56 THE ROYAL SOCIETY OF CANADA 



formation is shown in Fig. 1, with magnification 114 times, and in 

 Fig. II (mag. 25). 



The colloidal solution chosen for these experiments was a Bredig 

 copper hydrosol formed by making an arc under pure water with two 

 copper w4res in series with a resistance in a 110 volt circuit. As these 

 copper particles bear a positive charge they would absorb from a 

 solution of potassium chromate some of the chromate (negatively 

 charged) ions. 



Any theory explanatory of the Liesegang rings presupposes that 

 the potassium chromate is originally equally distributed through the 

 gelatine. The addition of copper colloidal solution to the gelatine 

 containing traces of potassium chromate would alter the continuity 

 of this distribution, if the copper particles strongly absorb the chromate 

 ions. One would expect under these circumstances that the rings 

 would not form. 



Gelatine solutions were made up similar to those used to produce 

 the rings shown in Figs. I and II, with the exception that a quantity 

 of copper colloidal solution was used in place of water. If the gelatine 

 solution was poured out as soon as the copper colloidal solution was 

 added, concentric rings like those shown in Figs. I and II were pro- 

 duced in films made from both these preparations, but if the films were 

 made several hours after the addition of the colloidal copper, the 

 precipitate was in the form of microscopic quantities scattered over the 

 plate around the central portion, some being gathered into piles, so 

 that the whole area presented a blotchy appearance under the low 

 power microscope. (Fig. III.) 



It is evident from these results that the positively charged 

 copper particles, as would be expected, do combine with the negatively 

 charged chromate ions, for the nature of the precipitate is entirely 

 changed when they are present. These results would also indicate 

 that a certain interval of time is necessary for the diffusion of the 

 copper particles thiough the gelatine solution and the formation of 

 the copper-chromate aggregates. 



The same idea was carried out with agar solutions. The agar was 

 prepared by washing and boiling and straining, and the solutions were 

 made up as were those containing gelatine. The phenomena ob- 

 served are .shown by micrographs. Fig. IV shows the outer boun- 

 dary of the precipitates formed by the diffusion of a drop of a solution 

 of ten gram molecular weight of silver nitrate per litre in a 1% agar 

 solution containing one one-hundredth gram molecular weight 

 potassium chromate per litre, and Fig. V shows the formation when 

 copper colloidal particles are present in the same chromate- 

 bearing agar solution. Some films containing only chromate showed 



