THE COLLOIDAL STATE 



The scattering of light by suspended particles has been made the basis of a method of 

 estimation by Theodore W. Richards (1906 ; also Biltz, 1907). Accurate determinations of 

 small amounts of precipitates can be made in this way. The instrument used is called, by 

 Richards, " Neplidometer." 



DIALYSIS 



One definition of the colloidal state is that, matter in this state does not 

 pass through such a membrane as parchment paper. The discovery of the fact 

 is due to Graham (1861, p. 186), as well as the application of it to the separa- 

 tion of colloids from crystalloids by the process which he called "dialysis." The 

 forms of apparatus which he used are shown in Fig. 39, and are in practice 

 very effective. 



I find that it is better not to allow the level of the liquid inside to rise above the upper 

 edge of the paper, since it is difficult to make a tight joint at the lower edge of the hoop or 

 glass bell. The sheet of paper taken should be large enough to be tied around the top of the 

 vessel. A continuous current of water may be caused to flow through the outer vessel, but a 

 given volume of distilled water is more effective if used in several changes of the whole volume 

 of liquid in the outer vessel. 



Crystalloids pass very 

 rapidly through parch- 

 ment paper. Graham 

 showed that 96 per cent, 

 of the salt content of a 

 2 per cent, solution of 

 sodium chloride passed 

 through in twenty-four 

 hours, when the volume 

 of the water outside was 

 ten times that of the 

 solution and was changed 

 once. Dilute hydro- 

 chloric acid applied to 

 one side of the paper 

 reddened litmus paper on 

 the opposite side in 5 '7 

 seconds. A point to be 

 remembered is that the 

 paper itself is altered by 

 the action of alkali, ex- 

 panding more than by the 

 action of water alone. 

 This will affect its per- 

 meability, and, in fact, I have noticed that Congo-red, which passes very slowly 

 through some samples of the paper, is accelerated in this process if the solution 

 is slightly alkaline. 



Other forms of dialyser will be found described in the practical handbooks, 

 such as the article of Zunz (1912, pp. 478-485). 



J. J. Abel (1913 and 1914) has applied the process of dialysis to the investiga- 

 tion of chemical changes occurring in the whole organism of the higher animals or 

 to those occurring in individual organs. The blood, issuing from an artery through 

 a canula, is made non-coagulable by the addition of small amounts of extract of 

 the heads of leeches, run into it from a side tube, and is then caused to pass 

 through a series of collodion tubes, immersed in warm Ringer's solution. 

 Collodion, like parchment paper, is impermeable to colloids. After passing these 

 tubes the blood is returned to a vein and thus is kept in continuous circulation 

 through the dialyser. In its passage, it gives up the diffusible substances which 

 it contains to the outer fluid, in so far as they are not already present in equal 

 concentration therein. By sufficiently long continuation of the process, these 

 substances pass out until they are in equal concentration in the blood and 

 in the outer liquid. If the maximum degree of dialysis is required m a 



Fie. 39. VARIOUS FORMS OF DIALYSING VESSELS 

 USED BY GRAHAM. 



(Pp. 556 and 573 of his "Collected Researches.") 



