THE CONSTRUCTION OF PEOTEINS 163 



3. META-PROTEINS. These are insoluble in distilled 

 water or in solutions of neutral salts. They are readily 

 soluble in very dilute acids and alkalies, and their solutions 

 do not coagulate on boiling. They are precipitated by care-* 

 fully neutralising their solutions, and when they are boiled 

 in the resulting state of suspension they are converted into 

 coagulated protein, and will not re-dissolve on the addition 

 of either dilute acid or alkali. 



They are readily prepared from either albumins or 

 globulins by warming them in the presence of a little acid 

 or alkali, preferably at about 60 C. Alkali-albumin may 

 be prepared by acting on albumin with fairly strong caustic 

 potash in the cold. 



The meta-proteins are not of frequent occurrence in 

 plants, but may be met with in certain seeds. 



4. PROTEOSES or ALBUMOSES. These are generally 

 soluble in distilled water, though some are less so than 

 others. They can be precipitated from their solutions by 

 saturating the latter with neutral ammonium sulphate. 

 They differ from the members of the first two classes by 

 not being converted into coagulated protein on boiling. 

 Their characteristic reaction is that they give with nitric 

 acid, or with potassium ferrocyanide in the presence of acetic 

 acid, a precipitate which dissolves on warming the liquid 

 and reappears as it cools. Unlike any of those of the 

 preceding groups, they have the property of dialysing 

 through a parchment membrane, but only very slowly. 



5. PEPTONES. These are much like albumoses, but do 

 not give a precipitate with nitric acid or with potassium 

 ferrocyanide in the presence of acetic acid. They are not 

 precipitated by saturation of their solutions with ammo- 

 nium sulphate, nor are they coagulated on boiling. Their 

 power of dialysis is much greater than is that of the 

 proteoses. 



Neither peptones nor proteoses occur very plentifully 

 in plants, and they are probably formed in them only from 

 the decomposition of the more stable forms of globulin and 



