40 PHYSICAL CHEMISTRY IN BIOLOGY. 



organic colloids (gelatin, haemoglobin, seralbumin, proteoses, dextrin). 

 Still it must be remarked that according to ZSIGMONDY 1 the size of the 

 particles of the same colloid are larger in one preparation than in another 

 and that the size can change on keeping. 



On filtering proteose solutions through filters of unequal thickness 

 BECHOLD was able to show that the larger the particles of the proteoses, 

 the easier are they precipitable by ammonium sulphate. 



Optical Properties. Colloidal solutions are opalescent by reflected 

 light, which depends upon the fact that the light is reflected by the sus- 

 pended particles. The reflected light is partly polarized. This phenom- 

 enon, called TYNDALL'S phenomenon, depends upon the presence 

 of small particles in the liquid, and is considered as a test for colloid 

 solutions. Still there are colloid solutions (certain gold solutions, ZSIG- 

 MONDY), which do not give TYNDALL'S phenomenon, and on the other hand 

 we also have solutions of certain high molecular crystalloids (cane 

 sugar, raffinose), which produce this phenomenon. 2 



With the aid of the uttramicroscope of SIEDENTOPF and ZSIGMONDY, 

 it has been made possible to directly see the colloidal particles. 3 In 

 this apparatus the colloidal particles are strongly illuminated by direct 

 light, so that no ray of the light directly falls in the eye of the observer. 

 The particles are hereby made visible on account of the formation of 

 diffraction disks which are visible by the miscroscope. In colloidal 

 solutions where the particles are close together a more or less intense, 

 homogeneous, polarized sphere of light is seen in the microscope where 

 the individual particles cannot be distinguished from each other. This 

 is possible on diluting the solution. Those particles which are only 

 made visible by dilution are called submicrons, while those that gradually 

 disappear on dilution are called amicrons. 



If the quantity of metal and the number of particles are determined in the 

 unit volume of a metallic sol, then from this the size of the particles can be approx- 

 imately calculated under the assumption that the density of particles is the same 

 as the metal. The amicrons must not be considered in such measurements. 

 In this manner the following lineal dimensions have been found for the sub- 

 microns of certain metals, bearing in mind that 5 /*/* is about the lowest limit 

 of the ultramicroscope for these particles. 



Gold 6-130 fifi* 



Silver 50- 77 " 



Platinum 44 " 



1 Zur Erkenntnis d. Roll., 1905, 104, as well as Zeitschr. f. Elektrochem., 12, 631, 

 1906. 



2 Lobry de Bruyn and Wolff, Rec. trav. chim. des Pays-Bas., 23, 155, 1904. 



3 Zsigmondy, Zur Erkenntniss der Roll. Jena, 1905, 83. 



4 According to Zsigmondy (I.e. page 124) only those gold solutions are stable 

 whose average particles have at least a size of 60 ////. When they are greater than 

 75 n(jL then the particles begin to settle. 



