Black’s account of fixed air and its properties is the first example we possess of a clear and well-reasoned series of experimental researches, where nothing was taken on trust, but everything was made the subject of careful quantitative measurement. It was not long since Hales had pronounced air to be a chaotic mixture of effluvia. Black showed that common air contains a small amount of fixed air, and that fixed air must be considered as a fluid differing in many of its properties from common air, especially in its being absorbed by quicklime and by alkalies. It must be remembered that at that time carbon was not recognised as an element; and hence, though Black knew that fixed air was a product of the combustion of charcoal, he did not attribute it to the union of carbon with oxygen, although the sentence quoted above closely approaches to the truth. The discovery of nitrogen was next in the order of time. It was made by Daniel Rutherford, a pupil of Black’s, and at his instigation, and its description formed a thesis for his degree of Doctor of Medicine. Daniel Rutherford was born at Edinburgh on November 3rd, 1749. He was the son of a medical man, Dr. John Rutherford, one of the founders of the Medical School in that city. He was educated at Edinburgh University, and after graduating in Arts, became a medical student, taking his degree of M.D. in 1772. His diploma was obtained on 12th September. He then travelled for three years in England, France, and Italy, and in 1775 he returned to his native town, where he practised his profession. In 1786 he succeeded Dr. John Hope in the Chair of Botany in his University, but he did not on that account resign his practice. He was president of the Royal College of Physicians of Edinburgh from 1796 to 1798. During the greater part of his life he suffered from gout; he died in 1819, at the age of seventy. Rutherford does not seem to have pursued the study of chemistry further: his duties led him into other fields. His genial, pleasant face, seen in the portrait by Raeburn, shows him to have possessed a happy disposition; and he is said to have maintained until his death his friendship with Black, and his interest in the progress which science was then rapidly making. The title of Rutherford’s dissertation, of which I have been able to find a copy only in the British Museum, is _Dissertatio Inauguralis de aere fixo dicto, aut mephitico_. It was published at Edinburgh in 1772, seventeen years after Black’s memorable dissertation on Fixed Air. As will be seen shortly, it precedes Priestley’s and Scheele’s writings by a year or two. Evidently Black had noticed that a residue was left after the combustion of carbonaceous bodies in air, and absorption of the fixed air produced by the combustion, and had suggested to Rutherford, then a student of his, the advantage of further investigating the matter, and ascertaining the properties of the residual gas. [Illustration: DANIEL RUTHERFORD.]