224 CARBON-DIOXIDE HAEMOGLOBIN. 



should first be freed from oxygen by a current of hydrogen and care 

 must be taken to neutralise the nitrous acid formed during the process. 



5. Carbon-dioxide haemoglobin. The possible union of 

 carbon-dioxide with haemoglobin has already been referred to (p. 222), 

 and more recent researches have thrown further, though still far from 

 complete light upon this possibility. There appears to be no doubt 

 that a solution of haemoglobin takes up a much larger volume of 

 carbon-dioxide than can be accounted for as the result of a merely 

 physical absorption. Thus in one set of experiments it was found 1 

 that 1 gr. of haemoglobin could unite with 2-366 c.c. of the gas at 

 a temperature of 18'4 and partial pressure of 31-98 mm. of Hg, 

 the latter being a mean average partial pressure of carbon-dioxide 

 in venous blood according to the older established data 2 , while that 

 in arterial blood is 2 1-28 mm. 3 . It is further stated that the stronger 

 solutions of haemoglobin absorb relatively less carbon-dioxide than the 

 weaker, and that, as in the case of oxy-haemoglobin (see p. 222) various 

 modifications of haemoglobin exist possessing different powers of 

 uniting with this gas. On comparing the amounts of carbon-dioxide 

 and of oxygen or CO or NO which may unite with a given weight of 

 haemoglobin it is at once evident that the mode of union of the 

 former gas must be different from that of the latter three, with 

 which, as already stated, haemoglobin unites molecule for molecule. 

 This difference in behaviour is very probably due to the decomposition 

 which haemoglobin undergoes when a current of carbon dioxide is 

 passed through- it 4 , and indeed it is hence probable that the so-called 

 carbon-dioxide haemoglobin is rather a compound of the -gas with 

 a coloured product of the decomposition of haemoglobin, viz. haemo- 

 chromogen, which has been shown by Hoppe-Seyler to unite with 

 carbon-monoxide (see below). The compound, whatever be its true 

 nature, is stated to exhibit a one-banded absorption spectrum closely 

 similar to that of haemoglobin, but the centre of the band lies slightly 

 more towards the violet end of the spectrum 5 . Bohr states that 



1 Bohr, see Beitrage z. Physiol, Ludwig, gewidmet, 1887, S. 164. Centralb. f. 

 Physiol. Bd. iv. (1890), S. 253. Skandinav. Arch. f. Physiol. Bd. HI. Hf. 1, 2 

 (1891), S. 47. See also Jolin, Arch. f. Physiol. Jahrg. 1889, Sn. 277, 285. 



3 See Wolffberg, Pfliiger's Arch. Bd. vi. (1872), S. 23. Strassburg, Ibid. S. 65. 

 Nussbaum, Ibid. Bd. vn. (1873), S. 296. 



3 But cf. Bohr, Centralb. f. Physiol. Bd. i. (1887), S. 293, n. (1888), S. 437, who 

 makes it much less. According to this observer the partial pressure of C0 2 in 

 blood is less than that of expired air, and that of oxygen is greater. If this should 

 prove to be the case on further investigation it would appear that the gaseous 

 interchange which takes place in the lungs cannot be the result of a purely diffusive 

 process, as it is now held to be ( 354 357). 



4 Torup (Swedish). See Abst. in Maly's Jahresb. 1887, S. 115. 



5 Torup, loc. cit. and see also "Ueber die Kohlensaurebindung des Blutes," 

 Kopenhagen, 1887. 



