METHODS OF COLLOID RESEARCH 115 



water bath must be stirred and a thermometer must be placed in 

 each. The test tube must be illuminated by a uniform and pro- 

 tected source of light. It is advisable to make a number of prelimi- 

 nary determinations of the coagulation point before making the 

 final reading. 



WOLFGANG PAULI distinguishes the following different appear- 

 ances in coagulation: clear, opalescent, slightly cloudy, milky trans- 

 lucent, milky opaque, finely, medium or coarsely flocculent in slightly 

 cloudy or clear fluid. These various aspects are strongly dependent 

 on the dilution and the salt content of the solution, and the latter 

 has the greater influence on the temperature of coagulation. 



Melting and Solidification Temperature. To determine the 

 melting point of gelatin, agar, etc., W. FAULT and P. RONA* used an 

 apparatus that is similar to that of E. BECKMANN for determining 

 the freezing point. The melting point is the temperature at which 

 the layer surrounding the thermometer melts. 



H. BECHHOLD and J. ZiEGLER* 2 used an air bath, in which a tube 

 containing the jelly is placed alongside the thermometer. The solid 

 jelly is weighted with 5 gm. of mercury. 1 The melting point is the 

 temperature at which the mercury breaks through the jelly. Since 

 it is difficult to observe the melting of the jelly and the thermometer 

 at the same time, the authors use an acoustic device (metronome) 

 which is described in the original papers and which is recommended 

 for similar observations. 



Swelling. 



The methods of measuring swelling, i.e., the water taken up by a 

 gel, are very inexact. The increase of volume, the gain in weight or 

 the pressure of swelling may be determined. 



Volume Increase. Equal quantities of fibrin may be placed in 

 test tubes and covered with different solutions; we then observe 

 how high the fibrin rises upon swelling (M. H. FISCHER, see p. 68, 

 Fig. 7). The increase in volume consists of the decrease in volume 

 of the swelling gel plus the volume of the water, so that the determi- 

 nation has an error, inasmuch as the contraction of the gel during the 

 swelling is unknown. This error is negligible in comparison with 

 the other experimental errors. 



Increase of Weight. This method introduced by P. HOFMEISTER 

 is somewhat more accurate. The total solids of the swelling sub- 

 stance (gelatin, muscle, etc.) are determined and the substance either 

 in a dry or a swollen state is placed in a solution. The weight deter- 



1 This apparatus is made by C. Gerhard, Bonn, Germany, dealer in chemical 

 utensils. 



