460 METILEMOGLOBIK [BOOK n. 



haemoglobin), but though belonging to the globulin family, has 

 characters of its own ; it is possibly a mixture of two or more 

 distinct proteids. It has been provisionally named globin and 

 is said to be free from ash. 



282. Haematin when separated from its proteid fellow, and 

 purified, appears as a dark -brown amorphous powder, or as a 

 scaly mass with a metallic lustre, having the probable composi- 

 tion of C 32 , H 34 , N 4 , Fe, O 5 . It is fairly soluble in dilute acid 

 or alkaline solutions, and then gives characteristic spectra 

 (Fig. 90, 1, 2, 5). 



An interesting feature in haematin is that its alkaline solu- 

 tion is capable of being reduced by reducing agents, the spectrum 

 changing at the same time (Fig. 90, 3), and that the reduced 

 solution will, like the hsemoglobin, take up oxygen again on 

 being brought into contact with air or oxygen. This would 

 seem to indicate that the oxygen-holding power of haemoglobin 

 is connected exclusively with its haematin constituent. 



By the action of strong sulphuric acid haematin may be 

 robbed of all its iron. It still retains the feature of possess- 

 ing colour, the solution of iron-free haematin being a dark rich 

 brownish red ; but is no longer capable of combining loosely 

 with oxygen. This indicates that the iron is in some way asso- 

 ciated with the peculiar respiratory functions of haemoglobin ; 

 though it is obviously an error to suppose, as was once supposed, 

 that the change from venous to arterial blood consists essentially 

 in a change from a ferrous to a ferric salt. 



Though not crystallizable itself, haematin forms with hydro- 

 chloric acid a compound, occurring in minute rhombic crystals, 

 known as hcemin crystals. 



When blood is left until it decomposes, the haemoglobin is 

 very apt to become changed into a peculiar body known as 

 methcemoglobin, in the spectrum of which a very conspicuous 

 band is seen in the red between C and D (see Fig. 90, 4). The 

 same change may be brought about by the action of weak acids, 

 such as carbonic acid, by ozone, and by other agents such as 

 nitrites and potassium permanganate. When a stream of car- 

 bonic acid is driven through blood or through a solution of 

 haemoglobin the band in the red characteristic of methaemoglo- 

 bin soon makes its appearance. Methaemoglobin differs but 

 little if at all in elementary composition from haemoglobin ; it 

 is maintained that it contains the same quantity of oxygen as 

 oxyhaemoglobin but in a more stable condition, more intimately 

 associated with the molecule. 



^ In conclusion, the condition of oxygen in the blood is as 

 follows. Of the whole quantity of oxygen in the blood, only a 

 minute fraction is simply absorbed or dissolved according to the 

 law of pressures (the Henry-Dalton law). The great mass is 

 in a state of combination with the haemoglobin, the connection 



