  The flow of heat, from top to bottom, in a column of light water can be
decreased by over 1000% with the addition of heavy water. A column of light
water cools from 25 C to 0 C in 11 hours, however, with the addition of heavy
water it takes more than 100 hours. There is a concentration dependence where
the cooling time increases as the concentration of added (D2O) increases, with
a near maximum being reached with as little as 2% of (D2O) added. This
phenomenon will not occur if the water is mixed after the heavy water is added.
