86 PROPERTIES OF CERTAIN AMINO-ACIDS. [CH. IV. 



(xxi.) Grind the precipitate with a litre of water, transfer to a 

 flask and decompose by means of sulphuretted hydrogen 

 gas. As the precipitate is somewhat difficult to decom- 

 pose it is necessary to pass the gas for 8 to 10 hours. It 

 must not be assumed the decomposition is complete as 

 soon as the precipitate has blackened. On no account 

 must the solution be heated. 



(xxii.) Filter off the mercuric sulphide, wash it with small 

 quantities of hot water, adding the washings to the bulk 

 of the fluid. 



(xxiii.) Concentrate the solution to a syrup in an evaporating 

 basin on a boiling water bath. 



(xxiv.) Whilst still hot, add boiling 97 per cent, alcohol, with 

 continuous stirring, until there is a faint permanent 

 turbidity. Allow to stand over-night. 



(xxv.) Filter off the crystals of histidine hydrochloride, 

 C 6 H 9 N 3 2 ,HC1,H 2 0. 



(xxvi.) Repeat (xxiii.) to (xxv/) to obtain a second crop. 



Recrystallisation. 



(i.) Dissolve the crystals in twelve times their weight of 65 per 

 cent, alcohol, by heating on a boiling water bath under a 

 reflux condenser. 



(ii.) Add a little charcoal and boil again. 



(iii.) Filter through a pleated paper on a hot water funnel, and 

 allow the filtrate to cool. The histidine hydrochloride 

 separates in the form of beautiful white glistening plates. 



Yield: 6 to 8 grams. 



Properties o! histidine. Histidine is readily soluble in 

 water, but very slightly soluble in alcohol.- The aqueous 

 solution has an alkaline reaction. It crystallises from 

 alcohol in platelets, which melt with decomposition at 

 about 253 C. 



The most convenient salt for the isolation of histidine 

 is the monohydrochloride. It is readily soluble in water. 



