. PEOTEIDS. 



of sodium chloride, but the ash <>t' tin- proteids of muscle contains an 

 exceixs of potash salts and phosphates. The nature of the connection 

 of the ash with the proteid is still a matter of obscurity, and it is not 

 known whether they constitute an integral part of its molecule or are 

 merely adherent impurities. There is a certain amount of probability 

 that the latter is the case, inasmuch as an increasing number of pro- 

 teids have in recent times been obtained practically free from any 

 ash-residue on ignition. It is, however, possible that in their natural 

 condition as constituents of the animal tissues and fluids the proteids 

 are combined with salts, the separation of which we are now speaking 

 being an artificial result of the processes employed to effect that 

 separation. The sulphur in proteids is present partly in a stably 

 combined condition, partly loosely combined. The latter is removed 

 by boiling with alkalis, the former is not. The proportions of the 

 two differ in the several proteids. 1 



Proteids met with in the animal body are all amorphous, the 

 only apparent exception being haemoglobin : this substance is 

 however not a pure proteid but a compound of a proteid globin 

 with the less complex haematin. It is to the latter that the 

 power of crystallising is due. 



Some are soluble, some insoluble in water, some are character- 

 istically soluble in moderately concentrated solutions of neutral 

 salts, and all are for the most part insoluble in alcohol and ether ; 

 they are all soluble in strong acids and alkalis, but in becoming 

 dissolved mostly undergo decomposition. Their solutions exert 

 a left-handed rotatory action on the plane of polarisation, the 

 amount depending on various circumstances, and differing for the 

 several proteids. 



Crystals into whose composition certain proteid (globulin) elements 

 largely entered were long since observed in the aleurone-grains of 

 many seeds. 2 Similar crystalloid compounds are also described as 

 occurring occasionally in the egg-yolk of some animals (Amphibia and 

 Fishes). By appropriate methods they may be separated and re- 

 crystallized from their solution in distilled water, most readily by 

 Drechsel's method of alcohol dialysis. 8 The crystals consist in no 

 case of pure proteids, but are always compounds of the latter with 

 some inorganic residue such as lime or magnesia. These recrystal- 

 lized, and hence presumably pure, compounds have been frequently 

 analysed with a view to establishing a formula for proteids which 

 should give some clue to their molecular magnitude. An excellent 

 summary of the endeavours to arrive at a definite formula for proteids. 

 based on the above analyses and on those of haemoglobin and certain 

 compounds of egg-albumin with salts of copper and silver is given by 



1 A. Kriiger, Pfliiger's Arch. Bd. XLIII. (1888), S. 244. 



2 For literature down to the year 1877, see Weyl, Zt. f. physiol. Ch. Bd. r., S. 84. 

 See also Hoppe-Seyler's Handbuch, Ed. v. p. 259." Vines, Jl. of Physiol. Vol. in. 

 (1880), p. 102. Chittenden and Hartwell, .//. of Ph i/siol. Vol. xi. (1890), p. 435. 



3 .//. f. i>rakt. Cliem. N. F. Bd. xix. (1879), S. 331. 



