137 



pears, that the inhalation of any gas, unfit to carry 

 on respiration, will, in some persons, produce effects 

 precisely similar to those which have been ascribed 

 to an absorption of nitrous oxide ; and, on the other 

 hand, that no such stimulant effects are produced in 

 other persons by the inhalation and supposed ab- 

 sorption of that gas. In no respect, therefore, can 

 we trust to the effects, which succeed to the respira- 

 tion of this oxide, as affording any decisive proof of 

 its absorption and commixture with the blood. 



112. It is, as we have seen, the opinion of Mr Da- 

 vy, and of some other authors, that air passes entire 

 into the blood-vessels, and is decomposed during the 

 circulation of that fluid. Now, the gases expired 

 are, and must be admitted to be, formed out of those 

 previously inspired, and, in natural breathing, the 

 air suffers but a small diminution (85.) in bulk ; 

 and the time of an ordinary inspiration occupies 

 from ~ to -^ part of a minute. To suppose the 

 inspired air to enter through the moist coats of the 

 cells and blood-vessels, su. er decomposition in the 

 blood, and be again returned through these vessels 

 and cells, with so small a difference in bulk, and in 

 so short a time, seems not only without proof, but 

 against all probability, especially if we consider the 

 extreme minuteness of the pulmonary vessels, and 

 the great rapidity with which the blood is transmit- 

 ted through them. If indeed, the reciprocal chan- 

 ges which take place between the blood and the air 

 were effected during the circulation of that fluid, 

 and it were only after that period the expired airs 

 could be rendered, we ought not to expect them for 

 nearly three minutes ; for suppose one ounce and a 



