CH. xxv.] THE PROTE1DS. 399 



large quantity of alkali) may be formed by adding strong potash 

 to undiluted white of egg. The resulting jelly is called Lieber- 

 kiihn's jelly. A similar jelly is formed by adding strong acetic 

 acid to undiluted egg-white. 



The halogens (chlorine, bromine, and iodine) also form albumin- 

 ates, and may be used for the precipitation of proteids. 



B. THE CONJUGATED PROTEIDS. 



These complex substances are compounds of albuminous sub- 

 stances with other organic materials, which are, as a rule, also 

 of complex nature. They may be divided into the following 

 groups : 



1. HAEMOGLOBIN and its allies. These are compounds of 

 proteid with an iron-containing pigment. All will be fully 

 discussed under Blood. 



2. GLUCO-PROTEIDS. These are compounds of proteid with 

 members of the carbohydrate group. This class includes the 

 nine-ins and substances allied to mucins called mucoids. 



Mucin. This is a widely distributed substance, occurring in 

 epithelial cells or shed out by them (mucus, mucous glands, 

 goblet cells), and in connective-tissue, where it forms the chief 

 constituent of the ground substance or intercellular material. 



There are several varieties of mucin, but all agree in the 

 following points : 



(a) Physical character. Viscid and tenacious. 



(b) Precipitability from solutions by acetic acid. They are 

 soluble in dilute alkalis, like lime water. 



(c) They are all compounds of a proteid with a carbohydrate 

 called animal gum, which by treatment with dilute mineral acid 

 can be hydrated into a reducing but non-fermentable sugar. 



The name animal gum is only a provisional name for the carbohydrate 

 radicle of the mucins. It is probable that the carbohydrate radicle may 

 differ in different mucins ; in some cases it is certainly the case that the 

 so-called sugar derived from it is not sugar, but a nitrogenous derivative of 

 sugar called glucosamine i.e., glucose in which HO is replaced by NH a 



The Mucoids differ from the mucins either in not being precipit- 

 able from alkaline solutions by acetic acid, or in being readily 

 soluble in excess of that acid. One of these (ovo-mucoid) is 

 found in white of egg, and others (pseudo-mucin and paramucin) 

 are occasionally found in dropsical effusions. 



Dr. Pavy has shown that a small quantity of a similar carbo- 

 hydrate can be split off from various other proteids, which wo 

 have already classified as simple proteids. 



