SOME years ago there were many blue-stained flints on a road near Cambridge. Lime from gas-works was about to be mixed with the flints used as road-metal, and the two different materials had lain for some time in heaps by the roadside. The blue colour, in some instances very intense, was developed wherever a heap of flints and one of lime touched each other; from which I surmised that the calcium sulphide of the gas-lime had reacted with an aluminium compound present in the flints, producing a substance akin to ultramarine.