WATER, ITS PROPERTIES AND FUNCTIONS 243 



electrolytes or non-electrolytes, which may raise or depress viscosity in the most 

 varied manner. A particularly striking instance of large changes in viscosity 

 produced by small changes in temperature is shown by such colloids as gelatines 

 which form gels, and also by those which coagulate on heating. As an illustra- 

 tion we may take the change in the viscosity of a dilute albumin sol when heated 

 (Fig. 68, from the paper by Wo. Ostwald). From 50' to 57 the viscosity 

 decreases regularly. At 57 '5, just before the appearance of turbidity, a large 

 increase occurs, which, at 60, gives place to an equally steep decrease. After 

 that, the curve forms practically a continuation of the direction of the first part 

 below 57, as if nothing had happened in the meantime. 



In the case of agar, the effect of concentration is very marked; from to 

 1 per cent, the viscosity in- 

 creases from that of water 



to several thousand times Coagulation of Albumin. 



this value. 



The general theory of the vis- 

 cosity of such two-phase systems 

 has been treated by Hatschek 

 (1910-1913). Certain conclusions 

 may be given here. Suppose the 

 particles themselves are unde- 

 formable, then the viscosity is 

 independent of their size and is a 

 linear function of the volume of 

 the dispersed phase only. The 

 matter is more complicated in the 

 case of two liquid phases, emul- 

 sions or emulsoid colloids, and 

 the change of shape due to the 

 shearing force must be taken into 

 account. With emulsoids above 

 a certain concentration, there is a 

 very rapid rise of viscosity with 

 further increase in concentration. 

 The particular concentration at 

 which this effect begins to show 

 itself varies with different colloids 

 and serves as a measure of their 

 "lyophilic" properties or affinities 

 for the solvent. With caseinogen 

 it begins at 5 per cent., with 

 glycogen at 25 per cent., with 

 india-rubber at 0'4-0 - 5 per cent. 

 The great swelling of india-rubber 

 in its solvents, before the hydrosol 

 is formed, is a familiar fact. It 

 will be clear that, in the investi- 

 gation of such systems, the rate 

 of shear is an important factor, 

 since on this depends the degree to which the deformed droplets are able to return to their 

 normal resting shape, spherical or polyhedral, according to the relative volume of the two 

 phases. Hatschek (1913) has improved the apparatus of Couette, in which this rate of shear 

 can be altered at will. It consists essentially of two concentric cylinders, the outer one of 

 which can be rotated at a desired rate, while the inner one is suspended by a wire. The 

 liquid is in the space between the two and the degree of torsion of the wire is measured by 

 the deflection of a beam of light reflected from a mirror attached to the cylinder. 



In this connection, some observations by Arisz (1913) are of interest. These experiments 

 were made to determine the fluidity, that is, the inverse or reciprocal of viscosity, as a 

 function of temperature in the case of the sol and gel of gelatine. It was found that a 

 continuous curve is given, so that there is no break at any point and the process is a uniform 

 one. The intensity of the Faraday effect and the elasticity were found to show similar 

 continuity. The method used in the case of the gel was to determine the viscosity by the 

 rate of change of shape under the action of a constant force. 



SUMMARY 



Water, of all substances known to us, is endowed with the most remarkable 

 combination of properties, all of which play a part in contributing to the import- 

 ance of its association with living processes. 



Temperature 



FIG. 68. CHANGES IN VISCOSITY OF ALBUMIN IN THE 

 PROCESS OF COAGULATION. 



Abscissae temperature. 



Ordinates logarithms of the time of flow through the capillary 



tube of the viscosimeter. 



(Wo. Ostwald, 1913.) 



