558 INFECTION AND RESISTANCE 



when the oil is very little, if any, dispersed, while in contact 

 with soap solution equilibrium is reached only when the oil 

 is highly dispersed. ' From the surface tension point of view this 

 would be expressed by saying that in contact with pure water 

 the surface tension is large and positive, while in contact with 

 soap solution the surface tension is large but negative. It is im- 

 possible to say to what extent these effects may be due to obscure 

 chemical action. 



The Preparation of Colloidal Solutions 1. This subject might 

 perhaps upon logical grounds have best been treated in an opening 

 paragraph. However, with the conclusions that have been reached 

 in the foregoing discussion the whole matter may be dismissed very 

 briefly. The conditions which must be fulfilled in order to obtain 

 colloidal solutions are in the main summarized in the following 

 paragraphs : 



2. A medium must be chosen in which the given substance does 

 not reach to any great extent the maximum, molecular degree of dis- 

 persion that is, in which what is usually called true solution does 

 not occur to any great extent. While, as pointed out at the begin- 

 ning of this discussion, there is no sharp line to be drawn between 

 colloidal and true solutions, the substances that are distinctly recog- 

 nizable at the present time as colloidal solutions carry particles which 

 are in the neighborhood of one thousand times as large as average 

 molecules. From media in which the dispersion is approximately 

 molecular the dispersed substance usually shows a strong tendency to 

 separate in the crystalline form, although it may frequently, if not 

 generally, first appear in the colloidal form, then more or less rapidly 

 becoming crystalline. The only distinction to be drawn is this: 

 from solutions in which the dispersion is very great, approximating 

 the molecular, separation in short time in the crystalline form is 

 favored ; from solutions in which the dispersion does not approximate 

 the molecular separation persistence in the amorphous state is fa- 

 vored. 



3. A colloidal sol or gel, one or the other, may generally be pro- 

 duced from any substance in any medium in which the amount of 

 molecular dispersion is at a minimum by means of any reaction 

 whereby the new substance is produced from solution. Thus, for 

 example, arsenic sulphid does not disperse ' in water to molecular 

 extent in any considerable degree. Therefore, by mixing together 

 a solution containing an arsenic compound which is soluble, and a 

 solution of hydrogen sulphid, the resulting arsenic sulphid will 

 appear in the colloidal state. Whether it appears in the dispersed 

 state as a sol or in the flocculated state as a gel will depeud mainly 

 on the electrolyte content of the solutions which are used. Tf a solu- 

 tion of arsenic chlorid be used the resulting solution will contain 

 .considerable free hydrochloric acid, and the tendency wil* be for the 



