It is easier to create heat than cold. For thousands of years, for example, people have been boiling kettles to make a pot of tea or something similar, but it is only in fairly recent times that we have mastered the art of refrigeration to make a lump of ice. Long before this was possible, it was realised that while there was no theoretical limit to the maximum temperature achievable, there must be a definite lowest temperature below which it is not possible to go.
The concept of absolute zero was first suggested by the French physicist Guillaume Amontons in 1702. It had been noted in the previous century that the pressure exerted on its container by a fixed volume of any gas is directly proportional to its temperature; an increase of one degree increases the pressure by a certain amount, an increase of two degrees increases the pressure exerted by exactly twice as much, and so on.
The reverse applies as a gas cools, and it followed that if it were possible to reduce the temperature of a gas indefinitely, a point would be reached when it would exert no pressure at all on its surroundings. This, it was argued, must be the lowest possible temperature achievable by any means, and in due course it was calculated that absolute zero, as it came to be called, must be around minus 273 degrees. In recent years scientists have succeeded in producing temperatures in the laboratory within 170 billionths of a degree of absolute zero, but in the 19th century there was no known method of even approaching it. Meteorologists, however, had hopes that it might lie within their sphere of influence. They were aware from measurements taken at various points on the slopes of high mountains that temperature decreases gradually with height, the average rate of decrease being of the order of two degrees for every 1000 feet. It was generally assumed that this decrease continued on indefinitely, until somewhere out in space, perhaps 25 miles above ours heads, might lie the Holy Grail of zero absolute.
But it was not to be. In 1902 another Frenchman, Leon Teisserenc de Bort, made what has since been called "the most striking discovery in the whole history of meteorology". Using recording instruments carried aloft by hydrogen-filled balloons, he found that the fall in temperature with height continues to an altitude of about eight miles and a minimum temperature of between minus 50 and minus 60 degrees. Above that, the temperature stops falling, and may even begin to rise a little.