The next stage in the history of our subject is the consideration of the work of Stephen Hales and of Joseph Priestley. Both of these philosophers were essentially experimentalists. While both discovered gases and prepared them in a more or less pure state, Hales had no theory to guide him, and concluded as the result of his researches that air was possessed of “a chaotic nature”; for he did not recognise his gases as different kinds of matter, but supposed them all to be modified air. Priestley, on the other hand, was an adherent of the theory of phlogiston, and interpreted all his experiments by its help. Hales was a country clergyman, interested in botany, and undertook researches on air in order to gain knowledge of the growth and development of plants. Priestley was also a divine, who amused himself with experiments during the intervals of composing sermons or writing controversial pamphlets on disputed doctrines. Both possessed the experimental faculty, and both employed it to good purpose. Hales’ chief work is entitled “Statical Essays, containing Vegetable Staticks; or an account of Statical Experiments on the Sap in Vegetables, being an Essay towards a Natural History of Vegetation: of use to those who are curious in the Culture and Improvement of Gardening, etc.: Also, a specimen of an attempt to analyse the air by a great Variety of Chymiostatical Experiments, which were read at several meetings before the Royal Society. By Stephen Hales, D.D., F.R.S., Rector of Farringdon, Hampshire, and Minister of Teddington, Middlesex.” In his “Introduction” Hales reveals his method of research. The determination of weight and volume was at that date especially necessary; for want of numerical data the experimental researches of the time were of a somewhat vague character, and it often happened that the conclusions drawn from them were incorrect. Hence it is with a feeling of satisfaction that we read (vol. i. p. 2):-- “And since we are assured that the all-wise Creator has observed the most exact proportions of _number_, _weight_, and _measure_ in the make of all things, the most likely way, therefore, to get any insight into the nature of those parts of the creation which come within our observation must in all reason be to number, weigh, and measure. And we have much encouragement to pursue this method of searching into the nature of things, from the great success which has attended any attempts of this kind.” For God has “comprehended the dust of the earth in a measure, and weighed the mountains in scales, and the hills in a balance.”