70 PRINCIPLES OF GENERAL PHYSIOLOGY 



be in a state of non-dissociable chemical combination with the substance to which 

 the membrane is impermeable. 



If a surface which has adsorbed a particular substance be exposed to a solution 

 of another one which has a greater power of lowering surface energy than the first, 

 there is a more or less complete displacement of the less powerful one by the other. 



This is shown in an interesting way in the experiments of Schmidt-Xielsen (1910, p. :U2). 

 When rennet is shaken up in solution, it is more or less inactivated by adsorption on tho 

 surface of the froth produced. This inactivation is completely absent if a little saponin be 

 added, although the foam is even greater than before. Saponin, in fact, lowers surface 

 energy more than does rennet, hence it obtains possession of the surface. The same fact is 

 seen in the driving out of rennet from its adsorption by charcoal in the experiments of 

 Jahnson-Blohm (1912). Charcoal added to rennet prevents its action on milk (acting as an 

 anti-enzyme), but, if saponin be added to such an inactive mixture, it becomes active owing 

 to the driving off by the saponin of the rennet from its " combination with the antibody." 



This fact, that one substance can displace another from adsorption, is of importance with 

 respect to the turning out of oxygen from oxvhsemoglobin by exposure to carbon monoxide 

 (see Chapter XXL). 



DYEING AND STAINING 



A short account may be given here of the bearing that the facts of the present 

 chapter have on the nature of the processes involved in the dyeing of fabrics, and 

 in the similar art of staining histological preparations, as no further opportunity 

 will present itself. 



Much controversy has taken place between advocates of chemical and physical 

 theories. It may be taken as established that a physical theory, based only on 

 coefficients of partition, due to greater solubility of dyes in the tissues than in the 

 staining solution, is inadequate. On the other hand, many facts have been 

 mentioned in the preceding pages which indicate the important part that surface 

 action, or adsorption, must play, as well as the probability that chemical reaction 

 may, in many cases, follow it, although adsorption is the controlling factor. 



Some additional points may be recorded here. 



Weber (1894) finds that the amount of dye taken up by cellulose is in proportion to the 

 extent of surface presented by the latter. Precipitated cellulose takes up more than does an 

 equal weight of compressed paper. Dinitrocellulose, freshly precipitated, adsorbs in about the 

 same degree as ordinary cellulose ; but in the form of a coherent film little or none is 

 taken up. 



In discussions on the subject of staining, the use of the names " basic " and 

 " acidic " is liable to lead to some misconception. With one or two exceptions, 

 all dyes are neutral salts ; the distinction is that the so called " basic " dyes are 

 salts of an organic coloured base with an inorganic acid, usually h3 T drochloric, 

 although sometimes salts with acetic acid are met with. The "acid" dyes, on the 

 other hand, are salts of a coloured organic acid with an inorganic base, usually 

 sodium. 



Bearing this fact in mind, it is clear that, if a " basic " dye stains a parti- 

 cular cell constituent, it does not directly follow that this constituent is an acid. 

 If such, it must be a stronger acid than that combined with the colour base 

 of the dye, usually hydrochloric. Double decomposition may occur, of course, 

 if the cell constituent in question is a salt. This will be more complete the 

 less soluble the compound between dye and tissue is. Similar statements 

 apply, mutatis mutandis, to " acidic " dyes. Since most of the staining bodies 

 in cells are colloids and with negative charges, it is easy to understand why 

 electro- positive dyes, such as many of the " basic " ones are, should be adsorbed. 

 It is also suggestive that haemoglobin, one of the few electro-positive colloids 

 of the organism (Iscovesco, 1906), takes up "acid" dyes, such as eosin and 

 acid fuchsin. Moreover, when the dye salts are electrolytically dissociated, 

 as in most cases, the positive ion is the coloured one in the "basic" dyes, and 

 will be taken up by negative surfaces, while the negative ion of the "acid" 

 dyes will be taken up by the positive surfaces. The " basic " dyes are frequently 

 hydrolytically dissociated, with formation of electro-positive free bases in the 

 colloidal state. 



