From experiments on the rise of sap in plants, many of them very ingenious and well adapted to secure their end, and which are still regarded by botanists as classic, Hales noticed that a quantity of air was inspired by plants. In order to ascertain the composition and amount of this air, the process of distillation was resorted to; for Hales remarks: “That elasticity is no immutable property of air is further evident from these experiments; because it were impossible for such great quantities of it to be confined in the substances of animals and vegetables, in an elastick state, without rending their constituent parts with a vast explosion” (Preface, p. viii.). Hence, concluding that the air absorbed by plants and animals could be recovered by their distillation, Hales proceeded to distil a great number of substances of animal and vegetable origin, such as hogs’ blood, tallow, a fallow-deer’s horn, oystershell, oak, wheat, peas, amber, tobacco, camphor, aniseed oil, honey, beeswax, sugar, Newcastle coal, earth, chalk, pyrites, a mixture of salt and bone-ash, of nitre and bone-ash, tartar, compound aquafortis, and a number of other substances. He collected the “air” in each case over water, and gave numerical data to show what proportion the air bore by weight to the substance from which it had been obtained. He even tried to compare the weight of ordinary air with that of air from distilled tartar; but his experiment led to no positive conclusion, because of the crudeness of his appliances. The compressibility or “elasticity” of the air from tartar, however, was found to be identical with that of common air. Hales does not appear to have made any special experiments on the properties of his various airs, by trying whether they supported combustion, whether they were themselves combustible, etc. We see from this list that he had under his hands mixtures of hydrocarbons, carbon dioxide, probably sulphur dioxide, hydrochloric acid and ammonia (both, however, dissolving in water as they were formed), oxides of nitrogen, possibly chlorine, and, as minium or red-lead was one of the substances he tried, oxygen in a more or less pure state. It must be remembered that in all cases the gas obtained was mixed with the air originally present in the retort. He next proceeded to produce “air” by the fermentation of grain, of raisins, and of other fruits; this “air” obviously was carbon dioxide more or less pure. It is curious to note here that he anticipated Lord Kelvin in devising a sounding-lead which should register the depth of the sea by the compression of air, the distance to which the air had receded along the tube being shown by the entry of treacle. He successfully carried out a sounding by means of his apparatus.