COLLOlt) CHEMISTRY. 203 



Von Weimat'u ' lias shown that even the most definitely crystalloid 

 bodies may be obtained in colloidal or gelatinised solution if produced 

 in solutions in which they are very insoluble. An ordinary pre- 

 cipitate, such as barium sulphate, is at first formed by the combination 

 of the ions in very minute crystal elements which are sufficiently soluble 

 in water rapidly to unite to form crystals, which gradually increase in 

 size by absorbing the smaller crystal elements. It is well known in 

 ordinary analytical work that such precipitates will pass at fii'st through 

 any filter, but gradually become filterable by the effect of time and 

 warmth. If, however, their insolubility is sufficiently increased by large 

 excess of a common ion, or by efTecting the combination in a medium, 

 such as methyl alcohol, in which sulphates are extremely insoluble, the 

 molecules cannot coalesce in the crystalline form, and a colloidal sol 

 results. Even sodium and potassium chlorides have been obtained in 

 colloidal form by the double decomposition of their organic compounds 

 in organic media in which the salts were sufficiently insoluble. 



Though inorganic sols are precipitated by the addition of an electro- 

 lyte, the presence of free dissociated ions in small quantity seems 

 essential to their formation and existence. If a solution of ferric chloride 

 be dialysed through a colloid membrane into water, the hydrochloric acid 

 formed by hydrolysis passes out into the water, leaving the ferric oxide 

 in the dialyser as a colloid sol, and this goes on till the sol is prac- 

 tically free from chlorine, but if absolutely the last traces are removed, 

 the ferric oxide flocculates and precipitates. Conversely, many oxide or 

 hydroxide sols may be obtained by washing the precipitated hydroxide 

 sufficiently free from electrolytes, or the precipitated and washed hydroxide 

 may be redissolved by boiling with a very small quantity of neutral salt 

 or free acid, when the precipitate will gradually' pass into colloidal solu- 

 tion. This process (peptisation, Anatzung) has been extended to the 

 colloidal solution of mechanically finely powdered oxides by the agency 

 of small quantities of acids or salts, and has been employed by Kuzel ^ 

 to bring many metallic oxides into the plastic condition requisite for the 

 formation of the so-called ' colloid ' electric-lamp filaments, now rapidly 

 superseding the carbon fibre. Colloid sols are frequently formed by 

 the free dilution of salts liable to hydrolytic dissociation, and especially 

 salts of weak organic acids which from their low electrolytic dissociation 

 do not readily cause precipitation. Even ferric chloride at a dilution of 

 1 : 125000 is completely dissociated in the cold in twenty -four hours, and 

 much stronger solutions suffer partial dissociation, especially on heating. 

 This is the cause of the darkened colour of hot ferric solutions. Acetates 

 are still more readily hydrolysed, and especially by boiling, by which the 

 free acetic acid is volatilised. Many solutions of hydroxides in alkalies 

 are colloidal, but those in ammonia are u.sually true complex salts. Colloidal 

 solutions are obtained of sulphides of metals of which the oxides, like those 

 of arsenic and antimony, are soluble in water, by precipitation with H.,S, 

 only water and the sulphide being formed in the reaction ; and metallic 

 sols of gold, silvei-, platinum, and other noble metals are formed by reduc- 

 tion of their dilute solutions, particularly with organic reducing agents. 

 Many sols may also be made by precipitation in presence of organic 



> Z. Ch. u. Iiul: d. Kolloide, ii. 1907, p. 76. 



= D. R. P. Ammeld, Ug. K. 30900 of Dec. 12, 1905. 



