xxxiv. Chemical Section. [January 30th, 1920. 



laboratory tests. These are, however, quite inadequate for this 

 object, and special tests are really required. Though for 

 ordinary commercial purposes the distillation test is sufficiently 

 accurate it is really only being used as a measure of the vola- 

 tality, which is correctly measured by the vapour tension. Also 

 in the past little notice has been taken of the importance of the 

 ignition pointy which is an indication of the maximum engine 

 compression permissible, of which the thermal efficiency is a 

 direct function. 



The use of mixed fuels is yet in its infancy, but if maximum 

 economy is to be obtained it will be necessary to employ a 

 standard mixture as motor spirit for general use and to adjust 

 the engine compressions to the corresponding permissible value. 

 At present about 25 per cent, of the power available in benzol 

 is wasted by the use of fuel in engines which are primarily 

 designed for petrol. 



Another advantage of the employment of mixtures is that 

 usually two chemically different bodies of approximately the 

 same vapour tension yield mixtures, the vapour tensions of 

 which are higher than those of either constituent. The use of 

 mixtures has already been initiated by the air forces, who have 

 adopted a 20 per cent, benzol and 80 per cent, petrol mixture 

 which possesses several advantages over normal petrol." 



Ordinary Meeting, January 30th, 1920. 



Mr. R. H. Clayton, B.Sc (Chairman), in the Chair. 



Dr. R. S. Willows, M.A., opened a debate on " Recent 

 Work on Colloids." 



For the purpose of the address colloids were denned as matter 

 in a fine state of subdivision, which consequently possesses a 

 very large surface area. E.g., if a cube of 1 cm. side is divided 

 into cubes whose sides are one-millionth of a cm. the total 

 surface is 60 sq. metres. With each sq. cm. of surface a definite 

 amount of energy is known to be associated, and the difference 

 between colloidal and other matter lies in the fact that the 

 possible changes of this energy may entirely govern the physical 

 and chemical behaviour of the colloid although they are negli- 

 gible for matter in bulk. If the presence of a solute lowers the 

 surface energy of a solvent, the solute will concentrate itself in 

 the surface, thereby causing a maximum energy decrease. This 

 excess concentration is called adsorption. 



Instances of this adsorption effect which have important 



