COLLOIDAL MIXTURES. 275 



under others; the same is true of blue colloidal gold, which can be 

 produced in a variety of ways, for example, by, the reduction of gold 

 chloride solution by hydrazine. In most cases, however, the colloidal 

 particles can not be seen even under the best conditions ; they are, there- 

 fore, smaller than one seventh of a micron (1/7000 mm.), which is 

 about the limit of microscopic visibility. It will be of interest to 

 determine whether they can not be detected in many other cases with 

 the help of the new Zeiss microscope, which, by employing quartz 

 lenses and ultraviolet light (having a much shorter wave-length than 

 ordinary light) and obtaining the image photographically, extends the 

 limit of visible diameters to about one half of its present value. With 

 such an ' ultramicroscope ' a German investigator, Eaehlmann, has 

 already observed the suspended particles in an albumen solution. By 

 the optical method of Sidentopf and Zsigmondy, in which the colloidal 

 mixture is intensely illuminated by a thin beam of light, and the dif- 

 fused light reflected from the suspended particles at right angles to the 

 beam is viewed with a powerful microscope, the presence of still smaller 

 particles having a diameter of 1/100 micron has been detected in red- 

 gold suspensions and in other colloidal mixtures. 



A strong indication of the heterogeneity of colloidal suspensions is 

 furnished also by the familiar optical phenomenon, which is often 

 called the Tyndall effect, and is observed when a beam of light is passed 

 through any medium containing particles in suspension. The beam 

 becomes visible, as does a sunbeam in dusty air, owing to a diffuse 

 reflection of light from the particles. This can readily be shown to 

 occur with colloidal suspensions of gold and of arsenious sulphide. 

 Moreover, in every case where reflection takes place from non-metallic 

 surfaces the reflected light is polarized, and this is found, in fact, to 

 be true of the rays diffusely reflected from a colloidal suspension by 

 examining them with a rotated Nicol prism. It has been shown, to 

 be sure, that not only colloidal solutions (colloidal mixtures of the 

 first class), but also ordinary solutions of some substances with com- 

 plex molecules like sugar, exhibit this phenomenon, so that it is not a 

 decisive criterion of a suspension. They do so, however, in an incom- 

 parably less degree than do typical colloidal suspensions, so that it at 

 least furnishes evidence that the particles in the latter mixtures are 

 of much larger size than are those in the former. 



Whether the well-defined colloidal suspensions possess in appreciable 

 degree what may well be regarded as the best single criterion of a true 

 solution — a measurable osmotic pressure — does not, in spite of its 

 importance, seem to have been the subject of investigation by the 

 direct osmotic method. Nor is there conclusive evidence that they 

 show the closely related phenomenon of diffusion. If the existence of 

 these properties to an extent corresponding at all to the size and num- 

 ber of the particles should be demonstrated, it would, of course, prove 



