THE CHEMICAL COMPOSITION OF DYES 87 



cell are capable of being dyed to some extent by most dyes, if 

 sufficient time is allowed. 



For the purposes of microtechnique, dyes may be defined thus. 

 They are aromatic, salt-like, crystalline solids, that dissolve in 

 water or aqueous solutions in the form of ions ; either the cations 

 or the anions (occasionally both) are coloured; the coloured ions 

 can link themselves chemically with proteins (and generally with 

 other tissue-constituents as well); when the linkage takes place, 

 the ions do not lose colour, and usually they do not change it. 



Two questions present themselves. What causes dyes to be 

 coloured, and what causes their coloured ions to attach them- 

 selves to tissue-constituents ? The first question will be considered 

 here, the second in chapter 8. 



Many of the familiar colours of nature — the brilliant wings of 

 many butterflies, for instance, and the iridescent feathers of 

 birds — owe their colour to parallel plates or parallel striations, 

 separated by distances varying from about a quarter of a wave- 

 length to a few wave-lengths of visible light. These 'structural' 

 colours disappear when the substance is dissolved, because they 

 are not due to any colour intrinsic in the molecules or lesser 

 particles of the substance. It is only when such particles them- 

 selves show a differential transmission of light of diff'erent wave- 

 lengths, that a substance remains coloured on solution. 



If all substances that remain coloured on solution are examined, 

 it is found that most of them fall into a few major groups. It is a 

 familiar fact that the salts of chromium, iron, and cobalt are 

 coloured, and that colour is retained when they are dissolved. 

 These are only examples of the wide generalization that the ions 

 and ionic complexes formed by the transition elements are 

 coloured; or, to dig a little more deeply into causes, we may say 

 that the elements that give coloured ions are those metals that 

 possess an incomplete shell of electrons inside their outermost 

 shell. That is to say, the cause lies in intra-atomic structure. No 

 dye, however, owes its colour to the possession of a transition 

 element, nor indeed to the possession of any particular atom as 

 such: its colour is due to its inter-atomic, not its intra-atomic 

 structure. 



