COLLOIDS 547 



which, in turn, is inversely proportional to the square roots of the 

 masses of the diffusing bodies. 



^Measurements of osmotic pressure in solutions also give an accu- 

 rate measure of the relative masses of dispersed systems where such 

 measurements can be successfully carried out, and a great deal of 

 work has been devoted to attempts to measure the osmotic pressure 

 of colloidal solutions. Great difficulties both of experimentation and 

 of interpretation are encountered in this field. As will soon be seen 

 a colloid particle stands in a very complex relationship to its sur- 

 rounding liquid, and furthermore it is a matter of extreme difficulty 

 to obtain a colloid solution free from electrolytes, which themselves 

 may create osmotic pressure or otherwise affect the measurements. 

 About the only conclusion which it is safe to draw at the present time 

 is that if colloid solutions show osmotic pressure at all the value of 

 it is very small compared to that shown by crystalloidal solutions of 

 substances of more or less like formula weights. This leads to the 

 conclusion that the particles in a colloid solution are in a state of 

 dispersion far less than that found in a typical crystalloidal solution. 

 For a most excellent resume of the present state of our knowledge in 

 this field the reader is referred to a recent book by Dr. L. Casuto, 

 of Pisa, entitled "Der Kolloide Zustand der Materie" (Steinkopf, 

 1913). 



When the size of the disperse particles is sufficiently great they 

 may, of course, be measured under the microscope, and with the 

 advent of the ultramicroscope the limits of visibility of small bodies 

 has been very notably extended. The ultramicroscope is known in 

 several forms, the first having been devised by Siedentopf and Zsig- 

 mondy. All depend upon the production of powerful rays of light 

 in directions parallel to the surface of the microscope slide. In such 

 a field there will be no luminosity, provided the field is optically 

 empty, that is, contains no particles of sufficient size to produce a 

 dispersion of light. If, on the other hand, such particles are present, 

 the effect observed will be an illumination whose character will de- 

 pend upon the size of the particles. If the particles are of sufficient 

 size the illumination will show them individually as bright points 

 of dispersion, even though the particles are too small to be observed 

 of themselves, just as the stars are visible from the light which they 

 disperse, but cannot of themselves be seen.' If the particles are so 

 small that they are no longer able to disperse sufficient light to make 

 each particle appear as a bright point, there will, nevertheless, pro- 

 vided the particles are present in sufficient numbers, be produced a 

 diffuse luminosity throughout the field. These phenomena are 

 wholly analogous to those observed when a beam of light is passed 

 through a dark room in the atmosphere of which fine dust particles 

 are found. The path of the whole beam is made apparently uni- 

 formly luminous by the smaller particles, while occasionally there 



