446 CHEMICAL DISCOVERY AND INVENTION 



the dangerously poisonous effects of breathing an atmosphere 

 containing even a small quantity of that gas. 



Oxyhaemoglobin is crystallisable, and the form of the crystals 

 differs in the blood of different animals. Haemoglobin and its 

 oxidised form both consist of a compound of a protein and a 

 coloured substance called haematin, which is said to have the for- 

 mula C 32 H 30 N 4 Fe0 4 . If blood is treated with acids the iron is 

 removed from the red colouring matter, and a new substance 

 called haematoporphyrin C 16 H 18 N 2 3 results. The views of the 

 experts engaged in this interesting problem being still unsettled 

 it is not advisable to attempt in these pages a display of the 

 constitutional formulas attributed to these compounds. The 

 only point which appears clearly established is the identity of 

 the ultimate pyrrolic products of vigorous oxidation or reduction 

 obtained from the green matter of the leaf, and the red colouring 

 matter of blood. This close similarity of chemical structure has 

 led to speculations as to the changes which must have come 

 about in the early stages of organic evolution. It may be sup- 

 posed that the common colouring matter prevailing in the 

 protozoa as well as all the early algae and other organisms living 

 in water was a substance essentially the same as the chlorophyll 

 now prevalent. It must therefore be inferred that at some time, 

 when worms or other creatures of distinctly animal character- 

 istics began to appear, the provision of much iron in the soil or 

 water inhabited by them led to a modification in the composition 

 of the protoplasmic material of their tissues, and the change 

 may have been further promoted by the exclusion of light from 

 their muddy or earthy habitations. Whatever may have been the 

 change of conditions which led to the change of composition 

 from magnesian chlorophyll to ferruginous haematin the retention 

 of the same fundamental atomic structure in the molecules of 

 these two substances now so widely separated in function as 

 well as in colour can only be regarded as strongly indicative of a 

 common origin. No other colouring matter found in animal 

 matters has the same importance and interest as haemoglobin, 

 and in nearly all cases our knowledge of their composition and 

 properties amounts to very little. The plumage of many birds, 

 the hair of man and many animals, and the skin of large sections 

 of the human race contain a dark or black pigment, but it may 

 be safely said that nothing important is known of the composi- 

 tion of this substance or mixture of substances, or whether, for 



