258 CHEMISTRY OF THE PROTEIDS CHAP. 



flow of the electrical current, owing to their increased bulk and 

 diminished surface. 



5. They undergo hydrolytic dissociation, as in the case of arsenic 

 trisulphide (see below). 



6. They are rendered more colloidal and are readily made insoluble 

 by electrolytes the potent ion of which l has an electrical sign the 

 opposite to that carried by themselves, and they are made less colloidal 

 by the addition of ions of the same sign. 



7. Colloids of opposite electrical sign precipitate one another if 

 they are in equivalent amounts, but if either of the two colloids is 

 added in excess, then the colloidal precipitate, which was formed in 

 the first instance, may redissolve. 



8. One colloid in solution does not penetrate another colloid which 

 forms a rigid system, or, in other words, colloids do not pass through 

 animal or vegetable membranes. 



9. As a rule they do not crystallise readily. 



POLARISATION -PHENOMENA. To make polarisation of light the 

 criterion as to whether a substance is or is not a ' colloid ' is not 

 permissible for these reasons : when in Tyndall's experiment a beam 

 of light is passed through a solution, its track may either become 

 very evident owing to the partial reflection of the light, or the beam 

 is hardly visible. In the former case light appears polarised because 

 the mean wave-length of light visible to our eye has its straight course 

 interfered with by the presence of particles, each of which is at least 

 one-half, and may be many times the diameter of the mean wave- 

 length. 2 If we put the scale of light visible to our eyes as lying 

 between the Fraunhofer lines A and K, i.e. between A7606 and A3934 

 (see p. 479), then it must be admitted that wave-lengths below and 

 above these limits may also be polarised by particles of an appropriate 

 size, although these wave-lengths are not visible to us directly. It 

 follows, therefore, that when we call a substance a colloid, because it 

 shows a beam of transmitted light, we are interpreting physico- 

 chemical phenomena from a narrow point of view. Because we can 

 see particles of a certain size interfering with light visible to our eyes, 



1 The potency of an ion is determined by the degree to which its electro-affinity is 

 satisfied by the other ion with which it is linked together. If both ions have strong 

 electro-affinities, as in the case of potassium chloride, then neither ion can exert its 

 influence readily ; but if one of the ions is weak, as, for example, the C0 3 radical in 

 potassium carbonate, NagCOg, and the Hg-radical in corrosive sublimate, HgCl 2 , then the 

 stronger ion causes the hydrolysis of water, or may act on other substances of the 

 opposite electrical sign which are dissolved in the water along with itself. 



2 See Stokes (Phil. Trans. 1852, p. 463), Strutt [Lord Rayleigh] (ibid. 41. 107, 

 274, 447), and Lommel (Pogg. Ann, 131. 105). 



