THE WATER OF CRYSTALLISATION, 



205 



of that moiety of the molecule on which its colour and its physiological func- 

 tion depends. At the same time, there is such a difference in the ratio of 

 S : Fe in the haemoglobin of certain animals as renders it highly probable, or 

 rather certain, that, in the haemoglobin of different animal groups, the albu- 

 minous moiety of the complex molecule differs. Such being the case, it is not 

 surprising that certain of the physical characters of haemoglobin, such as 

 crystalline form and solubility, should exhibit variations. 1 Nor can we lose 

 sight of the possibility, to which I have already drawn attention, that the 

 differences in the haemoglobins of certain animals may be due to their being 

 formed by the linking of the iron-containing molecule with different polymers 

 of the same albuminous group. The existence of haemoglobins varying some- 

 what in their percentage of iron renders this view highly probable. 



2. Quantity of water of crystallisation. Remarkable difficulties 

 encounter the observer in his attempts to determine the amount of 

 water of crystallisation of oxyha^moglobin, and considerable discrepancies 

 are to be noticed in the results obtained by different processes. 



In order to make the determination, pure oxyh&moglobin is dried 

 in vacuo at C., and after ceasing to lose weight under these conditions 

 it is heated to a temperature of 115 C. 



The following are some of the principal and most reliable results 

 obtained : 



According to Bohr, 2 the water of crystallisation of oxy haemoglobin may 

 vary in amount between 1*2 and 6 '3 per cent., but these results, like others 

 obtained by the same author, and to which reference has been made (see p. 

 192), are explicable by the fact that his preparations of haemoglobin did not 

 represent the pure substance, and contained products of decomposition. 



Without taking Bohr's results into consideration, there can be no 

 doubt that crystals of oxyhremoglobin of different animals exhibit 

 differences in the amount of water of crystallisation. Assuming the 

 above results to be correct, the highly soluble oxyhgemoglobiii of the 

 pig, which crystallises in rhombic prisms, possesses the same amount of 

 water of crystallisation as the very sparingly soluble oxyhamioglobin of 

 the guinea-pig, separating in the form of tetrahedra. 



3. Solubility. The difficulties which encounter the observer in 



1 The reader is referred to an admirable account of all the researches on the Crystal- 

 lography of Haemoglobin, up to the date of its publication (1871), to the chapter entitled 

 " Krystallformen des Blutroths," in Preyer's work, "Die Blutkry stalle. " Very fine 

 coloured engravings of the haemoglobin crystals of various animals amongst others, of 

 man, the guinea-pig, and the squirrel are to be seen in Funke's "Atlas of Physiological 

 Chemistry," being a Supplement to Lehmann's "Physiological Chemistry,' London, 

 printed for the Cavendish Society, 1853. See plate x. and pp. 15-17 of the appended 

 letterpress. 



2 "Exp. Untersuchungen it. die Sauerstoffaufnahme des Blutfarbstoffes, " Copenhagen, 

 1885. 



