143 



bronchial cells of those organs. This leads us, 

 therefore, in the next place, to speak of the nature 

 and extent of this decomposition ; in other words, 

 of the changes in quality which the air of our atmo- 

 sphere experiences in the lungs during its respira- 

 tion. 



116. It has been already shewn (96. et seq.}, that 

 when the dark crassamentum of venal blood is 

 placed in contact with atmospheric air, or with oxy- 

 gen gas, it speedily assumes a florid colour : that 

 the bulk of air is, in a small degree, diminished : 

 that a portion of the oxygen disappears, and a quan- 

 tity of carbonic acid is produced : and these effects 

 have all been considered to arise from the combina- 

 tion of the oxygenous portion of the air with carbon 

 supplied by the blood. In the living body, the ve- 

 nal blood experiences the same change of colour 

 (113.) when atmospheric air, or oxygen gas, is're- 

 ceived into the lungs : the air itself also is somewhat 

 diminished in bulk : a portion of its oxygen gas, as 

 we shall hereafter see, disappears : and that carbo- 

 nic acid is, in like manner, produced, was first pro- 

 ved by the experiments of Dr Black. " So early 

 as the year 1757," says this distinguished philoso- 

 pher, " I convinced myself that the change produ- 

 ced on wholesome air by breathing it, consists chief- 

 ly, if not solely, in the conversion of fiart of it into 

 fixed air : for I found, that by blowing through a 

 pipe into lime-water, or a solution of caustic alkali, 

 the lime was precipitated, and the alkali was render- 

 ed mild *." At a later period, Mr Bewley detect- 



* Black's Lectures by Robison, vol. ii. p. 87. 



