METHODS OF COLLOID RESEARCH 109 



J. TRAUBE has used the falling drop method with his stalagmom- 

 eter for numerous researches. M. ASCOLI has likewise used it in his 

 meiostagmin reaction in cancer. [CLOWES employed it in his studies. 

 See p. 39. Tr.] 



A given quantity of fluid volume is sucked up into the stalagmom- 

 eter tube and the number of drops required to empty it, dropping it 

 drop by drop, are counted. 



The stalagmometer is an instrument which requires most careful manipulation 

 to obtain reliable results. It is especially important to keep it scrupulously 

 clean, rinsing frequently with distilled water followed by hot potassium hydrate 

 solution. The apparatus is then placed over night in a hot mixture of concen- 

 trated sulphuric acid and potassium bichromate. Before use it is again thor- 

 oughly rinsed with distilled water. The dropping surface must be absolutely 

 horizontal; this is accomplished by placing it on a stand which can be adjusted 

 in all directions. There must be no bubbles on the dropping surface or in the 

 tubes. Before each initial measurement the fluid to be measured must be sucked 

 up and allowed to flow out again. The number of drops of the fluid is com- 

 pared with the number of the same volume of water. The speed of flow is so 

 gauged that no more than 20 drops fall in a minute. This is best regulated by a 

 screw clamp on a rubber tube slipped over the upper end of the instrument. 



J. L. R. MORGAN has devised a splendid apparatus for determining 

 the weight of falling drops and with it he has measured the surface 

 tension of many substances. 



There are, however, valuable contributions to the utilization of 

 surface tension (milk investigations by H* ZANGGER; meiostagmin 

 reaction of M. ASCOLI). 



The separation of colloids and crystalloids by foaming as well as the 

 separation of colloids of different surface tension is described on 

 page 35. 



Adsorption. 



Adsorption experiments may be employed for various purposes. 

 They may be used to determine the distribution of a dissolved col- 

 loid between solvent and adsorbent, thus constituting the determi- 

 nation of a physical constant. In such, a case an absolutely chemically 

 indifferent substance, e.g., charcoal, is chosen as absorbent. Ad- 

 sorption presents a suitable means of determining the nature of the 

 electric charge of a dissolved colloid. Positively charged colloids 

 are adsorbed particularly strongly by electronegative suspensions 

 (e.g., kaolin, mastic suspensions); negatively charged colloids are 

 strongly adsorbed by positive suspensions (e.g., iron oxid, clay). 

 Occasionally it is of interest to determine the properties of a gel 

 when it is used as an adsorbent. 



If in all cases there occurred pure adsorption, whereby a dissolved 



