LIVING SUBSTANCE 105 



the size of the albumin molecule ; the latter is too large to pass 

 through the excessively fine pores of the membrane, while no 

 obstruction stands in the way of the small molecules of salt. This 

 property is of practical importance in the chemical investigation 

 of proteids, for by dialysis the proteids can always easily be sepa- 

 rated from the salts that may be present with them in solution. 



The fact that proteids and a host of other substances which 

 behave similarly do not diffuse through membranes, has led to the 

 idea that these bodies, in contrast to diffusible substances, dissolve 

 in water only apparently, and form no real solutions ; their appa- 

 rent solubility may be only a largely developed power of swelling. 

 Proteids in a dry state are, in fact, capable of taking up very large 

 quantities of water, and thereby gradually swelling. In 1861 

 Graham contrasted these bodies as colloid substances from 

 crystalloid substances ; and this distinction has been handed 

 down and been generally accepted. The colloids are said to be 

 capable of swelling only, not of crystallising ; the crystalloids, on 



FIG. 38. Crystals of haemoglobin. /, From man, //, from the guinea-pig, ///, from the squirrel. 



(After Kirkes.) 



the other hand, to be really soluble and capable of crystallisation. 

 But such a sharp distinction is scarcely admissible ; for in the 

 first place, proteids are known that can form genuine crystals, like 

 the above-mentioned proteids in squash-seeds, which occur wide- 

 spread in plant seeds as aleurone-grains, and like the haemoglobin 

 of the red blood-corpuscles. If, e.g., whipped blood from the guinea- 

 pig be shaken for a time with ether, by which the haemoglobin 

 is extracted from the substance of the red corpuscles and driven 

 out into the blood-serum, and a drop of this liquid be allowed 

 slowly to evaporate upon a glass slide, very delicate tetrahedral 

 crystals gradually separate (Fig. 38, //), which consist of pure 

 haemoglobin. In the second place, under the influence of certain 

 reagents, proteids can pass over into modifications that diffuse 

 through membranes, without losing in the process the chemical 

 characteristics of proteids. These modifications, which, e.g., proteids 

 undergo in the body under the influence of the digestive juices of 

 the stomach and the pancreas, are termed peptones : it is known 



