32 



• 

 of non-volatile substances of molecular dimensions. The concentration 

 of the disperse phase in hydrosols is most conveniently determined 

 by ultra-filtration through collodion membranes. The method used 

 by Zsigmondy" gives very accurate results and is less complicated 

 than the Bechold^ method. By suitably varying the composition 

 of the collodion (Schoep''^), a rough grading of a non-uniform disperse 

 phase is rendered possible, those colloidal particles above a definite 

 size only, remaining on the filter. Certain physico-chemical methods, 

 such as colorimetric determinations have been successfully used to 

 determine concentration and Marc^* has developed the interferometer 

 for technical use, whilst Mecklenburg^^, and more recently, Tolman^^^ 

 have used the intensity of the Ught scattered by a colloidal solution 

 (Tyndall Beam^i). 



(b) Size, Shape, and Internal Structure. 



The size of the particle more than any other property determines 

 .the behaviour of a particular solution, and its estimation is therefore 

 of considerable importance. 



According to Ostwald the dimension is best expressed as the 

 degree of dispersion, or, the ratio of the absolute surface of an 

 individual particle of the disperse phase, to the volume of that particle, 

 von Weimam^o, however, has pointed out that this ratio alone does 

 not sufficiently determine the properties of the system, on account 

 of the difference in the internal structure of two otherwise similar 

 particles. Moreover the shape and hence the value of the ratio, 

 depends upon the method used for the preparation of the colloid 

 (Svedberg, Gans^-). Since nothing definite is known of the internal 

 structure and shape of the particle under particular experimental 

 conditions, it is more usual to ex^jress the size in terms of the linear 

 dimensions of the particle. The method used for this determination 

 depends upon (1) the physical condition of the continuous phase and 

 the size of the particle, (a) microscopic, (6) sub-microscopic. Sus- 

 pensions containing particles as large as 10 ju.= -01 mm. exhibit 

 colloidal properties — e.g., Brownian movement, cataphoresis — and 

 for the purpose of classification, such particles are called m.icrons^^. 

 The lower limit for the micron is conventionally fixed at -2 yu. and 

 corresponds with the limit of microscopic visibility as determined by 

 Johnston Stoney". Sub-microns are detected by means of the ultra- 

 microscope, and range from • 2 ju, to the lower Umit 3fMix= ■ 000003 mm. 

 observed by King-" using the Zsigmondy^^ immersion ultra- microscope. 



I. — Continuous Phase Solid. 

 (a) Microns. 



Owing to the absence of Brownian movement, direct microscoi^ic 

 measurements can be made. Tinker^' using an oil immersion fluorite 

 objective, and improved illumination, was able to measure the 

 particles in a semi-permeable membrane up to the theoretical limit 

 of microscopic visibility. 



