_. These principles of van Helmont’s apparently suggested to his successors, Becher and Stahl, the notion of a principle inherent in every combustible substance, which was lost during combustion. The development of this--the phlogistic--theory is almost wholly due to the latter chemist, and indeed it is difficult to trace Becher’s share in it.