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[Nature, Vol. LXXXVI. pp. 416, 417, 1911.]
THE manner in. which aqueous vapour condenses upon ordinarily cleai surfaces of glass or metal is familiar to all. Examination with a magnifie shows that the condensed water is in the form of small lenses, often i: pretty close juxtaposition. The number and thickness of these lenses depen upon the cleanness of the glass and the amount of water deposited. In th days of wet collodion every photographer judged of the success of th cleaning process by the uniformity of the dew deposited from the breath.
Information as to the character of the deposit is obtained by lookin through it at a candle or small gas flame. The diameter of the hal measures the angle at which the drops meet the glass, an angle whic diminishes as the dew evaporates. That the flame is seen at all in goo' definition is a proof that some of the glass is uncovered. Even when bot sides of a plate are dewed the flame is still seen distinctly though wit' much diminished intensity.
The process of formation may be followed to some extent under th microscope, the breath being led through a tube. The first deposit occur very suddenly. As the condensation progresses, the drops grow, and man of the smaller ones coalesce. During evaporation there are two sorts c behaviour. Sometimes the boundaries of the drops contract, leaving th glass bare. In other cases the boundary of a drop remains fixed, while th thickness of the lens diminishes until all that remains is a thin lamim Several successive formations of dew will often take place in what seem to be precisely the same pattern, showing that the local conditions whic determine the situation of the drops have a certain degree of permanence.
An. interesting and easy experiment has been described by Aitke (Proc. Ed. Soc. p. 94, 1893). Clean a glass plate in the usual way until th breath deposits equally.bove holds for aerial vibrations which are symmetrical in all directions about a centre. Thus within the sphere of radius TT it is possible to have a motion which shall be strictly periodic and is such that the condensation is initially arbitrary at all points along the (7). Now if 6 be the angle between the normal to this plane and the radius vector R, r = JR, sin B, and the mean is