310 Royal Society : — 



The gelatine, when dried in the manner described upon plates of calc- 

 sparand mica, did not adhere to the crystalline surface, but detached 

 itself on drying. Polished plates of glass must not be left in con- 

 tact, 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 raid 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 condi- 

 tions. These characters appear in the viscidity of liquids, and 

 in the softness and adhesiveness of certain crystalline substances. 

 Metaphosphate of soda, after fusion by heat, is a true glass or col- 

 loid ; but when this glass is maintained for a few minutes at a tem- 

 perature some degrees under its point of fusion, the glass assumes 

 a crystalline structure without losing its transparency. Notwith- 

 standing this change, the low diimsiuility of the salt is preserved, 

 with other characters of a colloid. Water in the form of ice has 

 already been represented as a similar intermediate form, both col- 

 loid 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 pectiza- 

 tion of liquid silicic acid by alkaline salts, including some of very 

 sparing solubility, such as carbonate of lime, beyond stating that the 

 presence of carbonate of lime in water was observed to be incompa- 

 tible with the coexistence of soluble silicic acid, till the proportion 

 of the latter was reduced to nearly 1 in 10,000 water. 



Certain liquid substances differ from the salts in exercising little 

 or no pectizing influence upon liquid silicic acid. But, on the other 

 hand, none of the liquids now referred to appear to conduce to the 

 preservation of the fluidity of the colloid, at least not more than the 

 addition of water would do. Among these inactive diluents of silicic 

 acid are found hydrochloric, nitric, acetic, and tartaric acids, syrup 

 of sugar, glycerine, and alcohol. But all the liquid substances named, 

 and many others, appear to possess an important relation to silicic 

 acid, of a very different nature from the pectizing action of salts. 

 They are capable of displacing the combined water of the silicic acid 

 hydrate, whether that hydrate is in the liquid or gelatinous condition, 

 and give new substitution-products. 



A liquid compound of alcohol and silicic acid is obtained by adding 

 alcohol to aqueous silicic acid, and then employing proper means to 

 withdraw the water from the mixture. For that purpose the mixture 

 contained in a cup may be placed over dry carbonate of potash or 

 quicklime, within the receiver of an air-pump. Or a dialyzing bag 

 of parchment-paper containing the mixed alcohol and silicic acid 

 may be suspended in a jar of alcohol : the water diffuses away, 

 leaving in the bag a liquid composed of alcohol and silicic acid ouly. 

 A point to be attended to is, that the silicic acid should never be 



