B LO

BLO

( 151 )

ferum deprived of its albumen by coagulation,if evaporated to drynefs gives, befides gelatinand uncombined foda, a fmall quantity of muriatof foda, muriat of potafh, phofphat of lime,and phofphat of ammonia, all of which maybe detected by the proper chemical reagents.

The proportion of the conftituent parts ofblood is fo extremely variable as probably to befcarcely the fame in any two individuals. Thevarying quantity of craflamentum is the moftpalpable, and has long been obferved by phyfi-ologilts and medical obfervers. It appears tobe the part the mod intimately connected withthe ftrength and vigour of bodily habit, and isthe latell to return to its natural quantity afterany great lofs of blood, fevere evacuation, defi-cient nutriment, or d.ebilating illnefs. It is alfo,ceteris paribus , more abundant in adult than inyoung animals.

It has been doubted whether the red colourof blood was owing to the iron which it con-tains, though the proportional quantity of thismetal mixed with any colourlefs albuminousfluid, in the Hate of fub-phofphat with excefsof alkali, is fufficient to produce as full a body ofcolour as that of blood. The ingenious argu-ments brought againft this explanation of- thecaufe of the colour by Dr. Wells, 1 ' belong more tooptics and the theory of colours than to chemif-try, and we {hall therefore not enter into them.

But there is another part of this fubjedtproperly belonging to chemical phyfiology,which ihould not be entirely unnoticed. This isthe caufe of the difference between venous andarterial blood. The blood contained in all thearteries of the body derived from the aorta,, andalio in the pulmonary veins, has a bright floridor Vermillion colour, that of the other veinsand the pulmonary artery, on the other hand,is of a dark black red ; the former is termed thearterial, the latter the venous blood. The dif-ference between the colour of the two is atonce apparent when any deep wound has beeninflicted, when the arterial blood will be asmuch diffcinguiflied by its more florid red, as byits manner of flowing by jerks correfponding withthe pulfe. The caufe of this, difference of colouris unqueftionably owing to fome change pro-duced upon the venous blood as foon as receivedin the lungs, for the blood coming to the heartfrom the different veins of the body, goes fromthence to the lungs by the pulmonary artery,black-red, and returns from the lungs by thepulmonary veins, vermillion red. Ihis fact wasobierved long before the caufe was accounted* Plul.Trau, 1797, 1

for, though the circumftance of air'being con-ftantly admitted into the lungs made it probablethat the air was in fome W'ay or other the caufeof this change, and it had been thus conjecturedby feveral phyfiologifts.

The experiments of Dr. Prieftley are the firftwhich diredtly tended to confirm this very pro-bable opinion, by {hewing that the contact ofair will produce the fame change of colour outof the body as in it. This eminent philofophertook part of the craflamentum of the venousblood of a {heep, ‘ laid it on a net of gauze or.wire, and introduced it into different kinds ofair. Common air foon gave it a florid redcolour, but dephlogifticated (oxygen) air 'pro-duced this change in a much more compleat-and rapid manner. On the other hand thebrighteft red blood became prefently black, inhydrogen, nitrous gas, azot and any other air.unfit for refpiration. Now. as common-air iscompofed of oxygen andazot, and as the formergives, a. florid arterial- colour to venous blood,but the latter only renders it darker, approach-ing to black, it follows that it is only the oxy-genous part of the atmofphere which effectsthis - change of colour in the lungs. From thefame caufe it is owing that the furface of venouscraflamentum gradually becomes of a bright redin coagulation, whilft the under part is nearlyblack •, and if the caagulum is turned upfidedown, the part that was at the bottom, beingnow in contact with air, gradually affumes thefame florid hue.

The blood however is not in adtual contact,with the air in the lungs, being feparated fromit by the fine membrane (thinner than thethinneft bladder) which lines the air-cells andthe coats of the extreme ramifying veflels ofthe pulmonary artery and veins. Hence it isrequifite to do away the objection that mightarife from this fource againft the opinion of theaction of air on venous blood, to {hew that theinterpofition of a thin membrane does not pre-vent their mutual action. Dr. Prieftley accordsingly found that when black blood was tied upin a bladder, moiftened with ferum and expofedto air, the change to florid colour was producednearly as. foon as when freely immerfed in air.

Another objection to be removed was that,the colouring part of blood is in its naturalftate mixed with and immerfed in ferum, whichmull have fome effedtin feparating it from theadtual contact of air, but Dr. Prieftley alfofound that the fame change of colour happenedwhen a piece of craflamentum was covered evestExperiments on Aic, vol. iii.