It was a night without air conditioning for 6500 homes and businesses in downtown Toronto.  All electricity was restored by just after 4:00 this morning in an area bounded by Jarvis, the DVP, Carleton and Queens Quay.  The blackout began just after 8 last night, leaving downtown residents in the dark and the heat.   The power outage was caused by a fire in a circuit breaker.  A Toronto Hydro spokesperson says two to three days into a heatwave, increased load does not allow equipment to cool down, which increases the possibility of a failure.