106 



was necessary to filter the oil from an insoluble skin before the weight 

 of the drops of the exposed oil could be determined. 



The surface tension of lead drying oU against air is, however, 

 lower than that of linseed oil, from which it would follow that the 

 lead soap would tend to accumulate on the surface, whereby its 

 specific surface would be increased and consequently its chemical 

 activity. 



The whole subject requires further investigation, and it is much 

 to be deplored that so little attention has been paid to it. 



Varnishes. 



In a pajDcr on the viscosity of varnishes, Seaton, Probeck, and 

 Sawyer {Jour. Ind. Eng. Chem., 9, 35, 1917) state that varnishes show 

 two types of solutions, viz., true and colloidal; they may, under 

 certain conditions, show the Tyndall effect. 



The suspensoid and emulsoid classes differ in viscosity character- 

 istics. The systems consist generally of three components, resin, 

 oU, and thinner, i.e., resin incorporated by heat with oil and thiimer 

 added. The variation in the viscosity of the emulsoid type with 

 change in concentration is very great. Seaton shows that the 

 viscosity temperature curves (determined by the DooUttle method) 

 of the true solution type, containing soluble gum and low in polymerised 

 oil, are curves whilst varnishes containing highly polymerised oils 

 give straight lines ; moreover, determinations of the viscosity of 

 varnishes at various temperatures vnH give information as to the 

 nature of the varnish solution. 



Decrease in dispersion increases viscosity in emulsoid coUoids and 

 addition of thinner, increasing the dispersion of the polymerised 

 components, will lower it. 



If Seaton's view is correct the viscosity temperature curve before 

 addition of the solvent would be a straight line, but after addition 

 of the thinner it would be a curve. The examples given by Seaton 

 are strUiing, but a number of variables define the viscosity of emulsoid 

 colloids besides concentration, temperature, and degree of dispersity 

 (Wo. Ostwald, Trans. Faraday Soc, 1913, 9, 34); especially there 

 is to be considered solvate formation where the viscosity increases 

 with the amount of dispersion medium taken up by the disperse 

 phase. In view of the difficulty in deciding with accuracy the amount 

 and even presence of polymerised oils m varnishes such a relationship 

 as indicated by Seaton is of great value. Similar changes in viscosity 

 during ageing are of importance because unless, the viscosity apj)roaches 

 a constant value in a month's time the varnish may become unsuitable. 



The importance of viscosity measurement in the standardisation 

 of aeroplane dope and aircraft varnishes has been fully recognised 

 as a determining factor for flow and freedom of working of these 

 coatings. In view of the variety of composition of varnishes the 

 volatility of the thinner as affecting the flow is of considerable practical 

 importance. 



Varnishes often contain a disperse phase associated with the 

 continuous medium. 



