CHAP, ii.] RESPIRATION. 589 



crystals observed occasionally within the corpuscles with those obtained 

 in other ways shews that the haemoglobin as it exists in the corpuscle is 

 the same thing as that which is artificially prepared from blood, it is 

 evident that some peculiar relationship between the stroma and the 

 haemoglobin must, in natural blood, keep the latter from being dissolved 

 by the serum. Hence in preparing haemoglobin it is necessary first of 

 all to break up this connection and to set the haemoglobin free from 

 the corpuscles. This may be done by the addition of water, of ether, 

 of chloroform or of bile salts, or by repeatedly freezing and thawing ; 

 blood so treated becomes 'laky,' cf. 24. It is also of advantage 

 previously to remove, by the centrifuge, for instance, the alkaline 

 serum as much as possible so as to operate only on the red corpuscles. 

 The stroma and haemoglobin being thus separated, a solution of haemo- 

 globin is the result. The alkalinity of the solution, when present, being 

 reduced by the cautious addition of dilute acetic acid, and the solvent 

 power of the aqueous medium being diminished by the addition of one- 

 fourth its bulk of alcohol, the mixture, set aside in a temperature of 

 C. in order still further to reduce the solubility of the haemoglobin, 

 readily crystallizes, when the blood used is that of the dog, cat, horse, 

 rat, guinea-pig, &c. In the case of the dog indeed it is simply sufficient 

 to add ether carefully to the blood until it just becomes 'laky,' and then 

 to let it stand in a cool place; the mixture soon becomes a mass of 

 crystals. The crystals may be separated by filtration, re-dissolved in 

 water and re-crystallized. 



Haemoglobin from the blood of the rat, guinea-pig, squirrel, 

 hedgehog, horse, cat, dog, goose, and some other animals, crystal- 

 lizes readily, the crystals being generally slender four-sided prisms, 

 belonging to the rhombic system, and often appearing quite 

 acicular. The crystals from the blood of the guinea-pig are 

 octahedral, but also belong to the rhombic system ; those of the 

 squirrel are six-sided plates. The blood of the ox, sheep, rabbit, 

 pig, and man, crystallizes with difficulty. Why these differences 

 exist is not known. Not only the amount of water of crystalliza- 

 tion, but even the actual composition differs in the crystals obtained 

 from different animals. In the dog, the percentage composition of 

 the crystals has been given as C. 53'85, H. 7'32, N. 1617, 0. 21'84, 

 S. 0'39, Fe. '43, with 3 to 4 per cent, of water of crystallization. It 

 will thus be seen that haemoglobin contains, in addition to the 

 other elements usually present in proteid substances, a certain 

 amount of iron ; that is to say, the element iron is a distinct part 

 of the haemoglobin molecule : a fact which of itself renders haemo- 

 globin remarkable among the chemical substances present in the 

 animal body. 



345. The crystals, when seen in a sufficiently thick layer 

 under the microscope, have the same bright scarlet colour as 

 arterial blood has to the naked eye; when seen in a mass they 

 naturally appear darker. An aqueous solution of haemoglobin, 

 obtained by dissolving purified crystals in distilled water, has also 

 the same bright arterial colour. A tolerably dilute solution placed 



