ULTRAMICROSCOPY 71 



class are metallic hydrosols. Faraday prepared gold solutions 

 by reducing a solution of a gold salt. Another method of 

 preparation now usually employed is that due to Bredig. An 

 electric arc is formed in pure water or other liquid, between 

 terminals composed of the metal in question. A shower of 

 particles is given off from the cathode and these colour the 

 solution. The colour of the solution varies from red to blue, 

 and depends on the current used ; the latter affecting the size 

 of the particles produced. If the current is small comparatively 

 large particles are produced, which rapidly fall to the bottom ; 

 with larger values of the current, smaller particles are obtained, 

 which remain in suspension for a long time. 



In a similar manner hydrosols of silver can be prepared, the 

 colour in this case varying from reddish brown to olive-green. 

 They are very stable indeed, often lasting several years without 

 much change. 



Heterogeneous Nature of Colloidal Liquids. — The appearance 

 of these hydrosols under the ultramicroscope is very striking : 

 there is seen a number of brightly illuminated particles, which 

 differ from one another in colour; these are not at rest in 

 the field but display very clearly the well-known Brownian 

 movements. The particles are usually separate but sometimes 

 clusters are observed, the constituent particles often retaining 

 their individual motions. 



Returning to the case of ordinary colloidal liquids, such as 

 " solutions " of ferric hydroxide and arsenious sulphide, it is 

 found that they too, on examination with the ultramicroscope, 

 show a lack of homogeneity. Faraday and others noticed that 

 such liquids scatter a beam of light, the scattered light being 

 partially polarised ; and the former suggested that it was 

 probably due to the presence of small particles in the liquid. 

 But it was left to Szigmondy actually to demonstrate the truth 

 of this hypothesis. The particles, which break the homogeneity, 

 are of different sizes ; they vary from particles approaching a 

 microscopic size, to some which are only just visible with the 

 ultramicroscope, when sunlight is used ; and very probably 

 there are some still smaller. The smallest observed by Szig- 

 mondy have a radius of only 3 — 6 /a/j. or 3 — 6 x io~ 7 cm. Like 

 metallic hydrosols, they exhibit the Brownian movements. The 

 number and size of the particles are determined as in the case 

 of solids (see above). 



