164 



Dr. Thomas Thomson on 



[March. 



decomposed without yielding any ammonia, from which we 

 may conclude that it contains no azote. It dissolves readily 

 in water and alcohol, but not in ether. Acid gives it a 

 yellow tinge. Hence the reason why bitartrate of potash 

 must be added when we wish to dye scarlet with cochineal 

 alone. If we agitate newly precipitated alumina in an 

 aqueous solution of cochineal, the colouring matter com- 

 bines with the alumina, and gives it a fine red colour. The 

 paint called carmine is alumina impregnated with cochinealin.* 



16. Black Ground and White. 



The method of fixing the colouring matter in this case is 

 precisely similar to the two last examples : the sole diffe- 

 rence lies in the dye-stuff used. The calico is in the first 

 place impregnated with the aluminous mordant. The mor- 

 dant is afterwards discharged by means of citric acid from 

 those parts of the cloth that are to remain white. It is 

 then dyed to saturation in logwood. 



Logwood is the wood of the hematoxylon campeschianum, 

 a tree which grows to a considerable size in Jamaica, and 

 on the eastern shore of the Bay of Campeachy. It owes 

 its dyeing powers to a colouring matter which it contains, 

 to which the name of hematin has been given. If we digest 

 the raspings of logwood in warm water, evaporate the 

 infusion to dryness, dissolve the residue in alcohol, and 

 distill off the alcohol to a syrup, and set the syrup aside, 

 crystals of hematin are deposited. They are needles 

 arranged in sphericles, and having a fine scarlet colour. 

 Hematin is but little soluble in water ; but it dissolves in 



* This is the name given hy chemists to the colouring matter of cochineal. 



