202 REPORTS ON THE STATE OF SCIENCE. 



light. By this means the suspended matter in many colloid solutions 

 has been rendered visible and the number and size of the colloid particles 

 estimated. 



Since sodium and potassium chlorides have been obtained in colloidal 

 form in organic solvents, it is no longer possible to distinguish between 

 colloid and crystalloid substances, but only between the colloid and 

 crystalloid sfate. In the typical crystalloid solution of an electrolyte 

 the dissolved body is separated into its molecules, and to a large extent 

 into individual ions, while in the colloid sol the units of distribution are 

 either large and often conjugated molecules or more frequently minute 

 particles composed of many molecules united by cohesive attraction. 

 Whether there is in principle any clear line dividing these conditions, 

 or whether they do not rather shade off into each other by insensible 

 gradations is very doubtful, but in typical cases the distinction is 

 sufficiently marked. As osmotic pressure, and all its attendant effects 

 are proportional to the nuiaher of the dissolved molecules, and therefore, 

 for a given quantity of substance inversely to the mass of the molecules 

 or particles, these physical effects are almost absent in the colloid sol, 

 and in most cases diffusion, lowering of freezing point, raising of boiling- 

 point, and a Jorliori electric conductivity which demands ionic dissocia- 

 tion are barely, if at all, perceptible, and may perhaps be attributable 

 to the traces of electrolyte which appear essential at least to inorganic 

 colloid solution. In the cases of many organic colloids of known high 

 molecular weight, boiling- and freezing-point determinations have yielded 

 numbers not inconsistent with the view that the particles are actual 

 single molecules. While crystalloid bodies can usually only be separated 

 from solution by evaporation or freezing of the solvent, or by chemical 

 changes, colloid sols are unstable, and liable, either spontaneously or 

 through very slight disturbing causes — such as the addition of an 

 electrolyte, change of temperature, or concentration — to separate into an 

 amorphous precipitate, or 'gel,' and the practically pure solvent. While 

 crystalloid solutions are optically homogeneous, all colloid sols exhibit, 

 in a greater or less degree, the ' Tyndall effect ' of scattering and polarising 

 light, and, in most, minute suspended particles may be detected by the 

 ultra-microscope. The relation of colloid sols to mechanical suspensions 

 of very finely divided solids and to emulsions of immiscible liquids is a 

 Tery close one, and may depend merely on the size of the particles. 

 Many sols are visibly opalescent, but those with very small particles 

 may be perfectly transparent, and metallic .sols are often deeply coloured ; 

 gold sols vary from deep red to bluish violet. 



The organic colloid sols differ in many respects from the inorganic, 

 often approaching very closely to true solutions ; and being usually 

 bodies of high molecular weight, and little subject to electrolytic 

 dissociation, their true solutions would necessarily possess many of the 

 properties associated with colloids. It is in fact by no means impossible 

 that very large molecules or molecular aggregates may disperse and 

 polarise light, and may even be made visible by the ultra-microscope. 

 Organic sols are usually more stable than inorganic, and are flocculated 

 or precipitated by different causes, so that it will be appropriate to defer 

 their consideration till after tliat of the simpler and better investigated 

 inorganic, or, as they are sometimes called, ' suspension ' colloids. 



Inorganic colloid sols are invariably suspensions of very finely 

 divided substances in media in which they are extrein^ly insoluble, 



