It would be interesting to know the sentiments of Lord Shelburne, then in the cold shade of retirement, as he perused these passages, and whether he realised the truth of the little homily from his “tame philosopher.” The preface is followed by an introduction, in which Priestley gives a rapid and confessedly imperfect survey of the state of knowledge concerning “air” prior to 1774. He gives to Boyle the credit of first clearly recognising that elastic fluids exist differing essentially from the air of the atmosphere, but agreeing with it in the properties of weight, elasticity and transparency. But he also points out that two remarkable kinds of factitious air had long been known to miners, viz., _choke damp_, which is heavier than air, which lies at the bottom of pits, extinguishes flame and kills animals; and the other, called _fire damp_, which is lighter than common air, is found, therefore, near the roofs of subterraneous places and is liable to take fire and explode like gunpowder. “The word _damp_ signifies _vapour_ or _exhalation_ in the German and Saxon languages.” “Air of the former kind, besides having been discovered in various caverns, particularly the Grotta del Cane in Italy, had also been observed on the surface of fermenting liquors, and had been called _gas_ (which is the same with _geist_, or _spirit_) by Van Helmont and other German chemists; but afterwards it obtained the name of _fixed air_, especially after it had been discovered by Dr Black of Edinburgh to exist, in a fixed state, in alkaline salts, chalk, and other calcareous substances.” Black’s work is dealt with in half a dozen lines, and a passing reference is made to Macbride and Brownrigg. A very imperfect account is given of the work of Hales, although it is stated that “his experiments are so numerous and various that they are justly esteemed to be the solid foundation of all our knowledge of this subject.” This section concludes with the mention of Cavendish’s determinations of the relative weights of fixed air (carbon dioxide), and inflammable air from metals (hydrogen), and of Lane’s observations that water charged with carbonic acid will dissolve iron, “and thereby become a strong chalybeate.” Priestley was the last man in the world to seek to disparage the work of his predecessors or to minimise what was due to them. In reality he had the intention, as he distinctly states, to write at his leisure the history and present state of discoveries relating to air, in a manner similar to his _History of Electricity_, and of the _Discoveries Relating to Vision, Light and Colours_, when no doubt he would have done full justice to all concerned. In the meantime he gives only such particulars as are necessary, in his judgment, to the understanding of his own work.