The idea that scurvy, in common with other so-called putrid diseases, was due to an insufficient supply of “fixed air” in the animal economy, and that it might be cured by the administration of that gas, originated with Dr Macbride about the middle of the eighteenth century, shortly after Black had established the individuality of the gas, and it was current doctrine with the faculty at the time of Priestley’s experiments. The reasons which Macbride gave in support of his hypothesis are contained in his _Essays on Medical and Philosophical Subjects_, and are sufficiently ingenious to be worth stating as characteristic of much of the therapeutics of the time. Macbride assumed that substances held together, and acquired the quality of firmness, by virtue of containing a “cementing principle,” which ensured the perfect cohesion of their constituent particles, and that as putrefaction resulted in the decomposition and disintegration of substances, putridity was connected with the loss or disappearance of this cementing or cohering principle. He found that “fixed air” was invariably produced when animal and vegetable substances putrefy, that a greater amount of fixed air is produced from vegetable substances than from animal substances, and that animal and vegetable matters putrefy more rapidly when mixed than when separate, and yield more fixed air in conjunction than apart. On the basis of these observations Macbride proceeded to explain the well-established fact that a diet mainly composed of animal food is apt to produce sea scurvy, the remedy for which is a sufficient supply of fresh vegetables, by assuming that the virtue of the vegetables was due to the evolution of a greater amount of carbonic acid in the process of digestion, the fixed air so liberated in the body counteracting, by its antiseptic powers, putridity in the circulating fluids. We are not here concerned with the subsequent history of so-called ærated or soda-water, as it came to be called, but it is worth noting that Priestley’s account of his process contains one remark which is not without significance in view of latter-day developments. He says:— “I do not doubt but that, by the help of a condensing engine, water might be much more highly impregnated with the virtues of the Pyrmont spring, and it would not be difficult to contrive a method of doing it.”