THE PROTEIDS 21 



drops of sodium phosphate, colour with litmus, and neutralise as before. 

 Note that the alkali-albumin now requires more acid for its precipitation 

 than in A, the acid which is first added converting the sodium phosphate into 

 acid sodimn phosphate. 



Now remove C from the bath. Boil it. Again there is no coagulation, 

 the proteid having been converted into acid-albumin, or syntonin. After 

 cooling, colour with htmus and neutrahse with O'l -per- cent, alkali. At the 

 neutral point a precipitate is formed, soluble in excess of acid or alkali. 

 (Acid-albumin is formed more slowh' than alkaU-albiunin, so it is best to 

 leave this experiment to the last.) 



2. Take some gelatin and dissolve it in hot water. On cooling, the solu- 

 tion sets into a jeUy (gelatinisation) . 



Take a dilute solution of gelatin, and try all the proteid tests with it 

 enumerated on p. 19. Carefolly note down yoxu results. 



3. Add a few drops of acetic acid to some saliva. A stringy precipitate 

 of mucin is formed. 



4. A tendon has been soaked for a few days in Hme water. The fibres 

 are not dissolved, but they are loosened from one another owing to the solu- 

 tion of the interstitial or ground substance by the lime water. Take some 

 of the lime-water extract and add acetic acid. A precipitate of mucin is 

 obtained. The fibres themselves consist of collagen, which yields gelatin on 

 boUing. Vitreous humour or the Whartonian jelly of the umbilical cord is 

 much richer in ground substance than tendon, and, if treated in the same 

 way, a much larger yield of mucin is obtained. 



The Proteids are the most important substances that occnr in 

 animal and vegetable organisms ; none of the phenomena of life 

 occur \vithout their presence ; and though it is impossible to state 

 positively that they occur as such in living protoplasm, they are 

 invariably obtained by subjecting living structures to analytical 

 processes. 



Proteids are highly complex compounds of carbon, hydrogen, 

 oxygen, nitrogen, and sulphur occurring in a solid viscous condition 

 or in solution in nearly all parts of the body. The different members 

 of the gi'oup present differences in chemical and physical properties. 

 They all possess, however, certain common chemical reactions 

 and are united by a close genetic relationship. 



The various proteids differ a good deal in elementary composition. 

 Hoppe-Seyler gives the following percentages : — 



We are, however, not acquainted vnth the constitutional formula 

 of proteid substances. There have been many theories on the 

 subject, but practically all that is known with certainty is that many 

 different substances may be obtained by the decomposition of pro- 

 teids. How they are built up into the proteid molecule is unknown 



