50 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



water, which by its smell and properties shewed itself to be 

 slightly alkaline " (A. C. R. I. 23). 



Black had himself carried out the experiment of heating 

 magnesia (a compound resembling chalk, but more easily 

 decomposed by heat), and had " found only five drams of a 

 whitish water in the receiver," although the three ounces 

 (24 drams) of magnesia in the retort " had lost more than 

 the half of its weight" (A. C. R. I. 15). 



From these experiments it was clear that the burning of 

 chalk to lime resulted in the escape of an invisible gas ; to 

 this gas Black applied the name FIXED AIR. He regarded 

 chalk as " a peculiar acrid earth rendered mild by its union 

 with fixed air." According to his view : 



" When the calcareous earths are exposed to the action of 

 a violent fire, and are thereby converted into quick-lime, 

 they suffer no other change in their composition than the 

 loss of a small quantity of water and of their fixed air. The 

 remarkable acrimony which we perceive in them after this 

 process, was not supposed to proceed from any additional 

 matter received in the fire, but seemed to be an essential 

 property of the pure earth " (A. C. R. I. 22). 



The contrast between Black's theory and that which it 

 replaced may be expressed by the equations : 



Old theory : Limestone + phlogiston = lime. 

 New theory : Limestone -fixed air = lime. 



Lime combines with water and with fixed air. In 



the process of slaking, lime combines vigorously with water 

 to form slaked lime ; but in presence of fixed air it releases 

 the water, recombines with the fixed air and is reconverted 

 into chalk. Black writes : - 



" A calcareous earth deprived of its air, or in the state of 

 quick-lime, greedily absorbs a considerable quantity of water, 

 becomes soluble in that fluid, and is then said to be slaked : 

 but as soon as it meets with fixed air it is supposed to quit 



