42 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



the second. A further experiment showed that a mixture 

 of 8 cubic inches of the respirable air with 42 cubic inches 

 of non-respirable air behaved in all respects like ordinary 

 air. There was, therefore, no doubt that the air which 

 disappeared during calcination had actually combined with 

 the mercury, and had been released from it by heating it 

 more strongly. 



In order to complete the proof it was only necessary to 

 show that the loss in weight of the calx (45 grains), when 

 reconverted into mercury (41^ grains), was equal to the 

 weight of the gas which had been liberated : a knowledge 

 of the density of the active gas showed that the quantity 

 collected would weigh 3^ to 4 grains, agreeing closely with 

 the loss of 3 J grains already recorded. 



Lavoisier completes his " oxygen " theory of com- 

 bustion. Lavoisier's proof was now complete : he had 

 shown that the calcination of a metal meant the combina- 

 tion of the metal with an active constituent of the air, 

 which in the case of mercury could be recovered from the 

 calx by heating it : the calx was, therefore, a compound, and 

 the combustible metal a simpler substance. The same 

 constituent of the air was required for the burning of fuel 

 and for respiration, two processes which differed from the 

 burning of metals mainly in giving rise to gaseous instead of 

 solid products (see Chapter VI). 



Lavoisier showed that the same constituent of air was 

 also concerned in the burning of sulphur and of phosphorus; 

 but the products of combustion differed completely from 

 the metallic calces, dissolving readily in water and producing 

 acid solutions. Lavoisier regarded his active gas as an 

 essential constituent of these and of all other acids. He 

 therefore selected for it the name OXYGEN or "acid-pro- 

 ducer" (compare German, sauerstof), deriving it "from 

 the Greek words ous, acid, and yeivo/xat, I beget, on account 

 of the property of this principle, the basis of vital air, to 



