ELASTICITY 491 contiguous to the thin film. Tbfe following ta- ble contains some results obtained by Quincke ; the figures hold good for the temperature of 20 centigrade and the above units of weight and length : LIQUID. Tension of sur the liqu Air. face separating Id from Water. Water Grammes. 8-258 Grammes. o-ooo Mercury 55-08 42-58 OHve oil 3-760 2-10 3-080 1-18 The investigation of the superficial tension of liquids leads to the explanation of the phe- nomena of capillarity, evaporation, the wetting of surfaces, the condition of vapor in clouds, or of vapor vesicles if such there be, &c. ; and thus brings these diverse subjects within the range of the laws of elasticity. The elasticity of vapors, or more strictly their expansibility, is a matter of such importance in connection with the steam engine, &c., and in meteor- ology, that it will be specially treated of under those heads. By the removal of pressure or the communication of heat, or both, a solid or liquid may be converted into a more or less perfect gas, usually called a vapor. The vapors generally have coefficients of elasticity that depend almost wholly upon the temperature ; a few such are given in the following table, from Regnault : Temperature, centigrade. Alcohol, millimetres. Ether, millimetres. Water, millimetres. 20 8-8 68-9 0-9
12-7 184-4 4-6 + 20 44-5 482-8 17-4 40 183-7 907-0 54-9 60 350-2 1725-0 148-8 80 812-9 3022-8 854-6 100 1697-6 4953-3 760-0 120 3231-7 7719-2 1491-8 140 2717-6 The elasticity of a gas is its power to resist compression, and to restore either its original volume or its original tension. At ordinary temperatures all gases are perfectly elastic, so far as the most delicate measures have as yet been able to determine, for the most extreme changes of pressure. The limits of perfect elasticity are therefore practically infinite; thus justifying the law first announced by Boyle, though frequently spoken of as the law of Mariotte, because firmly established by him, that the density of a gas is directly, or its volume inversely, proportional to the pressure. The elasticity of gases, being to a high degree independent of temperature, finds an important application in mechanical engineering, in the transmission of power to great distances by means of compressed air. It is through this elastic property of the air that sounds are conveyed by it at the rate of about 1,100 feet per second in all directions. II. ELASTICITY OF MOLECULES AND ATOMS. The explanation of the phenomena of heat and light has led to the assumption of the existence throughout all space of a highly elastic gas, the ether of phy- sical science ; the vibrations of the molecules of this ether, being communicated to the nerves of the body, produce the sensations of heat and light. This gas is supposed to exist, though in a state of constraint, among the atoms of transparent bodies, and the math- ematical development of the nature of its mo- lecular vibrations offers a complete explanation of all the phenomena of optic and thermic sci- ence ; and indeed, according to Maxwell and Edlund, we can use this same ether for the ex- planation of the phenomena of electricity and magnetism. Its elasticity is such that heat, light, and electricity are transmitted by it at the rate of about 200,000 miles per second through the interplanetary spaces. It is in general homogeneous, but in the interior of many crystalline bodies its elasticity varies as does that of the material of the crystal, thus producing the phenomena of polarization, dou- ble refraction, &c. While heat itself apparent- ly consists of molecular vibrations depending on the elasticity of the ether, it on the other hand exerts a marked and highly important influence over the elasticity of bodies. So in- timate is this connection that it may indeed be doubted whether the elasticity of the ether can be properly spoken of as distinct from the elasticity of the more material bodies that are recognized by the methods of chemical science. In general an excess of heat induces such mo- lecular vibrations that the constituent atoms be- come separated from each other, producing in the chemical nature of the body the changes known as dissociation, changes that may be compared with the rupture or crushing of a solid mass when subjected to too great a stress. When, on the other hand, a more moderate degree of heat is applied, most interesting changes occur, which will be briefly indicated in so far as they affect the elasticity of the sub- stances. The elasticity of solid bodies is in gen- eral diminished by heat, thus tending to length- en the time of vibration of chronometer springs, tuning forks, &c., retarding the velocity of sound waves, contracting the limits of perfect elasticity, diminishing the force of cohesion or the strength of the material, increasing its plas- ticity, diminishing its viscosity, &c. Iron and steel are among the few known instances of departure from this general rule. The elasti- city of liquids is so affected by heat that, ac- cording to Gladstone and Dale, the index of refraction and the coefficient of dispersion di- minish as the temperature increases. An in- crease of temperature diminishes the compres- sibility of water, but increases that of ether, alcohol, and chloroform. With regard to va- pors and gases there seems to be one uniform rule, t. ., that an increase of heat increases the elasticity of these forms of matter. The pre-
