Hopes Apparatus

Hope's Apparatus is an instrument for showing the temperature of greatest density of water. It consists simply of a cylinder of water around the middle of which there is a belt-shaped hollow for the reception of a mixture of snow and salt. This mixture produces an intense cold, and so causes a lowering of temperature of the middle zones of the water in the cylinder. Inserted in the top and bottom of the cylinder are thermometers recording- the varying values of top and bottom temperatures. The middle regions cooling, the water there becomes denser and sinks; we thus see the lower thermometer falling. This goes on till 4° C. is there reached, after which further cooling diminishes the density of the water and it rises. The result is that the upper thermometer falls to 4° C. and then lower till freezing-point is reached, the lower thermometer remaining at 4° C. till nearly all the water in the cylinder is frozen. [Heat.]