138 



half of blood to be propelled at each contraction of 

 the heart, and each contraction to occupy ~ of a 

 minute, and the whole circulating mass to be about 

 324 ounces, then a complete circulation is perform- 

 ed in the time of 216 pulsations, that is, in three 

 minutes. The almost instantaneous return of the 

 inspired airs, however, is totally adverse to this fact : 

 and if this be true of one respiration, it must be true 

 of all, since the power acting, and the substances 

 acted upon, preserve always the same relative state 

 and circumstances, and the actions are performed 

 not in time only, but in succession also ; and thus, 

 as one cannot begin until the other is ended, each is 

 only a repetition of the same event, which, being 

 operated by like powers, and in like circumstances, 

 must afford invariably a like result. 



1 13. If then there be no proof that air, either by 

 absorption, or by chemical affinity, enters into the 

 blood-vessels of the lungs, the sensible effects which 

 it is there known to produce on the blood, must, in 

 some other way, be accounted for. The celebrated 

 Lower, not only first remarked the scarlet colour 

 which black blood acquired by exposure to the air, 

 but he first proved likewise, that the same change 

 was produced by the air received into the lungs. In 

 combating the opinion of Willis, that the florid co- 

 lour of the blood was derived from an innate fire 

 kept up in the heart, he asserted, that, if this were 

 the case, it should, from their similarity of structure, 

 be effected by the right, as well as by the left, side 

 of the heart, whereas the blood sent into the lungs 

 from the right heart was dark and venal. He pass- 

 ed a ligature also round the trachea of an animal, so 



