336 Mr. Graham on the Properties of Silicic Acid [June 16, 



The pectization of liquid silicic acid is expedited by contact with solid 

 matter in the form of powder. By contact with pounded graphite, which 

 is chemically inactive, the pectization of a 5 per cent, silicic acid is brought 

 about in an hour or two, and that of a 2 per cent, silicic acid in two days. 

 A rise of temperature of 1°*1 C. was observed during the formation of the 

 5 per cent, jelly. 



The ultimate pectization of silicic acid is preceded by a gradual thicken- 

 ing in the liquid itself. The flow of liquid colloids through a capillary tube 

 is always slow compared with the flow of crystalloid solutions, so that a 

 liquid-transpiration-tube may be employed as a colloidoscope. With a col- 

 loidal liquid alterable in viscosity, such as silicic acid, the increased resist- 

 ance to passage through the colloidoscope is obvious from day to day. 

 Just before gelatinizing, silicic acid flows like an oil. 



A dominating quality of colloids is the tendency of their particles to ad- 

 here, aggregate, and contract. This idio-attraction is obvious in the gradual 

 thickening of the liquid, and when it advances leads to pectization. In 

 the jelly itself, the specific contraction in question, or synceresis, still pro- 

 ceeds, causing separation of water, with the division into a clot and 

 serum ; and ending ia the production of a hard stony mass, of vitreous 

 structure, which may be anhydrous, or nearly so, when the water is allowed 

 to escape by evaporation. The intense synaeresis of isinglass dried in a 

 glass dish over sulphuric acid in vacuo, enables the contracting gelatin to 

 tear up the surface of the glass. Glass itself is a colloid, and the adhesion of 

 colloid to colloid appears to be more powerful than that of colloid to crystal- 

 loid. The gelatin, when dried in the manner described upon plates of calc- 

 spar and mica, did not adhere to the crystalline surface, but detached itself 

 on drying. Polished plates of glass must not be left in contact, as is well 

 known, owing to the risk of permanent adhesion between their surfaces. 

 The adhesion of broken masses of glacial phosphoric acid to each other is 

 an old illustration of colloidal synseresis. 



Bearing in mind that the colloidal phasis of matter is the result of a 

 peculiar attraction and aggregation of molecules, properties never entirely 

 absent from matter but greatly more developed in some substances than in 

 others, it is not surprising that colloidal characters spread on both sides 

 into the liquid and solid conditions. These characters appear in the 

 viscidity of liquids, and in the softness and adhesiveness of certain crystal- 

 line substances. Metaphosphate of soda, after fusion by heat, is a true 

 glass or colloid ; but when this glass is maintained for a few minutes at a 

 temperature some degrees under its point of fusion, the glass assumes a 

 crystalline structure without losing its transparency. Notwithstanding 

 this change, the low diffusibility of the salt is preserved, with other cha- 

 racters of a colloid. Water in the form of ice has already been repre- 

 sented as a similar intermediate form, both colloid and crystalline, and 

 in the first character adhesive and capable of reunion or " regelation." 



It is unnecessary to return here to the fact of the ready pectization of 



