22 A MANUAL OF PHYSIOLOGY 



thrown down. Add a little more of the alkali, and the precipitate is 

 redissolved. It can be again brought down by neutralizing with acid 



(/?) Heat a portion of the solution to boiling ; no precipitate is 

 formed. 



(y) Add strong nitric acid ; a precipitate appears, which dissolves 

 on heating, and the liquid becomes yellow. 



(b) Alkali-albumin. To a solution of egg-albumin add a little 

 sodium hydrate, and heat gently for a few minutes. Alkali- albumin 

 is produced. It can also be derived by similar treatment from any 

 albumin or globulin. 



(a) Neutralize, after colouring with litmus solution, by the addition 

 of dilute hydrochloric or acetic acid. Alkali-albumin is precipitated 

 when neutralization has been reached. It is redissolved in excess of 

 the acid. 



(ft) To another portion of the solution of alkali-albumin add a few 

 drops of sodium phosphate solution, then litmus, and then dilute acid 

 till the alkali-albumin is precipitated. More of the dilute acid should 

 now be required to precipitate the alkali-albumin, since the sodium 

 phosphate must first be changed into acid sodium phosphate. 



(y) On heating the solution of alkali-albumin there is no coagu- 

 lation. 



(3) Proteoses (Albumoses). For preparation and reactions, see 

 p. 377. They differ from group (i) in not being coagulated by heat, 

 and from group (2) in not being precipitated by neutralization. 

 They are soluble (with the exception of hetero- and dys-albumose), 

 in distilled water, and are not precipitated by saturation of their 

 solutions with magnesium sulphate or sodium chloride. Saturation 

 with ammonium sulphate precipitates them. With a solution of 

 commercial ' peptone,' which consists chiefly of albumoses, and 

 contains only a little true peptone, perform the following tests : 



(a) Boil the solution ; there is no coagulation. 



(/5) Biuret reaction, (3) p. 20. 



(y) Add to a little of the solution a drop of strong nitric acid by 

 means of a glass rod or small pipette ; a precipitate is formed, which 

 dissolves on heating, and reappears on cooling. 



(4) Peptones. For preparation and tests, see p. 378. They differ 

 from groups (i) and (2) in the same way as albumoses, and they 

 differ from albumoses in not being precipitated by ammonium 

 sulphate. Saturate the solution of commercial * peptone ' with 

 ammonium sulphate ; the albumoses are precipitated. Filter ; the 

 peptones are contained in the filtrate. On it perform the biuret 

 test, as described in (5), p. 21 ; and note that the pink colour is the 

 same as that given by albumoses. 



(5) Coagulated Proteids. These are divided into two classes : 



(a) Proteids coagulated by heat, such as boiled white of egg. 



(b) Proteids whose coagulation is determined by the action of 

 /erments. Of these, fibrin is a type. Both classes give such of the 

 general proteid tests, (i), (3), (4), p. 20, as with suitable modifica- 

 tions can be instituted on solid substances. Thus, in performing 

 (3), a flake of fibrin or a small piece of the boiled egg-white should 



