72 SCIENCE PROGRESS 



between the mixing of the solutions and the precipitation of the 

 sulphur. Biltz examined these solutions and found that, though 

 in some instances the ultramicroscope shows the commencement 

 of the action before the liquid appears turbid, yet for a short 

 time nothing seems to happen ; and then, quite suddenly, the 

 ultramicroscopic particles appear, increasing in number and 

 brightness. After a time the rings, characteristic of large 

 particles, appear. 



It is an interesting question whether action begins imme- 

 diately the solutions are mixed, or whether there is an interval 

 during which there is no change. The probability is that the 

 particles which are first formed are too small to be seen, even 

 with the ultramicroscope, and it is only after they have grown 

 sufficiently, that they become visible. This period of growth 

 would correspond to the temps mort (latent period). 



This view is supported by the experimental fact that the 

 presence of small quantities of the substances facilitates the 

 appearance of the particles. Thus, Cotton and Mouton dis- 

 covered that the interaction of sodium carbonate and calcium 

 nitrate appeared to be accelerated when the action took place 

 in a vessel in which it had previously taken place, the vessel 

 having been washed with distilled water in the meantime. 

 Similar results have been obtained by other observers. 



The particles in these solutions may remain in suspension 

 for a comparatively long time. Such suspensions can be 

 obtained in many liquids and may persist for several months 

 or even years. 



Colloidal Liquids. — An important class of such liquids is 

 formed by colloidal solutions. Many substances, which in the 

 usual sense of the word are insoluble in water — for example, 

 metallic gold and platinum or arsenious sulphide — can, by 

 appropriate methods, be brought into the form of a solution, 

 which, however, differs in many respects from a true solution. 

 These liquids diffuse much more slowly than solutions of 

 electrolytes, for example, and they do not, or only very slowly, 

 pass through certain membranes which are permeable to 

 electrolytes. Colloidal solutions may contain liquids other than 

 water as the " solvent," and Graham, who was one of the first 

 to investigate the subject, divided them into hydrosols, alco- 

 osols, etherosols, etc. 



Hydrosols. — Among the earliest known of the first-mentioned 



