APP.] OF THE ANIMAL BODY. 701 



PBOTEIDS. 



These form the principal solids of the muscular, nervous, and gland- 

 ular tissues, of the serum of blood, of serous fluids, and of lymph. In 

 a healthy condition, sweat, tears, bile and urine contain mere traces, if 

 any, of proteids. Their general percentage composition may be taken as 



O. H. N. C. S. 



From 20-9 6-9 15-2 51-5 0-3 



to 23-5 to 7-3 to 17-0 to 54-5 to 2-0 



(Hoppe-Seyler l . ) 



These figures are obtained from a consideration of numerous analyses, slight 

 differences in the various results being immaterial, where the purity of the substance 

 operated upon cannot be definitely determined. 



In addition to the above constituents, proteids leave on ignition a variable 

 quantity of ash. In the case of egg-albumin the principal constituents of the ash 

 are chlorides of sodium and potassium, the latter greatly exceeding the former in 

 amount. The remainder consists of sodium and potassium, in combination with 

 phosphoric, sulphuric, and carbonic acids, and very small quantities of calcium, 

 magnesium and iron, in union with the same acids. There is also a trace of silica 2 . 

 The ash of serum-albumin contains an excess of sodium chloride, but the ash of the 

 proteids of muscle contains an excess of potash salts and phosphates. The nature 

 of the connection of the ash with the proteid is still a matter of obscurity. Globin 

 from hasmoglobin is said to leave no ash on ignition. 



Proteids as met with in the animal body are all amorphous ; some 

 are soluble, some insoluble in water, 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 

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

 amount depending on various circumstances, and being, with one ex- 

 ception, viz. peptones, changed by heating. 



Crystals into whose composition certain proteid (especially globulin) 3 elements 

 enter were long since observed in the seeds of many plants; as yet they have not 

 been obtained sufficiently isolated or in quantities large enough to permit any 

 accurate analysis to be made. A method of isolating in quantity and recrystallizing 

 these substances has however 4 been indicated, and it seems probable that analysis 

 of these may lead to interesting information on the subject of the constitution and 

 combinations of proteids. 



The presence of proteids may be determined by the following tests. 



1. Heated with strong nitric acid, they or their solutions turn 

 yellow, and this colour is, on the addition of ammonia, or caustic soda 

 or potash, changed to a deep orange hue. (Xanthoproteic reaction.) 



1 Hdb. Phys. Path. Chem. Anal., Ed. rv. (1875) S. 223. 



2 See Gmelin, Hdb. Org. Chem., Bd. vin. S. 235. 



3 Vines, Jl. of Physiol. Vol. in. (1880) p. 93. 



4 Drechsel, Journ. f. prakt. Chem., N. F. Bd. xix. (1879) S. 331. 



