328 PHYSICAL CHEMISTRY [CH. XXIV. 



blood. Starling gives the osmotic pressure of the proteins of the blood-plasma as 

 equal to 30 mm. of mercury. We should from the theoretical standpoint find it 

 difficult to imagine that a pure protein can exert more than a minimal osmotic 

 pressure. It is made up of such huge molecules that, even when the proteins are 

 present to the extent of 7 or 8 per cent. , as they are in blood-plasma, there are 

 comparatively few protein molecules present, and these are in a state of colloidal 

 solution, not true solution. Still, by means of this weak but constant pressure it is 

 possible to explain the fact that an isotonic or even a hypertonic solution of a 

 diffusible crystalloid may be completely absorbed from the peritoneal cavity into 

 the blood. 



The functional activity of the tissue elements is accompanied by the breaking 

 down of their protein constituents into such simple materials as urea (and its 

 precursors) sulphates and phosphates. These materials pass into the lymph, and 

 increase its molecular concentration and its osmotic pressure ; thus water is 

 attracted (to use the older way of putting it) from the blood to the lymph, and so 

 the volume of the lymph rises and its flow increases. On the other hand, as these 

 substances accumulate in the lymph they will in time attain there a greater concen- 

 tration than in the blood, and so they will diffuse towards the blood, by which they 

 are carried to the organs of excretion. 



But, again, we have a difficulty with the proteins ; they are most important for 

 the nutrition of the tissues, but they are practically indiffusible. We must 

 therefore assume that their presence in the lymph is due to filtration from the blood. 

 The plasma in the capillaries is under a somewhat higher pressure than the lymph 

 in the tissues, and this tends to squeeze the constituents of the blood, including 

 the proteins, through the capillary walls. I have, however, already indicated that 

 the question of lymph-formation is one of the many physiological problems which 

 await solution by the physiologists of the future. 



Waymouth Reid finds that absolutely pure proteins exert no osmotic pressure ; 

 the pressure observed is due to saline and other materials from which it is difficult to 

 disentangle the protein. * Haemoglobin is an exception to this rule ; it exerts a small 

 osmotic pressure and forms a true solution with water. 



Colloidal Solutions. The study of colloids is important, seeing how many 

 important physiological substances belong to this class ; for instance, the proteins, 

 starches, and soaps. Their main characters are, that they do not pass the 

 membrane of a dialyser, their solutions are opalescent, they crystallise with 

 difficulty if at all, they have a tendency to form jellies (as in the case of gelatin), 

 or to coagulate under the influence of heat and other agents (as in the case of 

 most proteins), and they exert a low osmotic pressure. Inorganic substances 

 (e.g., several metals, and compounds such as silicic acid) may also assume a 

 colloidal condition ; these are in an unstable physical condition, passing from the 

 "sol*' (or soluble) to the "gel" (or jelly-like) condition under slight provocation. 

 This confers upon them their power to act as catalysts. 



The solutions formed by colloidal materials are not true solutions, even 

 although the highest powers of the microscope reveal no visible particles. Never- 

 theless efficient filters made of gelatin will not allow these substances to pass 

 through them. Colloidal solutions also show what is known as the Tyndall 

 phenomenon ; that is, the particles, though invisible, will nevertheless scatter light, 

 just as minute dust particles in the air are lit up by a beam of sunlight. This 

 test forms the basis of the instrument known as the ultra-microscope. Such 

 observations show that colloidal solutions are really suspensions of extremely 

 minute particles. 



Reaction Velocity. Most reactions in Inorganic Chemistry take place 

 between electrolytes substances which are good conductors of the electric current. 

 These may be considered as reactions between ions, and Ionic Reactions occur at 



* Bayliss has shown that the saline constituents found in a native protein are 

 not mechanically mixed with it, and are also not in true chemical combination with 

 it, but are in a condition intermediate between these two extremes, to which the 

 term adsorption is applied. Many dyes used for staining fabrics and histological 

 preparations are also adsorbed, 



