than all these spurious things I have named can recommend it.” It has been necessary to enter at some length into the nature of the phlogistic theory, because the discoveries of the time were expressed in its language. The _fire-air_ or _vital air_ of Mayow was termed dephlogisticated air; _i.e._ air wholly deprived of the power of burning, or air more capable of supporting combustion than ordinary air; while airs capable of burning were supposed to be more or less highly charged with phlogiston; indeed, at one time, it was imagined that hydrogen was phlogiston itself. It is to Joseph Black that the discovery of carbon dioxide, that constituent of air first to be definitely recognised, if we except Mayow’s early work, is generally ascribed. But we must remember that it had been prepared by Becher and by Hales, and had been doubtless obtained in an impure state by many others. It will be seen that Black’s work was so complete, and established the identity of this gas in so definite a manner, that his right to be named as its true discoverer can hardly be questioned. Black was born near Bordeaux in 1728. His father, a wine-merchant, was originally a native of Belfast, being descended from a Scottish family which had been settled there for some time. When twelve years of age. Black returned to Belfast, and received his education in the local grammar-school, afterwards proceeding to the University of Glasgow in 1746, at the age of eighteen. He was a pupil of Dr. Cullen, then Lecturer on Chemistry at the College there, who is mentioned by Professor Thomas Thomson, in his _History of Chemistry_, as an excellent and instructive lecturer. Black intended to choose the career of medicine, and he indeed practised occasionally as a medical man during the greater part of his life. [Illustration: JOSEPH BLACK.]