There is another reason why ionization tends to cool and chemical combination tends to warm a body; it is because of the increase or de crease in the mere number of separate particles. For temperature depends upon the average kinetic energy of the particles or molecules, small particles at a given temperature making up in speed for what they lack in mass. An increase in the number of particles involves, if no heat is added from the outside, a subdi vision of their energy, a smaller average and, hence, a lower temperature. When par ticles combine, their energies combine also, and so there is an increase in their average energy and a like increase in their temperature.
The temperature at which melting takes place depends upon external pressure. With a' solidlike paraffin, which expands on liquefying, high pressure, which resists expansion, stops melting until a temperature slightly higher than the ordinary melting point is reached. Paraffin that under ordinary conditions melts at 46.3° C. melts at 49.9° C. when subjected to the addi ticmal pressure of 100 atmospheres. In the case of ice, which contracts on melting, melting is favored by pressure. The addition of one atmosphere of pressure lowers the melting point of ice by 0.0072° C. This fact ac counts for the slipperiness of ice, when being skated upon. The sharp edge of the skate exerts great pressure on the ice be-, low it, which melts and furnishes a lubricating film of water. This film of water is cooler than the ice furnishing it, some of the heat of the ice having become latent, and as soon as the skate has passed over, the water imme diately resumes the solid state. This process of. freezing again is called regelation. Regelation, is an important factor in glacier motion. The ice as it follows down a tortuous valley is con tinually being cracked. After the settling fol lowing this cracking, the great pressure from the upper ice fields melts the ice at the points of contact of opposite sides of a fracture, and the escaping undercooled water freezes again, thus healing the fracture. In this the glacier appears to follow down the irregulari ties of a valley as would a very viscous mass.
as when it had the water evaporating into it, and so we would then pronounce the heated saturated vapor as unsaturated. On the other hand, if a mass of unsaturated water vapor be cooled, the density of the vapor will at a certain tem perature be sufficient to cause saturation. Be
low 'this particular temperature, called the some of the moisture will condense. In some cases, however, when there are no nuclei in the form of dust particles, free ions, etc.., the vapor may cool appreciably below the dew-point without immediate condensation. The vapor is then said to be supersaturated. The presence of air has only a very small influence on the density and pressure of saturated water vapor in contact with water, especially when the temperature is not high.
When the temperature of water or other volatile liquid is raised so high that the pressure of the saturated vapor becomes as great as that of the atmosphere, bubbles of the vapor begin to form in the body of the liquid. This constitutes the process of boiling. The temperature at which a liquid boils is much influenced by the external pressure. The boiling point is the same as the temperature at which the pressure of the saturated vapor equals the external pressure on the bubble. In the following table these tem peratures with their corresponding pressures are given for water.
Saturated and Unsaturated Vapors.— When a liquid, water for example, is placed in a vacuous enclosure kept at constant temper ature by artificial means, it immediately begins to evaporate, the vapor presently attaining a maximum density and pressure. The vapor as well as the space occupied by it is then said to be saturated. Before this maximum pressure was reached the vapor was unsaturated. If the temperature of the whole apparatus be now raised, more water will commence to evaporate, and the vapor will increase in density and pressure before it is again saturated. Had the saturated vapor formed in the first place been shut off from the water surface before raising the temperature, it would not become as dense A saturated vapor in contact with its liquid offers a beautiful instance of dynamic equilib rium. We conceive that molecules are ever ing the surface of the water, adding themselves to the vapor. At the same time molecules of the vapor coming near to the liquid surface or plunging into it are caught by the cohesion of the liquid, thus subtracting themselves from the vapor. A less dense vapor would lessen the lat ter process and would allow the vapor to grow denser; a denser vapor would increase it and allow the vapor to fall to a state—the saturated state—when the rate of evaporation is just equal to the rate of condensation.