286 PHYSICAL PROPERTIES 



determined by the rate at which the particles are penetrated 

 and wetted by the solvent, the process of chemical reaction 

 between the alkali and the casein taking place at a relatively 

 great velocity. 



It is, perhaps, not surprising that the factor which determines 

 the rate of solution of casein should be the velocity with which 

 it is wetted by the solvent, while that which determines the rate 

 of solution for a crystalloid is the velocity with which the dis- 

 solved substance diffuses out of a thin layer of saturated solu- 

 tion in immediate contact with the surfaces of the crystals. A 

 crystal is only wetted upon its external surface, and the wetting, 

 naturally, takes place instantly. A particle of casein (or, in 

 general, of the solid phase of any colloid) is, however, comparable 

 in structure with a sponge; the surface which may be wetted by 

 the solvent is, per unit volume, very much larger than that of 

 a crystal, and the solvent must, in wetting this surface, traverse 

 a relatively immense network of minute capillary pores. Under 

 such conditions the time occupied in wetting the surfaces of the 

 particles may well be great compared with the time required for 

 the dissolved substance to diffuse from these surfaces into the 

 solvent, or with the time required for the accomplishment of 

 the union between the protein and the alkali in the solvent. 



As might be expected, similar relationships are encountered in 

 the extraction of a protein from desiccated and finely divided 

 fragments of tissue. The rate of extraction of the protamin 

 salmin by dilute hydrochloric acid from the dried spermatozoa 

 of the salmon is determined primarily by capillary forces (87) 

 (88). The accompanying chemical phenomena (decomposition 

 of compounds of salmin within the tissue, formation of salmin 

 hydrochloride, etc.) occur at a relatively very great velocity and 

 hence do not affect the rate of extraction. 



The rate of extraction or passage of a protein from a colloidal 

 phase into a surrounding solvent may be determined either by 

 the rate of passage of the soluble protein compound from within 

 the colloid particles into the surrounding solvent, or by the rate 

 of penetration of the solvent into the colloid particles, or by 

 both of these processes. That the latter process, namely the 

 penetration of the colloid, is of very great importance in deter- 

 mining the observed time-relations may be inferred from the 

 fact that the absorption of acid from dilute acid solutions by 



