| Pressure in Atmospheres | Temperature, F. ° | Rise of Temperature for each additional Atmosphere |
|---|---|---|
| 1 | 212 | |
| 2 | 249·5 | 37·5 |
| 3 | 273·3 | 23·8 |
| 4 | 291·2 | 17·9 |
| 5 | 306·0 | 14·8 |
| 6 | 318·2 | 12·2 |
| 7 | 329·6 | 11·4 |
| 8 | 339·5 | 9·9 |
| 9 | 348·4 | 8·9 |
| 10 | 356·6 | 8·2 |
| 11 | 364·2 | 7·6 |
| 12 | 371·1 | 6·9 |
| 13 | 377·8 | 6·7 |
| 14 | 384·0 | 6·2 |
| 15 | 390·0 | 6·0 |
| 16 | 395·4 | 5·4 |
| 17 | 400·8 | 5·4 |
| 18 | 405·9 | 5·1 |
| 19 | 410·8 | 4·9 |
| 20 | 415·4 | 4·6 |
It may be seen from the above that, with the exception of one irregularity, there is a continual diminution of the additional temperature which is required to overcome an additional atmosphere of pressure, and if this goes on as the pressure and temperatures advance, we may ultimately reach a curious condition—a temperature at which additional pressure will demand no additional temperature to maintain the gaseous state; or, in other words, a temperature may be reached at which no amount of pressure can condense steam into water, or at which the gaseous and liquid states merge or become indifferent.
But we must not push this mere numerical reasoning too far, seeing that it is quite possible to be continually approaching a given point, without ever reaching it, as when we go on continually halving the remaining distance. The figures in the above do not appear to follow according to such a law—nor, indeed, any other regularity. This probably arises from experimental error, as there are discrepancies in the results of different investigators. They all agree, however, in the broad fact of the gradation above stated. Dulong and Arago, who directed the experiments of the French Government Commission for investigating this subject, state the pressure at 20 atmospheres to be 418·4, at 21 = 422·9, at 22 = 427·3, at 23 = 431·4, and at 24 atmospheres, their highest experimental limit, 435·5, thus reducing the rise of temperature between the 23d and 24th atmospheres to 4·1.
If we could go on heating water in a transparent vessel until this difference became a vanishing quantity, we should probably recognize a visible physical change coincident with this cessation of condensibility by pressure; but this is not possible, as glass would become red-hot and softened, and thus incapable of bearing the great pressure demanded. Besides this, glass is soluble in water at these high temperatures.
If, however, we can find some liquid with a lower boiling-point, we may go on piling atmosphere upon atmosphere of elastic expansive pressure, as the temperature is raised, without reaching an unmanageable degree of heat. Liquid carbonic acid, which, under a single atmosphere of pressure, boils at 112° below the zero of our thermometer, may thus be raised to a temperature having the same relation to its boiling-point that a red-heat has to that of water, and may be still confined within a glass vessel, provided the walls of the vessel are sufficiently thick to bear the strain of the elastic outstriving pressure. In spite of its brittleness glass is capable of bearing an enormous strain steadily applied, as may be proved by trying to break even a mere thread of glass by direct pull.
Dr. Andrews thus treated carbonic acid, and the experiment, as I have witnessed its repetition, is very curious. A liquid occupies the lower part of a very strong glass tube, which appears empty above. But this apparent void is occupied by invisible carbonic acid gas, evolved by the previous boiling of the liquid carbonic acid below. We start at a low temperature—say 40° Fahr. Then the temperature is raised; the liquid boils until it has given off sufficient gas or vapor to exert the full expansive pressure or tension due to that temperature. This pressure stops the boiling, and again the surface of the liquid is becalmed.
This is repeated at a higher temperature, and thus continued until we approach nearly to 88° Fahr., when the surface of the liquid loses some of its sharp outline. Then 88° is reached, and the boundary between liquid and gas vanishes; liquid and gas have blended into one mysterious intermediate fluid; an indefinite fluctuating something is there filling the whole of the tube—an etherealized liquid or a visible gas. Hold a red-hot poker between your eye and the light; you will see an upflowing wavy movement of what appears like liquid air. The appearance of the hybrid fluid in the tube resembles this, but is sensibly denser, and evidently stands between the liquid and gaseous states of matter, as pitch or treacle stands between solid and liquid.
The temperature at which this occurs has been named by Dr. Andrews the “critical temperature”; here the gaseous and liquid states are “continuous,” and it is probable that all other substances capable of existing in both states have their own particular critical temperatures.
Having thus stated the facts in popular outline, I shall conclude the subject by indulging in some speculations of my own on the philosophy of these general facts or natural laws, and on some of their possible consequences.
As already stated, the conversion of water into steam under ordinary atmospheric pressure demands 966·6° of heat over and above that which does the work of raising the water to 212°, or, otherwise stated, as much heat is at work in a given weight of steam at 212°, as would raise the same quantity of water to 1178·6° if it remained liquid.