The Latent Heat Saga

A n outstanding scientific and technological achievement of the twentieth century has been the evolution of nuclear energy which forms part of the mix of energy sources on which the continued growth of our civilization depends. In the twenty-first century we may be blessed with the arrival of another source of energy, of similar magnitude, which is more benign than nuclear energy and does not have the disadvantages of fossil fuels, that is their declining availability and their pollution of the atmosphere. The potentially new energy of the present century is hydrogen bond energy stored in ordinary liquid water. It is hard to believe that we can speak of new energy when its existence should have been recognized long ago during two centuries of water science. So far no sizeable attempt has been made to utilize this hydrogen bond energy for the generation of electricity or the propulsion of motor vehicles. Notwithstanding the great abundance of water on the surface of the earth and its scientific appeal for over two thousand years, water has somehow managed to conceal part of its internal energy with the confusing concept of " latent heat. " This latent or concealed heat is said to have been discovered 250 years ago, but still shrouds itself in mystery. How can something be discovered and remain a mystery? The story is told by the German scientist Stefan Emeis 1 in a recent issue of a meteorological journal. In 1758, Scottish chemist and professor at the University of Glasgow, Joseph Black, measured the temperature of a block of ice as it was steadily heated through the melting point of 0°C. First the temperature of the ice slowly increased until it reached 0°C. Continued heating had no effect on the thermometer until all the ice had been converted to liquid water. Black called the traceless disappearance of heat in the melting process the latent heat (of fusion). He made the same type of measurement of the reverse process of freezing water. As he extracted heat from warm water, the temperature of the liquid would drop gradually until it reached 0°C. At this point the temperature stopped falling below the freezing temperature while further heat was subtracted from the water. Not until all the water was converted to ice did the ice temperature fall below 0°C. The heat that disappeared without any temperature change in the cooling process Black also …