CHEMICAL BASIS OF THE ANIMAL BODY. 217 



difficulty owing to its extreme solubility in water. The crystals may 

 be prepared by sealing up a concentrated aqueous solution of oxy- 

 haemoglobin in glass tubes from which, if necessary, all remaining air 

 is displaced by hydrogen : on prolonged standing all the oxygen dis- 

 appears during the putrefactive reduction which ensues, and finally, 

 more readily on exposure to a low temperature, crystals of haemoglobin 

 make their appearance in the fluid 1 . A similar production and forma- 

 tion of crystals is frequently observed when crystals of oxy-hsemoglobin 

 are sealed up with Canada balsam under a cover-slip and kept for some 

 time 2 . The form of the crystals obtained from the blood of different 

 animals has not yet been fully investigated. They exhibit to a marked 

 degree the phenomena of pleochroism, being apparently trichromatic 3 . 



Pleochroism is that property possessed by many crystals of appearing to differ 

 more or less in colour, in accordance with the direction from which they are viewed 

 by transmitted light. The phenomena are usually investigated by means of a single 

 Nicol prism. For further details consult some special work on mineralogy or the 

 article on this subject in the Encyclopaedia Britannica, Vol. xvi. p. 375. 



As ordinarily seen the crystals of haemoglobin have a dark red 

 appearance, unlike the bright scarlet of oxy-hsemoglobin, with a strong 

 purple or bluish tint. They are extremely soluble in water, much 

 more so than the crystals of oxy-hsemoglobin. The optical properties 

 of solutions of haemoglobin have already been sufficiently described 

 (346 and see below Fig. 36, No. 5). One of the most remarkable 

 properties of haemoglobin is its power of uniting directly with any one 

 of several gases, such as oxygen, carbon monoxide, nitric oxide and, as 

 recent research has shown, possibly carbon dioxide; the compounds 

 which are thus formed have in the case of the first three gases a 

 definite and constant composition, crystallising more or less readily in 

 characteristic forms and showing in aqueous solutions absorption 

 spectra which are constant and characteristic for each. (See below.) 



The chemical composition of haemoglobin does not as yet admit of 

 being represented by any definite formula, and indeed its percentage 

 composition has not been determined by direct analysis. It must be 

 inferred from a knowledge of the probable composition of the more 

 stable and easily crystallisable oxy-haemoglobin and of the quantitative 

 relationships which hold good between haemoglobin and oxygen during 

 its conversion into oxy-haemoglobin. As will be seen later on analysis 

 of purified crystals of oxy-haemoglobin show that these probably differ 

 in composition as prepared from the blood of different animals, and the 



1 Hiifner, Zt. f. physiol. Chem. Bd. iv. (1880), S. 382. Cf. Nencki u. Sieber, 

 Ber. d. d. chem. Gesell. Bd. xix. (1886), Sn. 129, 410. 



2 A. Ewald, Zt. /. BioL Bd. xxn. (1886), S. 459. 



3 A. Ewald, loc. cit. 



