THE RESPIRATORY SYSTEM. 389 
carbonic oxide by allowing illuminating gas to pass through it, 
when a change of color to a cherry red may be observed, and 
which will remain in spite of prolonged shaking up with air or 
attempts at reduction with the usual reagents. Haemoglobin’ 
may be resolved into a proteid (globin) not well understood, 
and hematin. This happens when the blood is boiled (perhaps 
also in certain cases of lightning-stroke), and when strong acids 
are added. Hematin is soluble in dilute acids and alkalies, and 
has then characteristic spectra. Alkaline hematin may be re- 
duced; and, as the iron can be separated, resulting in a change 
of color to brownish red, after which there are no longer any 
reducing effects, it would seem that the oxygen-carrying power 
and iron are associated. This iron-free hematin is named 
hematoporphyrin or hematoin. 
Hemin is hydrochlorate of hematin (Teichmann’s crystals), 
and may be formed by adding glacial acetic acid and common 
salt to blood, dried blood-clot, etc., and heating to boiling. This 
is one of the best tests for blood, valuable in medico-legal and 
other cases. 
When oxy-hemoglobin stands exposed to the air, or when 
diffused in urine, it changes color and becomes, in fact, another 
substance—methe moglobin, irreducible by other gases (CO, etc.), 
and not surrendering its oxygen in vacuo, though giving it up 
to ammonium sulphide, becoming again oxy-hemoglobin, when 
shaken up with atmospheric air. Its spectrum differs from 
that of oxy-hemoglobin in that it has a band in the red end of 
the spectrum between the C and D lines. Hematoidin is some- 
times found in the body as a remnant of old blood-clots. It is 
probably closely allied to if not identical with the bilirubin 
of bile. 
Comparative.-— While hemoglobin is the respiratory agent in 
all the groups of vertebrates, this is not true of the inverte- 
brates. Red blood-cells have as yet been found in but afew 
species, though hemoglobin does exist in the blood plasma of 
several groups, to one of which the earth-worm and several- 
other annelids belong. It is interesting to note that the respir- 
atory compound in certain families of crustaceans, as the com- 
mon crab, horseshoe-crab (limulus), etc., is blue, and that in 
this substance copper seems to take the place of iron. 
The Nitrogen and the Carbon Dioxide of the Blood.—The little 
nitrogen which is found in about equal quantity in venous and 
arterial blood, seems to be simply dissolved. The relations of 
carbonic anhydride are much more complex and obscure. The 
