98 
SUMMARY OF CURRENT RESEARCHES. 
Reversible Sol-gel Transformation. — S. C. Bradford {Trans. 
Faraday Soc ., 16 , part 3, 1921). There is considerable evidence from 
microscopical work for regarding the reversible sol-gel transformation 
as an extreme case of crystallization due to the low diffusivity and 
minimum velocity of crystallization of the gel substance. F. I. G-. R. 
The Structure of Gels. — J. 0. W. Barratt (Trans. Faraday Soc., 
16, part 3, 1921). Application of the ultramicroscope has resulted in 
the view that gels are sometimes structureless, and that when structure 
is observed it consists, not of liquid droplets enclosed by solid septa, 
but of a mass of intersecting fibrils. F. I. G-. R. 
Precipitation of Disperse Systems. — R. S. Willows (Trans. 
Faraday Soc., 16, part 3, 1921). Zsigmondy finds it possible to dis- 
tinguish ultramicroscopically between the yellow-green primary and 
secondary particles in a gold sol as follows. The number of primary 
particles per unit volume of a red-gold sol is counted, electrolyte is then 
added in “ definite ” volume. When a certain easily recognized colour 
tint appears a solution of gum arabic is added, preventing further 
coalescence. A recount is now made of the primary particles or the 
total number of particles, primary, secondary, etc., which can be com- 
pared with theory. F. I. G. R. 
Abnormal Crystallization of Lead Azide by Protective Colloids. 
— A. G. Lowndes (Trans. Faraday Soc., 16, part 3, 1921). A very 
beautiful series of microphotographs of lead azide crystals. Some of 
the crystals show very perfect twinning. The explosive nature of this 
compound can be gauged by the sharp acicular outline of the crystals. 
The effect of adding a colloid to the solution is to give much more 
rounded, and presumably stabler, forms. F. I. G. R. 
Colloids in Electro-deposition of Metals.— W. E. Hughes (Trans. 
Faraday Soc., 16, part 3, 1921). In some cases, contrary to the usual 
opinion, the use of a colloid makes it easier to obtain a smooth, coherent 
coating in electro-deposition. Microscopical examination shows that 
the deposit is really crystalline. The subject is one of some importance 
to industry. F. I. G. R. 
