The beginning of a continuous buoyancy (heat) injection into a stationary fluid results in a cap of buoyant fluid followed by a steady plume. This cap undergoes a transition in which a breakup is observed at a height from the source which increases with decreasing buoyancy injection rates. Sequences of cine film frames are presented illustrating this transition. A numerical criterion for the transition was found, namely, that the product of the power injected times the time at which transition occurs is a constant (19 cal). This can be related to an effective Rayleigh or Reynolds number, but the transition mechanism remains unknown. A critical effective Rayleigh number for the thermal may be estimated from these results to be 105which in this case represents an injected heat of 10 cal.