ON March 24, a Royal Society Discussion was held on the subject of the physics and chemistry of hydrocarbon gels. Interest in these systems arose in the first instance from the desire to extend the range of frame throwers in which hitherto ordinary liquid hydrocarbons had been used. It was evident at a very early stage in the investigation that a possible improvement in range could be effected if the fuel could be made thixotropic in character, or at any rate if its intrinsic viscosity could be varied over wide ranges on changing the rate of shear. Further investigation revealed the fact that another physical property or combination of properties embraced under the term 'stringiness' or 'extensibility' had also to be taken into consideration.