84 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



sal-ammoniac, or the marine acid united to the volatile 

 alkali" (Experiments on Air, 1774, I. 169-170). 



Sal- volatile formed by the union of alkaline air with 

 fixed air. On mixing alkaline air with fixed air Priestley 

 again obtained a solid product which he identified as the 

 volatile alkali " sal-volatile." 



" Fixed air admitted to alkaline air formed long and 

 slender crystals, which crossed one another and covered the 

 sides of the vessel in the form of net-work. These crystals 

 must be the same thing with the volatile alkalis which 

 chemists get in a solid form, by the distillation of sal- 

 ammoniac with fixed alkaline salts " (Experiments on Air, 

 1774, I. 171). 



Apparatus for experiments on gases. The first 

 apparatus used for experiments on gases, as distinguished 

 from condensible vapours, was that of Mayow, Figs, n, 12, 

 13, 1 6. It should be noticed that, in preparing gases 

 artificially by the action of acids on iron, Mayow was not 

 able to collect the gas over water, but was obliged to fill 

 the whole apparatus (Fig. 14, p. 33) with acid. 



This disadvantage was removed by Stephen Hales 

 (1677 1761), botanist, chemist, and Vicar of Tedding- 

 ton, who was one of the first to separate the GENERATOR 

 from the RECEIVER of the gas. His Vegetable Statics (1727) 

 is chiefly concerned with hydrostatic experiments on the 

 pressure of the sap in plants. But the sixth chapter, occupy- 

 ing nearly one-third of the book, deals with "an attempt to 

 analyse the Air by means of a great variety of chymio- 

 statical experiments, which shew in how great a proportion 

 Air is wrought into the composition of animal, vegetable, 

 and mineral Substances, and withal how readily it resumes 

 its former elastick state, when in the dissolution of those 

 Substances it is disengaged from them." 



In order to measure the volume of gas set free by heating 

 animal, vegetable, and mineral substances, Hales used a 



