We may take X =6 X I o' 5 cms.,, u - I =0.0003; whence from (21) we obtain as the distance x, equal to I/h, which light must travel in order to undergo alteration in the ratio e : 1, x=4 4XIo 13 Xn (22) The completion of the calculation requires a knowledge of the value of n, the number of molecules in unit volume under standard conditions, which, according to Avogadro's law, is the same for all gases.
The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from 10 to 65 cms. The adults live in great numbers in the alimentary canal of some vertebrate, usually fish, the larvae are as a rule encysted in the body cavity of some invertebrate, most often an insect or crustacean, more rarely a small fish.
Soc., 1890, 47, p. 364), who found that a film of olive oil spread over the surface of water produced a perceptible effect on small floating pieces of camphor, at places at which the thickness of the film was io 6 X10 -8 cms., but produced no perceptible effect at all at places where the thickness of the film was 8.1 X10 -8 cms. Thus a certain phenomenon, of the nature of capillary action, is seen to depend for its existence on the linear dimensions of the film of oil; the physical properties of a film of thickness Io 6Xio 8 cms. are found to be in some way qualitatively different from those of a film of thickness 8.1 X io 8 cms. Here is proof that the film of oil is not a continuous homogeneous structure, and we are led to suspect that the scale on which the structure is formed has a unit of length comparable with 8 X10 -8 cms. The probability of this conjecture is strengthened when it is discovered that in all phenomena of this type the critical length connected with the stage at which the phenomenon changes its nature is of the order of magnitude of 10 -8 cms.
(If the molecules of air at normal temperature and pressure were arranged in cubical order, the edge of each cube would be about 2.9 X I o - ' cms.; the average diameter of a molecule in air is 2.8X Io - 8 cms.) Further and very important evidence as to the nature of the gaseous state of matter is provided by the experiments of Joule and Kelvin.
C =183,900 cms. per sec.
Thus if a fork Ut 3 = 256 is used, the length of pipe for the fundamental at o° C. is about 33,000/4X256 =33 cms. If a fork Solo= 768 is used the pipe resounds to it according to the mode of the first overtone.
Perot and C. Fabry, employing their interferometer methods, have compared the wave-length of the red cadmium line with the standard metre in Paris and found it to be equal to 6438.4696 A, the observations being taken in dry air at 18° C and at a pressure of 76 cms. (g = 980.665).
The disks were 10 cms. in diam., and nearly 2 cms. thick, plated with copper to a thickness of 2 mm.
The chief difficulty, as usual, was the determination of the gradient, which depended on a difference of potential of the order of 20 microvolts between two junctions inserted in small holes 2 cms. apart in a bar 1 .
5' cms. in diameter.
The Wire Had A Length Of 760 Cms., And The Potential Difference On Its Terminals Was Nearly 30 Volts.