Rumford's most important scientific work took place in Munich, and centred on the nature of heat, which he contended in "An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction" (1798) was not the caloric of then-current scientific thinking but a form of "motion". Rumford had observed the frictional heat generated by boring cannon at the arsenal in Munich. Rumford immersed a cannon barrel in water and arranged for a specially blunted boring tool. He showed that the water could be boiled within roughly two and a half hours and that the supply of frictional heat was seemingly inexhaustible. Rumford confirmed that no physical change had taken place in the material of the cannon by comparing the specific heats of the material machined away and that remaining.