102 



Portland cement, and the mineral matter afforded by that present 

 in the Trinidad lake asphalt cement which forms the cementing or 

 bmding material of the surface. Experience has shown that the 

 stability of such a surface under heavy travel is dependent on the 

 amount of surface energy developed by the mineral aggregate, that 

 is to say, by the state of sub-division of the particles composing this 

 aggregate. While this will depend upon the size of the sand particles 

 and of those composing the fiUer, it is also contributed to by the 

 highly developed surface of the coUoidal components of Trinidad 

 asphalt and to an extent which woidd be entirely lacking if the purer 

 forms of bitumen were used with the aggregate, a fact which has been 

 demonstrated by the difficulties which have been encountered in the 

 construction of asphalt surfaces with the residual pitches, free from 

 colloidal mineral matter, which have been met with in the past decade 

 in England, and which have necessitated the employment of various 

 expedients to overcome them. 



The relation of surfaces of sohds to films of Uquids, especially 

 when the surface is developed to such an extent as occurs in material 

 in a colloidal state, has been demonstrated, therefore, to be a matter 

 of supreme importance in carrying out successfully the construction 

 of asphalt roadways to carry intense traffic. 



[Note. — A more detailed account of the colloid chejnistry of asphalt is 

 given in the following paper : ' The Colloidal State of Matter in its Relation to 

 the Asphalt Paving Industry,' C. Richardson, Minnesota Engineering Society, 

 May, 1917. W. C. McC. L.] 



VARNISHES, PAINTS AND PIGMENTS. 



By R. S. MoBRELL, M.A.Ph.D., E.I.C, Chief Chemist, Mander Bros. 



Wolverhampton. 



In spite of the importance of the problems of surface it is surprising 

 that the scientific study of the class of products comprising varnishes, 

 paints and pigments, has been so much neglected. 



The primary components in some form or other, dissolved in a 

 suitable liquid or a finely ground pigment mcorjaorated with a medium 

 as in a paint, introduce a field of investigation of great practical 

 importance and of absorbing interest. If the medium contains 

 water, as in water paints, the properties of ordinary emulsions are 

 prime factors of success. Problems of viscosity arise in varnishes, 

 paints, dopes, and coatings containing cellulose esters; moreover 

 polymerisation .of drying oils confers valuable properties on many 

 varnishes and paints. The conditions of spreading on a surface 

 depend on the physical properties of the components and of the 

 mixtures. The changes on " drying " are essentially superficial, 

 involving questions of adsorption, oxidation, and polymerisation, 

 causing increases in viscosity. The permeabfiity to water and the 

 alteration in the appearance of films' introduce the study of the 

 properties of gels. 



The resins in their many forms are typically colloid bodies, and 

 their solutions show the properties of that class. The thickened oils 



