Dr. Spurgin on the Nature and Properties of the Blood. 419 



the blood to the air, some tune before the component parts of 

 the air were discovered : and Priestley fully proved that this 

 is owing to the oxygenous part of the air alone ; and that car- 

 bonic acid and azote have the contrai'y effect, reducing bright 

 scarlet blood to a purple colour. And it is conjectured that 

 this change is owing to the presence of iron, and experienced 

 by the red globules alone. 



The preservation of the life of the blood, and thence of the 

 body, would seem greatly to depend upon the change by which 

 this bright scarlet colour is constantly renewed and preserved ; 

 lor as the blood loses this colour by its circulation through the 

 body, it is made to pass through the lungs after its arrival at 

 the heart before it can be distributed again from the heart to the 

 body. Now, the structure of the lungs is such as to admit of a 

 large quantity of air being exposed to an extensive surface of a 

 most minute and vascular net -work, whereby the dark venous 

 blood comes almost into contact with the air admitted into the 

 pulmonary air-cells ; the consequence of this is, an immediate 

 change of the dark venous blood to a bright scarlet colour, or 

 to what is commonly termed arterial blood, because such is the 

 blood contained in the arteries. But not only does the air exert 

 such an influence on the blood both in and out of the bodjf, but 

 also certain other gases and certain salts will manifest a similar 

 effect: — among the gases, the nitrous oxide more especially; 

 and nitre, ammonia, and common salt, among the salts. 



Many discordant circumstances have also been stated re- 

 specting the appearance of these globules before the micro- 

 scope ; and different microscopic observers have described 

 them in a manner that might lead one to question, whether they 

 could have been engaged upon tlie same subject : for the evi- 

 dence of our eye-sight, and this assisted too by the magnifying 

 powers of an optical instrument like the microscope, ought to 

 be relied upon, if any satisfactory evidence at all can be ob- 

 tained for our guidance. But this instrument may be compared 

 in its power to the reasoning faculty of man ; and we thence 

 need no longer be surjirised, that the subjects it is employed 

 upon, shoukl, like the subjects of our reasoning powers, be dif- 

 ferently represented and differently apprehended by different 

 persons. Many of these anomalies have been attributed to the 

 instrument itself, and to its modification of the rays of light as 

 they j)ass througli it; or to its conveying the altered modifica- 

 tions induced by the subject under examination itself, whereby 

 it imparts a false im|)ression to the eye of the observer. And 

 certainly the instrument in question and the rational faculty are 

 nearly allied together in these respects ; for our minds are very 



:} II 2 apt 



