COU.OtD CHEMrSTMY, 213 



liave assUuicd \ariuiis netted, cellular, and micellar structures in gela- 

 tinous gels, and to a certain extent these must have a real existence, 

 though the extension of the theory to the true organic jellies is of doubtful 

 validity. Von Weimarn ' has shown that gelatinous precipitates are 

 obtained when concentrated solutions of crystalloids are mixed which 

 react with each other to produce a compound which is very insoluble in 

 the liquid ; as, for instance, when concentrated solutions of barium thio- 

 cyanate and manganese sulphate are shaken together. These 'jellies' 

 may even be perfectly clear and transparent, and consist of drops of 

 one or other solution, surrounded by a thin lilm of practically colloidal 

 precipitate, which prevents coalescence and further reaction with the 

 surrounding liquid. A very similar effect has been observed by the writer 

 when a concentrated solution of calcium phosphate in hydrochloric acid 

 is mixed with strong ammonia, when the mixture, though smelling 

 strongly of ammonia, may yet contain acid calcium phosphate in excess, 

 which is only slowly decomposed by the diffusion of ammonia into the 

 isolated droplets.- The colloid particles are necessarily extremely insoluble 

 in their media, and whenever precipitation occurs by the mixture of a 

 precipitating solution the gel will be likely to take such cellular forms, 

 the dimensions of which will depend on the conditions of precipitation. 

 In other cases where the precipitation is more gradual the particles will 

 still adhere to loose amorphoys masses, presenting a very large surface to 

 the liquid, and it is probable that where dilute colloid solutions coagulate 

 in which the surface-tension between the particles and water is con- 

 siderable they will cohere to a more or less molecular network, which 

 will contract in order to lessen surface and tear into flocculent masses, 

 entangling and carrying down with them, as albumen does, all suspended 

 particles in the liquid. Biitschli^ found that many gels showed network 

 structures under the microscope with magnifications of about 2,000 diams. 

 when suitably treated, and detected similar structure in gelatine when 

 hardened with alcohol or chromic acid ; but as the unhardened gelatine 

 appeared quite structureless it is very probable that the cavities were 

 produced by the hardening process, and it may be noted that gelatine 

 rapidly hardened by dehydrating agents is always more or less opaque and 

 non-homogeneous. 



The drying of gels is a continuous process, proceeding rapidly at first 

 with vapour pressures not materially below that of water, but gradually 

 becoming slower, and the vapour-pressure lower as dryness is approached. 

 Owing to the structure of the gel some time (days or weeks) is required 

 for the mass to reach equilibrium with any definite vapour-pressure. 

 The curve of vapour-pressures as compared with gradually diminishing 

 percentage of water, though continuous, is not always regular, and 

 does not appear to lie exponential, and the process is usually irre- 

 versible with inorganic gels, or at least partially so, indicating a per- 

 manent change of state, thougii in some cases it may be connected with 



' Z. Ch. 11. Tnd. d.Kollolde, 1907, ii. 7fi. 



" Cp. p 2 of reprint. Von Weimarn holds that the sole difference betwetn the 

 colloid and cr^ stalloid condition lies in the extreme insolubility of the minute crystal- 

 line elements tjrsi formed, which prevents thoir uniting to form larger crystals, and 

 considers that all matter is potentially crystalline. 



' Untersuchungen uber viikrosh)})' tche ScTiiLuine und das Protnplasma, Leipzig, 

 1892; Verb. dc9 natnrh.-njed. Vereins m Heidelbe^, iV'.Z^., 18"9Z, 5, 28-41, 42-43; 

 1894, 5, 89-102, 230-292; 1806, 5, 457-472. 



