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When an astronomer trains his telescope at one of the planets neighbouring the Earth, he always wonders about the presence of water and oxygen there. His interest is quite natural: should they exist in any quantity on the planet life there might be expected somehow to resemble ours. It is water that brought our Earth into existence, developed it to its present state, and created life. Above all else, water is the most wonderful substance on the Earth and the more we learn about it the more we marvel at it.

Hardly anybody has ever speculated on the remarkable properties of water, and no wonder: we encounter water everywhere, it is a common thing on our planet. Three quarters of the Earth’s surface is covered by water, with about one fifth of the land covered by solidified water (ice and snow); clouds of water vapour and tiny drops of water always shroud a good half of that land, where there are no clouds water-vapour is always present in the air. That’s how common water is on our planet: it constitutes as much as seventy-one per cent of the human body. And common things are never regarded as wonderful. Its very commonplaceness, however, is extraordinary. No other substance is more abundant on the Earth and none occurs in three states at the same time: solid, liquid and gaseous.

Water has conditioned the Earth’s climate. But for water our planet would have cooled long ago and all life would have disappeared. Water possesses a high heat capacity. When warmed it absorbs much heat, and cooling it loses it. Oceans, seas and all other reservoirs on our planet, as well as atmospheric vapour act as heat accumulators: in warm weather they absorb heat, and when it is cold they give it off, so warming the air and all surrounding space.

The cold of outer space would long ago have penetrated the Earth but for its warm coat. This coat is the atmosphere which surrounds our planet, the water vapour acting like a layer of cotton wool. Over deserts where water vapour is scarce the coat is full of holes. Thus unprotected the Earth is fiercely heated by the Sun in the daytime and cools off completely overnight. That is why temperature fluctuations in the desert are so great.

Nonetheless the Earth would freeze in the long run if it were not for another of water’s remarkable properties. It is common knowledge that almost all substances contract on cooling, but water expands. If it contracted, ice would be heavier than water and would sink. All the water would gradually turn into ice and the Earth would be left with a very light mantle of a gaseous atmosphere containing no water vapour.

Water has one more extraordinary characteristic: its latent heat of melting and evaporation is extremely high. It is only due to this that life is possible in a hot climate. Only by evaporating water (i.e. giving off a large amount of heat) can animals and men maintain their body temperature several degrees lower than that of the ambient atmosphere.

The role of water in nature is unique because life would be impossible without it. Life originated in primitive seas from the substances dissolved in them. Ever since then chemical reactions have been occurring in every cell of all animals and plants between the dissolved substances.

Perhaps the least known among the remarkable properties of water is its ability to form an extremely strong surface film resulting from the very powerful mutual attraction of molecules in the uppermost layers. Its surface tension is strong enough to support things which seemingly should not float.



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