IMPORTANCE OF WATER 103 



tion with one another. The coarsely dispersed systems are mechan- 

 ical mixtures whose heterogeneity may be readily observed under 

 the microscope. Such systems are usually unstable. Their 

 dispersed phase is either precipitated or rises to the surface on 

 standing. With sufficient subdivision of particles, however, a 

 greater stability is obtained. As examples of such stable mixtures 

 are fine suspensions of clay, which do not settle for many days, or 

 common India ink, a suspension of minute carbon particles in 

 water. Sometimes, the suspended particles are so small that they 

 remain imperceptible under a common microscope. In such 

 cases one has to resort to a so-called ultra-microscope, under 

 which the object is examined not with transmitted light, but 

 in the dark field of vision with very strong lateral illumination. 

 The solid particles reflect the rays of light and become visible as 

 bright dots or circles against the dark field of vision, showing the 

 heterogeneity of the medium studied. The reflection of light rays 

 from the sun or from a projecting lantern makes visible the minute 

 dust particles floating in the air, which are invisible in transmitted 

 or weakly diffused light. 



Similar to suspensions are emulsions, or mixtures of two immis- 

 cible fluids, such as, for example, water and oil. Ordinary milk 

 is such an emulsion, in which particles of fat swimming in water 

 may be seen under the microscope. Artificial emulsions are readily 

 obtained by shaking vigorously two fluids that as a rule do not mix, 

 or by diluting with water an alcohol solution of a substance that is 

 insoluble in water. In this manner, Perrin prepared the emulsion 

 of mastic on which he carried out his famous investigations on 

 Brownian movements. The dispersion of the substance attains 

 its maximum in a true solution, in the solvent of which are dis- 

 tributed separate molecules and even ions of the dissolved sub- 

 stance. At this degree of dispersion the stability of the system is 

 very high, and the dissolved substance may be separated from the 

 solvent only with great difficulty. Such separations can be 

 obtained only by means of freezing, or by evaporation of the solu- 

 tion. The resistance to separation is expressed by the lowering 

 of the freezing point or' the raising of the boiling point of the 

 solution. 



Colloidal systems occupy an intermediate position between 

 coarse mixtures and true solutions. The degree of dissociation in 

 colloids is considerably greater than in coarse mixtures, and the 



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