The remaining section of the introduction deals with his method of experimenting and with the apparatus he employed. It is of historical interest as containing a description of that most useful article of chemical furniture, his well-known pneumatic trough. He explains its use and gives details of his modes of manipulation. What an advance these were in simplicity, ingenuity and convenience can only be fully realised by comparing his methods with those of Hales. Not the least of Priestley’s services to science were the improvements he effected in that section of operative chemistry which is concerned with the preparation, collection and storage of gaseous substances. The main body of the volume is divided into two parts—the first dealing with observations made in and before 1772, the second with observations made in the year 1773 and in the beginning of 1774. In the outset Priestley finds himself at a disadvantage in regard to the only terms at that time in vogue for the factitious airs, viz., _fixed_, _mephitic_ and _inflammable_, which, he rightly says, are not sufficiently characteristic and distinct. Strictly speaking, any two of these terms might be applied to any one of the “airs” then known. The inflammable air from metals, as well as choke damp, is noxious, and therefore mephitic, as is fixed air, and since the inflammable airs are, apparently, capable of being imbibed by certain substances they may equally be considered fixable. The term _fixed air_ had, however, acquired a distinctive meaning, and rather than introduce a new term or change the signification of an old one, he would, with his contemporaries, restrict the term to the air which had been made the subject of Black’s memorable investigation. The first paper in this section deals with fixed air; it is practically a reprint of that in the _Phil. Trans._ and which has already been described in sufficient detail. In the course of his experiments he says he once thought that the readiest method of procuring fixed air, and in sufficient purity, would be to heat pounded lime-stone in a gun barrel, “making it pass through the stem of a tobacco pipe or a glass tube carefully luted to the orifice of it.” “In this manner I found that air is produced in great plenty; but, upon examining it, I found to my great surprise that little more than one half of it was fixed air, capable of being absorbed by water; and that the rest was inflammable, sometimes very weakly, but sometimes pretty highly so.”