SYMPOSIUM ON COLLOIDS 



43 



OUTLINE OF THE PHYSICAL CHEMISTRY OF 



COLLOIDS 



D. A. MacInnes, University of Illinois. 



In the short time at our disposal it will be possible only to 

 discuss some of the outstanding features of the physical chem- 

 istry of colloids. Since the first use of the term "colloid" by 

 Thomas Graham in 1861 the subject has advanced so rapidly 

 that no one but a specialist could hope to read all the literature 

 connected with it. There are at present two journals devoted 

 to colloids; the "Kolloide Zeitscrift" and the "Kolloide Bei- 

 heft," both edited by Wolfgang Ostwald. Extended treatises 

 on the subject have been written by Ostwald, Zsigmondy, Hat- 

 scheck and others, and scientific journals of the most diverse 

 character will be found to contain articles pertaining to colloids 

 at least a few times a year. 



In order to get a rough idea of the place colloids occupy in 

 nature let us make the experiment of shaking up some soil in 

 water. There will be a certain portion that will settle out al- 

 most as soon as the shaking ceases. Another portion will re- 

 main suspended in the water for a short time, but will be sep- 

 arated if the mixture is run through a filter paper. This is 

 usually termed a "coarse suspension." The turbid fluid that 

 has gone through the filter is a colloidal solution. The solid it 

 contains differs from the portion that remained on the filter 

 paper chiefly in the fineness of division of its particles, or in 

 terms more usual in this branch of science, the degree of dis- 

 persion of the particles. We may then, following Wo. Ost- 

 wald, take the size of the largest particle that will go through 

 a filter paper as the upper limit of the size of colloidal par- 

 ticles. The arbitrary nature of this division will be evident to 

 everyone, but it is in accord with the general usage of chem- 

 ists. If a precipitate is not stopped by a filter paper a chemist 

 makes expressive remarks concerning colloids. The lower limit 

 of the degree of dispersion of colloids we may also take from 

 Ostwald. This may be given as the size of the smallest par- 

 ticle that will be briefly described later. Now the diameter of 

 the particles that an ordinary filter paper will pass is about 

 100 /Jifji, or 1-1000 millimeter, and the ultramicroscope 

 can make evident to the eye particles of a diameter of 6/j.fi, 

 or six millionths of a millimeter in diameter. We can then 

 agree for the present to consider solutions containing particles 

 of sizes that come within this range as colloidal solutions. 



