iv THE BLOOD: FORMED CONSTITUENTS 109 



with sulphuric acid, the haematin loses its iron and takes up water, 

 turning into haematoporphyrin, an iron -free pigment somewhat 

 resembling haemoglobin in colour. 



Another iron -free derivative of haemoglobin, which forms 

 spontaneously in a crystalline form in the corpora hi tea and in 

 old haeinorrhagic foci, is haematoidin (Virchow), now regarded by 

 chemists as identical with bilirubin, one of the principal bile- 

 pigments (Fig. 34). 



It seems clear that all the colouring matters of bile and urine 

 are derived from successive transformations of blood pigment; but 

 with the exception of bilirubin, which forms spontaneously, only 

 one of the urinary pigments, urobilin, has at present been produced 

 artificially from haemoglobin or haeuiatiu. 



Many of the pigment substances above recorded, haemoglobin, 

 oxyhaemoglobin, carboxyhaenioglobin, methaemoglobin, haemochro- 

 mogen, haematin, haematoporphyrin, urobilin, possess the important 

 property, when examined in layers of known thickness and con- 

 centration, of absorbing well-determined and distinct zones of the 

 spectrum in aqueous solutions, acid or alkaline, as shown in Fig. 35. 

 It is important to note that while haemoglobin shows a single 

 absorption band between the Fraimhofer D- and E - lines, 

 oxyhaemoglobin and carboxyhaenioglobin show two bands that 

 almost coincide in the two cases, lying practically within the 

 same region of the spectrum. Apart from the different tint 

 exhibited by oxy- and carboxyhaenioglobin, the former being the 

 pinker, they can, however, readily be distinguished by adding a 

 reducing substance, e.g. carbon disulphide, to the two solutions, 

 when the spectrum of oxyhaemoglobin is speedily transformed 

 into that of haemoglobin, while the spectrum of carboxyhaenio- 

 globin undergoes no modification. 



To determine the relative quantity of haemoglobin contained in a given 

 quantity of blood, several instruments have been adopted. The simplest and 

 most convenient Haemoglobinometer is Gowers' apparatus, provided with a 

 standard solution of CO-haernoglobin. The method, as accurately described 

 by Haldane, 1 gives extremely good results. 



For more accurate quantitative determination, either of the haemoglobin 

 or of the pigments derived from it, the Spectro-Photometric Method must 

 lie employed. 



This method is based on the law that the coefficient of extinction of any 

 coloured solution is (for any given zone of the spectrum) directly proportional 

 in its concentration, i.e. C : E = C' : E', when C and C' indicate the correspond- 

 ing coefficient of extinction By coefficient of extinction of a fluid is meant 

 the negative logarithm of that intensity of light which remains after it has 

 traversed a liquid stratum of the depth of 1 c.c. (Kriiss, Kolorimetrie n. quantit. 

 Spectralanalyse, 1891). 



From the above equation it follows that -n--:^-; this ratio, known as 

 that of absorption, is a constant for the same colouring substance. Xow, if 



1 Junr/1. of Ptiysiol. xxvi. 497. 



