90 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



when a dry powder had to be surrounded by gas drawn from 

 a vessel standing in water. 



The eudiometer? 15, was used for mixing minute quantities 

 of nitrous air and common air ; two bubbles were introduced 

 into the narrow tube a with the help of the iron-wire 

 plunger b, then measured, mixed, and measured again. 



Apparatus for sparking small quantities of gas is shown in 

 the second plate at 16, 17, 18, 19, anticipating in part the 

 apparatus used by Volta (Fig. 27) and by Cavendish 

 (Fig. 39). 



SUMMARY AND SUPPLEMENT 

 A. FIXED AIR AND INFLAMMABLE AIR 



Van Helmont (b. 1577 ; d. 1644) recognised the existence of 

 two "gases," a poisonous "gas sylvestre" and an inflammable 

 " gas pingue," analogous with the " choke-damp " and " fire 

 damp " of miners. 



John Mayow, Medico-physical Works, 1674, collected gases 

 prepared artificially by the action of iron on aqua fortis and on 

 dilute oil of vitriol contained in an inverted flask ; he showed 

 that they possessed the same elastic properties as common air, 

 but was not sure that they were essentially different from it. 



Stephen Hales, Vegetable Statics, 1727, by heating different 

 substances, prepared many gases which he collected over water 

 in a separate "receiver"; he also mixed different gases and 

 studied the changes of volume produced in them by the action 

 of different agents ; but he did not recognise these gases 

 as distinct substances, and supposed that all had the same 

 density. 



Joseph Black, Experiments upon Magnesia Alba, 1755, 

 proved that chalk and the alkalis contain a gas which he called 

 fixed air (carbonic anhydride or carbon dioxide, CO 2 ), but he 

 did not at that time collect or handle it. 



Henry Cavendish, On Factitious Air, 1766, measured the 

 density of fixed air from a variety of sources, and of in- 

 flammable air (hydrogen) prepared by the action of acids on 

 different metals, thus : 



1 This name was first used in 1776 (Experiments on Air], 1777, III. 

 379 and 380). 



