COAL-TAR 65 



But these are hard to see on a microscope slide where 

 they are mixed up with all sorts of similar cells and 

 tissues and may be quite invisible. It was fortunately 

 found that the aniline dyes were useful in bringing out 

 the various substances, for some would be stained with a 

 particular colour while other things on the slide were 

 unaffected. Those of you who have tried home dyeing 

 will have found that in a piece of cloth composed of 

 mixed cotton and wool, the dye is apt to attach itself 

 to one kind of thread and leave the other untinted. 



One day Dr. Koch was being shown through the 

 Breslau laboratories, and as he passed a table where a 

 young student was busily engaged in staining micro- 

 scope slides, he was told: "This is our little Ehrlich. 

 He is a first-class stainer of tissues, but he will never 

 pass his examinations." In fact, he never did, but his 

 "staining of tissues" led to the new science of chemo- 

 therapy which has given remedies for diseases hitherto 

 incurable. He found first that fuchsine, a familiar red 

 dye, would stain the tubercle bacilli so that they could 

 be seen on a miscroscope slide. Later he found that these 

 stains would act even in the living cell. He discovered 

 that methylene blue, a common colouring matter, 

 \vould seek out and destroy the parasite that causes the 

 quartan type of malarial fever. With this as a clue he 

 set about making molecules that would not only search 

 out and attach themselves to the pernicious parasite, 

 but carry along a dose of poison. For instance salvar- 

 san, otherwise known as "606," or as it has been re- 

 christened in America since the war, arsphenamine, con- 

 sists of two aniline rings with arsenic atoms attached. 



