Chap. 2.] Why purs Air is eflentlal to Life. 379 



three, or four times better than common air. Dr. 

 Prieftley mentions Tome extracted from red lead five 

 times as pure as common air. 



Pure or oxygen air is not abibrbed by water, nor 

 foluble in it j but it may, as was juft intimated, be al- 

 moft entirely condenfed by nitrous gas, with which it 

 combines, as will be proved when treating of that 

 fluid i and this combination is foluble in water, 

 and forms nitrous acid} for this acicj is compofed of 

 the bafis of nitrous gas combined with oxygen, the, 

 whole being diffolved in water. 



The reafon that pure air is the moft eifcntial of all 

 the fluids to the fupport of life is, probably, becaufe a 

 great quantity of heat is neceffary for this purpofe, and 

 becaufe this fluid contains it in great quantity, and 

 parts with it very freely when it meets with any fub- 

 jftance for which it has itfelf a (Irong attraction. But as 

 its bafis (oxygen) combines itfelf very eafily with the 

 )bafis of coal which is found in the blood and lungs, and, 

 during this combination, lofespart of its caloric or heat, 

 which goes to the fupport of life, the remainder of 

 the caloric and oxygen, combined with the coal, form 

 t,he carbonic acid gas or fixable air, which is always 

 found to exift in a larger quantity in air which has 

 been refpired, than in atmofpherical air which has 

 not been fubfervient to that function. Of this a very 

 eafy experiment affords fufficient proof j it is founded 

 on the property which the carbonic gas has of render- 

 ing lime-water turbid. A crooked tube open at both 

 ends is partly filled with lime-water j a perfon applies 

 his mouth to one end of the tube, and infpires, by 

 drawing the air through the lime-water contained in 

 it. By this the tranfparency of the lime-water is not 

 affe&ed ; but it becomes turbid as foon as the perfon 

 xpires, which is owing to the carbonic acid formed 



in 



