110 



layer which does not clear on drjdng and becomes very granular 

 with eventual loss of cohesion. Like varnish films shellac gives a 

 semipermeable membrane and wdth half normal solutions of salts 

 the absorption of water is practically inliibited. 



In a varnish film such an equiUbrium would leave the film clear, 

 but in shellac there is a persistent cloudiness indicating that the film 

 is becoming granular. The examination of the properties of shellac 

 films is of interest in comparison with oil rosin films. In some respects 

 there is much in common, but in the impregnated shellac film, water 

 is probably the continuous medium. As in the case of oil varnishes 

 a certain per cent, of water can be absorbed without opalescence 

 appearing. 



Natanson (Z. phys. Chem., 38, 690, 1901) has followed up Poisson's 

 researches of 1829 in which it was stated that when a liquid is subjected 

 to deformation a certain time is necessary for obtaining equilibrium, 

 different for different liquids. Liquids have a very small relaxation 

 value. For castor oil from G. de Metz's results the relaxation time 

 is 0-0031 sees., and for solutions of tragacanth and collodion, values 

 of the same order. 



Reiger [Physik Zeitschrift, 8, 537, 1907 ; and Annalen der Physik, 

 4, 31, 51, 1910) has shown that fluid mixtures of rosin and turpentine 

 have a possible elastic reaction by an oscillatory viscometric method 

 provided due allowance be made for siurface forces. 



De Metz {Comptes Rendus, 136, 604, 1903) has examined the very 

 slow relaxation in the double refraction of a copal varnish induced 

 by mechanical deformation caused by pressure or extension. The 



phenomenon of relaxation in a varnish lasts long enough to be observed 



^ ^1 



in the fall in the double refraction. T = , , -, where log is 



log A — log A^ ° 



to base e and rj = nT (Maxwell, Phil. Mag., 4, 25, 129, 1868). 

 T = time of relaxation, A and a^ the difference of path of two rays at 

 times t and t^; r] == coefficient of internal friction of the varnish and 

 n = modulus of rigidity. 



The modulus of rigidity of a liquid varnish calculated on the 



above formula is n = 0- 12 - , at 22° C, and is of the same order as 



cm^ , 



that of gelatine in water calculated by another method (c./. Schwedoff, 



Jorir. d. phys., 8, 341, 1889, and 9, 34, 1890). 



Paints and Pigments. 



In the literature on oil pamts the appUcation of the principles 

 of coUoid chemistrj'^ is very scanty. 



H. A. Gardner {Jour. Ind. Eng. Chem., 8, 794, 1916) discussing 

 the physical character of pigments and paints, points out that the 

 opacity of pigments generally increases mth fineness of division. 

 As the refractive index of the vehicle approaches that of the pigment 

 the opacity diminishes. Hence, in turpentine and in linseed oil 

 the opacity will be less than in water as those media have higher 

 refractive indices. 



