APPENDIX. 



jical interest. The physiological function of any substance must depend 

 ultimately on its molecular (including its chemical) nature ; and though at present 

 our chemical knowledge of the constituents of an animal body gives us but little 

 insight into their physiological properties, it cannot be doubted that such chemi- 

 cal information as is attainable is a necessary preliminary to all physiological 

 study. 



PROTEIDS. 



These form the principal solids of the muscular, nervous, and glandular 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 



0. 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 



From 50.0 6.8 15.4 22.8 0.4 



to 55.0 to 7.3 to 18.2 to 24.1 to 5.0 



(Hoppe-Seyler. 1 ) 

 (Drechsel.) 



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 def- 

 initely 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 potas- 

 sium, 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 quan- 

 tities 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 pro- 

 teids 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 haemoglobin 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 alkalies, but in becoming dissolved mostly 

 undergo decomposition. Their solutions possess a left-handed rotatory action on the 

 plane of polarization, the amount depending on various circumstances, and being, 

 with one exception, 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 s'ubstances 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 

 color is, on the addition of ammonia, or caustic soda or potash, changed to a deep 

 orange hue. (Xanthoproteic reaction.) 



2. With Millon's reagent they give, when present in sufficient quantity, a pre- 

 cipitate, which turns red on heating. If they are only present in traces, no precip- 

 itate is obtained, but merely a red coloration of the solution. 



3. If mixed with some concentrated solution of sodic hydrate, and one or two 

 drops of a solution of cupric sulphate, a violet color is obtained, which deepens in 

 tint on boiling. 



The above serve to detect the smallest traces of all proteids. The two following 

 tests may be used when there is more than a trace present, but do not hold for 

 every kind of proteid. 



4. Render the fluid strongly acid with acetic or other acid, and add a few drops 

 of a solution of ferrocyanide of potassium ; a precipitate shows the presence of 

 proteids. 



5. Render the fluid, as before, strongly acid with acetic acid, add an equal volume 

 of a concentrated solution of sodic sulphate, and boil. A precipitate is formed if 

 proteids are present. 



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



2 See Gmelin, Hdb. Org. Chem., Bd. viii. S. 285. 



3 Vines, Journ. of Physiol., vol. iii. (1880), p. 93. 



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



