Page:Popular Science Monthly Volume 41.djvu/834

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this first step was soon followed by another, even more important step, when Van der Waals—also a Dutch chemist still more effectively bridged over the gap between the gaseous and liquid condition of matter. He studied that state of a gas when, under an increasing pressure and a decreasing temperature, it becomes a liquid; and he found a mathematical expression (an equation) which very approximately represents the mutual dependence between the volume occupied by the gas under a given pressure, its temperature, the volume occupied by its particles, and their mutual pressure. He thus expressed in a more comprehensive way how, in proportion as the lengths of the paths of its particles decrease, a gas becomes a liquid.[1]

The long-since suspected continuity between the gaseous and liquid states of matter was thus demonstrated once more, and rendered easy to investigate; and the importance of these conclusions was still more enhanced by Clausius, when he demonstrated that a slight alteration of Van der Waals's equation makes it also represent the absorption or dissipation of heat-energy which always takes place when a body passes from the liquid to the gaseous state, or vice versa.

And, finally, another step in the same direction was made by the French physicist, Raoult. "We all know that if some table salt, or saltpeter, or some other salt, be added to water, the water may be cooled below zero without freezing. Its freezing temperature is lowered. Now, Raoult studied the lowering of this temperature caused in water and other liquids by the addition of various amounts of various salts, and he came to a most remarkable result. It appeared that, whatever the nature of the dissolved salt may be, the freezing temperature of a solution will always be lowered by the same amount (nearly six tenths of a degree) if we add one molecule of the dissolved body to each hundred molecules of the solvent,[2] Thus, again, a purely physical fact, such as freezing, proves to be dependent upon a purely chemical fact the molecular weights of the solvent and the dissolved body; and this physical law is so general that it has become a very accurate means for determining such chemical data as molecular weights. Chemistry and physics appear again so closely interwoven that there is really no means of separating them.

  1. See the interesting discussions which took place upon this subject in the Physical Society, in October and November last.
  2. Thus, if table-salt be used, the weight of its molecule (compared with a molecule of hydrogen) is 581/2; while the weight of a molecule of water (also compared with hydrogen) is 18. So that, if we add 581/2 ounces of table-salt to each 1,800 ounces of water, we shall lower its freezing temperature by 0·62° of the centigrade scale. The same result will be obtained if we take 741/2 ounces of potassium chloride, or 101 ounces of saltpeter, to the same amount of water.