112 CYTOLOGICAL TECHNIQUE 



Restrainers such as ethanol and glycerol are particularly useful in 

 the dyeing of whole mounts, but they sometimes serve a useful 

 purpose with sections also, especially when dyes are used 

 progressively. 



The various tissue-constituents mentioned on p. 101 are 

 coloured by purpural exactly as though it were a basic dye (p. 90). 

 There is, however, this difference, that whereas basic dyes can be 

 washed out of the tissues by neutral alcoholic solutions (50% or 

 70% ethanol, for instance), mordanted purpurine cannot. 



Other soluble salts of aluminium (the chloride, for instance) 

 may be substituted for the sulphate: the action is again that of a 

 basic dye. If, however, sodium sulphate is used instead of the 

 aluminium salt, there is no mordanting and purpurine acts as a 

 very weak acid dye. Thus it is the cation that mordants. 



It will be remembered that metallic aluminium has three elec- 

 trons in its outer 'shell' (the third or 'M' shell). In its salts these 

 are lost to the anion, and the cation is left with the same orbital 

 electrons as the inert gas neon. The cation has, however, the 

 ability to accept electrons from suitable donor atoms, and in the 

 presence of water it accepts no fewer than 12, two from each of 

 6 oxygens in water molecules. In solution the cation consists of 

 aluminium in the centre, with 6 molecules of water surrounding 

 j^- 45, 173 jj^g 5 dative covalency bonds are arranged in opposite 

 pairs, each pair being at right-angles to the other two pairs. Each 

 valency bond holds a molecule of water (fig. 6, left). 



It may be wondered why exactly 6 bonds are formed, involving 

 the donation by water of 12 electrons. This is controlled partly by 

 the geometrical fact that 6 water molecules fit round a central 

 atom or ion more easily than (say) 5 or 7, but there is also another 

 cause. The reader may recall that the M shell will hold a total of 

 18 electrons, but these cannot all be accepted when the atomic 

 nucleus is that of aluminium. Twelve have the faculty of 'hybrid- 

 izing' with one another, without distinction as to whether they 

 belong to the s,p, or c^orbitals of the shell; and these 12 give the 

 dative covalency of six. 



The complex water/aluminium ion, having 3 electrons less than 

 the total number required to neutralize the protons of its atomic 



