144 LEAVES. 



in all its interesting relations, we must premise a few 

 observations respecting the constitution of the atmos- 

 phere. It is composed of three invisible fluids, two of 

 which possess very peculiar properties. One of these 

 is oxygen or vital air, the great supporter of combustion 

 and animal life, without which neither can for a moment 

 exist. Combustible bodies introduced into a portion of 

 this gas, burn with increased splendour ; its volume is 

 diminished, and they acquire an additional weight, pro- 

 portioned exactly to the quantity of gas absorbed. Its 

 union with inflammable bodies, is usually attended with 

 an evolution of heat and light, constituting in ordinary 

 language, combustion or fire ; but sometimes this union 

 is effected without the appearance of flame. 



If 30 grains of powdered charcoal are inflamed in 100 

 cubic inches of this gas, both the charcoal and the oxy- 

 gen disappear, a combination of their elements is ef- 

 fected, and a new product is formed, possessing very 

 different properties from either. This is carbonic acid 

 gas, or fixed air, and it constitutes an essential, though 

 a very small portion of the atmospheric air. The res- 

 piration of animals is one of its most fruitful sources, 

 and hence we find that the atmosphere of a close and 

 crowded room soon becomes contaminated, and unfit to 

 sustain the ordinary functions of life ; and this contami- 

 nation arises entirely from the repeated respiration of 

 confined air, by which it is deprived of its oxygen, and 

 impregnated with carbonic acid. 



A diligent inquirer after truth, would ask how the pu- 

 rity of the atmosphere is preserved, while millions of 

 animals are continually breathing out their noxious ex- 

 halations. Even the light which guided the immortal 

 Priestly through his evening labours, was contaminating 

 the air, while it enabled him to point out the renovat- 



