1294 METHAEMOGLOBIN. 



doubt that a solution of haemoglobin takes up a 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 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 press- 

 ure of carbon-dioxide in venous blood according to the older 

 established data, while that in arterial blood is 21-28 mm. It 

 is farther stated that the stronger solutions of haemoglobin absorb 

 relatively less carbon-dioxide than the weaker, and that, as in 

 the case of oxy-haemoglobin 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 decom- 

 position which haemoglobin undergoes when a current of carbon- 

 dioxide is passed through it, and indeed it is hence probable 

 that the so-called carbon-dioxide haemoglobin is rather a com- 

 pound of the gas with a coloured product of the decomposition 

 of haemoglobin, viz. haemochromogen, which has been shewn 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. 



6. Methaemoglobin. When blood or solutions of haemoglobin 

 which have been exposed to the air for some time are examined 

 with the spectroscope they are frequently found to exhibit, in 

 addition to the more or less persistent absorption bands of oxy- 

 hemoglobin, a marked, band of absorption between C and J), 

 closely resembling but differing slightly in position from the 

 band which haematin shews in acid solution (see Fig. 90, No. 4). 

 This band may also frequently be observed in many patholog- 

 ical fluids, such as those from ovarial cysts, etc., which are 

 coloured by blood, and in urine when similarly coloured. The 

 substance to which the band is due is known as methaemo- 

 globin. It may be readily prepared in the laboratory by the 

 action of many reagents such as acids or alkalis, or more con- 

 veniently of certain salts, on solutions of oxy-haemoglobin. Of 

 these salts those which may perhaps on the whole be most 

 advantageously employed to obtain the spectrum of methae- 

 moglobin are nitrites, potassium ferricyanide, or potassium 

 permanganate. With the two latter salts the spectrum of 

 methaemoglobin may be obtained as follows. To 10 c.c. of 



