172 DYEING 



blue WS, and light green — are preferable for this purpose. Acid 

 fuchsine and light green are also available as background dyes. 



HAEMATEIN 



This single acid dye has no relative that is commonly used in 

 microtechnique, but it is so important that rather a full description 

 is necessary. Indeed, it is probably the most useful of all dyes in 

 microtechnique. It is used with intermediaries (mordants) between 

 it and the tissues (p. 207). The importance of haematein derives 

 from the variety of different objects that can be dyed by it, the 

 ease with which dyeing can be controlled, the insolubility of the 

 colour in neutral aqueous and alcoholic media after dyeing, and 

 the possibility of obtaining a jet black that is permanent in Canada 

 balsam (such a black being very convenient in photomicrography). 



Haematoxylon campechianum Linn, is a small, spreading tree 

 with crooked branches (fig. 21, a), thorny when young and re- 

 markable for the gnarled appearance of the stem of old specimens 

 (fig. 21, c). The stem attains a circumference of about two feet. It 

 is a leguminous plant, belonging with the tamarind to the group 

 Caesalpiniaceae, in which the sepals are nearly or quite separate, 

 instead of being fused as in the pea and its relatives. The pods and 

 heart-shaped leaflets are shown in fig. 21, B. For a fuller botanical 

 description see Bentley and Trimen.^^ 



The sap-wood is white, but the heart- wood red. The latter is 



". . . what Campeachy's disputable shore 

 Copious affords to tinge the thirsty web." 



(From Dyers' Fleece, quoted by Bancroft.^'^) 



The dyeing property of the heart- wood was known to the natives 

 of Campeche before the arrival of Europeans. The Spaniards 

 brought the wood to Europe soon after the discovery of America. 

 It seems to have been brought to England early in the reign of 

 Queen Elizabeth. Strangely enough, the dye was thought to fade 

 and a law prohibiting its use, under severe penalties, was in force 

 for nearly a century.*^ Its virtues were eventually recognized, 

 however, and the tree was introduced in 171 5 into Jamaica, where it 

 is still cultivated at the present day ; for haematein still survives the 

 severe competition of modern synthetic products in the dyeing of 

 black on wool, silk, leather, and nylon. Extracts of logw^ood were 

 first used in microtechnique in the eighteen-forties.^^^ 



