The fourth volume made its appearance in March 1779, with a dedication to Sir George Savile, who had rendered Priestley the service of introducing him and his invention of soda-water to the notice of the Admiralty. In the preface, which is commendably short, he makes some reference to the respite which he had promised himself and his readers, but trusts, by way of extenuation, “it may be sufficient to allege the instability of human purposes and pursuits.” He had intended to devote himself to metaphysics. “But that kind of writing,” he says, “is a thing of a very different nature from this. I can truly say ... that single sections in this work have cost me more than whole volumes of the other; so great is the difference between writing from the head only and writing, as it may be called, from the hands.” The fact was Priestley could not keep away from his laboratory. “Having acquired a fondness for experiments, even slighter inducements than I have had would have been sufficient to determine my conduct.” The preface is noteworthy for its plea for the position of experimental science in the scheme of general education. “If we wish to lay a good foundation for a philosophical taste, and philosophical pursuits, persons should be accustomed to the sight of experiments and processes in early life. They should, more especially, be early initiated in the theory and practice of _investigation_, by which many of the old discoveries may be made to be really _their own_; on which account they will be much more valued by them. And, in a great variety of articles, very young persons may be made so far acquainted with everything necessary to be previously known as to engage (which they will do with peculiar alacrity) in pursuits truly original.” In the course of some observations on the effect “of impregnating oil of vitriol with nitrous acid vapour” he discovered _nitrosulphuric acid_, the so-called “Leaden Chamber Crystals,” whose properties and behaviour with water he describes with accuracy and even eloquence. Of these crystals he says: “A more beautiful appearance can hardly be imagined, and I am afraid I shall never see the like again.” He also noticed the formation of the dark brown compound which nitric oxide forms with a solution of green vitriol, and adds:— “To determine whether the phenomena attending the impregnation of the solution of green vitriol with nitrous air depended in any measure upon the seeming _astringency_ of that solution ... I impregnated a quantity of _green tea_, which is also said to be astringent, with nitrous air, but no sensible change of colour was produced in it.”