16 LABORATORY WORK IN PHYSIOLOGICAL CHEMISTRY. 



glass rod moistened with HC1 at the mouth of the test-tube. 

 What is the reaction which takes place? 



(6) Warm together carefully in a dry test-tube a few 

 particles of the dry substance and a 3-4 mm. cube of metallic 

 sodium. (Caution: Do not place the tube in the flame. 

 Gases are evolved which are explosive unless the temperature 

 is kept moderate.) When the fusion' is complete and the 

 tube has cooled somewhat plunge the end into a small amount 

 of water placed in a suitable vessel, preferably a conical glass. 

 The glass of the test-tube will probably break, and unless the 

 sodium has been completely fused a slight explosion will 

 result. When the water has thoroughly impregnated the 

 fused mass, filter and to the filtrate add a few drops of ferric 

 chloride and of ferrous sulphate solution. Upon acidifying 

 with HC1 a blue precipitate of Prussian blue is obtained. 

 Write all the reactions which take place in this manipulation. 



Sulphur. The sulphur in the protein molecule exists in 

 part in such a form that upon boiling with caustic soda it is 

 easily split off as sodium sulphide. This in the presence of 

 a lead salt forms black lead sulphide. On this account such 

 sulphur has assumed the name of loosely combined or lead- 

 blackening sulphur. If the quantity of sulphur obtained in 

 the above manner be multiplied by |, the product is the 

 amount of sulphur considered to be present in the cystine 

 nuclei of the given protein. In some proteins keratin, 

 serum albumin, serum globulin the total sulphur and cystine 

 sulphur content are the same and we are justified in assum- 

 ing that all the sulphur in the molecule exists in the form of 

 cystine. In other proteins only | of the total sulphur is to 

 be accounted for as cystine S; hence it must be concluded 

 that sulphur exists in some atomic complex which yields, upon 

 the decomposition of the molecule, substances like a-thio- 

 lactic acid, mercaptans, ethyl sulphide, etc. Sulphur of this 

 type may be styled, in contradistinction, -firmly combined 

 sulphur. The sum of the loosely and firmly combined sul- 

 phur equals the total sulphur of the substance. 



