34 DYNAMICS OF LIVING MATTER 



loids and colloids. The former diffuse easily through animal mem- 

 branes, the latter only with difficulty, or not at all. It is, however, 

 well to remember that there exist transitions between both groups. 

 According to Krafft,* the colloidal character of sodium soaps increases 

 I with the size of the acid molecule. Thus sodium acetate possesses 

 the qualities of a crystalloid in a watery solution, while sodium stearate 

 belongs to the colloids. The proteids, certain carbohydrates like 

 starch or glycogen, and higher fats, belong to the colloids; while their 

 products of cleavage, e.g. dextrose, may belong to the crystalloids. 

 According to Krafft, there is therefore a steady transition from the 

 crystalloids to the colloids-f 



In physical chemistry, as a rule, a different idea of the colloidal 

 solution is given, i.e. that they are no real solutions, but suspensions 

 of small particles in a liquid, or a system of two phases. We have 

 already mentioned the fact that not only organic substances may form 

 colloidal solutions, but also many inorganic substances, and even pure 

 metals, such as platinum, gold, silver, etc. All these colloidal sub- 

 stances alter the freezing point or boiling point of the liquid not at all 

 or but little. From this the conclusion is drawn that no work, or but 

 little, is required to separate the solvent from the dissolved colloidal 

 particles. It must, however, be stated that according to some authors 

 the proteins dissolved in the blood serum have a definite osmotic pres- 

 sure which is far from being a "quantite nSgligeable." Starling J found 

 by a direct measurement — the freezing-point determinations fail in 

 such cases — an osmotic pressure of the colloids of as much as 30 

 to 40 mm. of mercury. This pressure plays, according to Starling, 

 a definite and important r61e in phenomena of lymph formation, oedema, 

 etc. In view of Starling's observations it is doubtful whether we still 

 have a right to maintain that colloidal solutions behave like suspensions, 

 inasmuch as the latter differ from real solutions through the fact that 

 they possess no measurable osmotic pressure. § 



A second argument in favor of a principal difference between col- 

 loidal and crystalloidal solutions lies in the fact that dissolved particles 

 in a colloidal solution have, as a rule, a definite electrical charge. The 

 same is often found in the particles which form suspensions in water. 

 The existence of the electrical charge can be demonstrated if an 



* Krafft, Zeitsch. fur physiologische Chemie, Vol. 35, pp. 364 and 376, 1902. 



t This idea receives still further support from the fact that if a salt solution is exposed 

 to the action of a centrifuge, the concentration at the periphery becomes larger than at the 

 center. 



% Starling, /»«?•. of Physiology, Vol. 19, p. 312, 1895. 



§ More recently Reid has reached the conclusion that the colloids in Starling's experi- 

 ments were not free from salts, and that be in reality measured the osmotic pressure of the 

 latter. 



