WHY THE SKY IS BLUE.

Auckland Star, Volume LVI, Issue 192, 15 August 1925, Page 31

WHY THE SKY IS BLUE.

THE. WOEK OF MOLECULES. Why the sky Is blue was explained by Sir Ernest Butherford (Cavendish Professor of Experimental Physics in the University of Cambridge), and New Zealand's most famous man), In the second lecture of the series on "The Counting of the Atoms," which te delivered at the Royal Institution.

The cause of the blue colour of the sky, ■aid Sir Ernest, had fascination for physicists from Newton onwards. Lord Baylelgb's theory of the blue of the sky waa that lt was caused by the scattering of the light of the sun by the molecules of air in Its path.

Sir Ernest showed by experiment the polarisation, .of light by liquids and gases containing fine particles In suspension, and also the cause, of the red sunset. The present Lord Rayleigh, and afterwards Cabannee, had shown experimentally that this scattering of light by gases could be measured in the laboratory. The Brownian Movement. The incessant movement of minute particles In a liquid was first observed by an Englishman named Brown in 1827, and was now called the "Brownian" movement. It was now known that this constant agitation was a necessary consequence of the invisible and irregular movements of the molecule of the surrounding fluid to be expected on the kinetic theory of matter. For a long time this Brownian motion remained an Isolated observation and merely a little curiosity- It was not until 1880 that the XuH. importance of the discovery became manifest.

Examples, .of the Brownian motion shown by- minute" colloidal particles viewed through a high-power microscope were fflostrate'd -by Sir Ernest by a cinematograph film, the amaing movements of the particles being shown clearly. The behaviour of a large number of equal spheres, said Sir Ernest, obtained from gum arable was carefully examined by Perrin. who showed that they behaved In exactly the same way as an atmosphere of molecules of very great size and weight. Experiments had given a complete verification of the kinetic theory of gases, so that the speed and number of collisions a mole- ■ cule made per second could be estimated ~WitS . certaint^.