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eurface-tension, as in that of swollen gelatine drying in saturated watet*- 

 vapour, which will be mentioned later. Silica-jelly, which was very fully 

 investigated by Van Bemraelen,' may be taken as an example. The fresh 

 gel was dried at a temperature of 15°, at which water has a vapour- 

 pressurejof 12*7 mm., in a series of exsiccators containing sulphuric acid 

 diluted to known vapour-pressures, until equilibrium was established. 

 The fall of vapour- pressure was at first very gradual, but became more 

 marked at 1 1 mm. when a water-content of 3*7 was reached. (Tt is not 

 clear from the diagram whether this is 3"7 mols. per mol. of SiO.2 or 

 3"7 grm. per grm., but probably the former.) At a pressure of about 

 5 mm. and a water- content of about 1'5 the gel, which had previously 

 been transparent, became opalescent, and finally porcelain-white, and the 

 curve ran horizontally, that is, the vapour-pressure remained constant 

 until a water- content of 1 was reached. This constant pressure indi- 

 cates some change, chemical or physical, which takes place at this 

 pressure, and if the proportions are molecular, might correspond to the 

 decomposition of a definite hydrate (Si0.2)20H., with a vapour-pressure 

 of 5 mm., or to a change of form to one with lessened attraction for 

 water. The opacity would also correspond to some definite change, as 

 during the horizontality of the curve the gsl would be a non-homo- 

 sreneous mixture of the two substances or two forms. Below 5 mm. 

 the gel again becomes homogeneous and transparent. Van Bemmelen 

 supposes that during the change a further coagulation takes place with 

 contraction of the gel and the production of air-spaces. Up to this 

 point the curve is irreversible, that is, the gel will not absorb moisture 

 in the same way, but only a much lessened quantity in saturated water- 

 vapour. Probably in the earlier part of the curve the surface-tension 

 between the water and silica is different from its surface-attraction for 

 water-vapour, and the change indicated by the opaque state is not 

 likely to be a reversible one. Below 4 or 5 mm. pressure the drying 

 is a reversible one, and of the character common to all porous or finely 

 divided substances. The drying of ferric hydroxide is very .similar in 

 character, but has no marked horizontal in its curve, which is throughout 

 irreversible. From some, perhaps rather fanciful, resemblance of the 

 curves produced to those caused by the hysteresis or ' lag ' in the magneti- 

 sation and demagnetisation of iron, the phenomenon has been described 

 by that name. It produces some curious consequences, of which space 

 does not permit discussion. 



Most organic colloids seem to differ somewhat widely from the inor- 

 ganic sols and gels which have been described, and in many cases their 

 solutions, though possessing colloidal properties, approximate closely to 

 what we should anticipate of true solutions of bodies of the high mole- 

 cular weights which many of them are known to possess. They present 

 more analogy to emulsions of immiscible liquids which become more 

 miscible at higher temperatures than to suspensions of solid particles 

 like the metallic sols, and have hence been called ' emulsion colloids.' 

 Their sols usually show the Tyndall effect, and frequently, but not 

 always, submicrons are visible to the ultra-microscope ; but both may 

 disappear with rise of temperature or the addition of a ' common 

 (solvent.' They are not usually sensitive to small additions of an electro- 



» Z, anorg. Ch., 1896, 13, 233-256. 



