78 



KNOWLEDGE. 



[April 1, 1896. 



physical properties of this element, and the native mate- 

 rials from which it is produced. 



Aluminium is one of the most widely distributed of the 

 chemical elements, but, owing to its strong affinity for 

 oxygen, is never found in the free state, but exists in the 

 form of alumina — its only oxide, Al.jOj, which, combined 

 with silica and the alkaline oxides in various proportions, 

 forms ihe chief constituents of most rocks. Boauxite, a 

 hydrated oxide of aluminium, and cryolite, a double fluoride 

 of aluminium and sodium, also exist native in considerable 

 quantities, and being comparatively free from impurities 

 form the more usual source of the metal at the present 

 time. Large deposits of beauxite are found in France — 

 chiefly near the town of Beaux in Styria, in Wocheim, 

 iiud in various places in the North of Ireland; and last year 

 a large factory was erected at Lame for the purpose of 

 purifying the beauxite derived from the Antrim mines 

 before shipment to Scotland. Cryolite exists in almost 

 inexhaustible quantities on the west coast of Greenland, 

 and is shijiped from there to the principal aluminium pro- 

 ducing centres. 



Metallic aluminium has a beautiful white silver lustre, 

 which becomes especially apparent when held against a 

 polished silver plate, as the latter is then seen to have a 

 distinct yellow colour. When impure, aluminium has a 

 grey or bluish colour, the latter being caused by the pre- 

 sence of silicon. A dull matt surface can be obtained by 

 washing the metal in a solution of caustic soda, and then, 

 after being washed with cold water, dipping the plate in a 

 bath of nitric acid. 



It is ductile, malleable, very sonorous, and a good con- 

 ductor of heat and electricity. It is as light as 

 glass or porcelain, having a specific gravity of 

 2-56. It melts at about 700° C, and is not 

 appreciably volatile. The metal is not readily 

 oxidized, and is permanent in air, wet or dry, 

 and, when pure, is not acted upon by water, even 

 at a red heat. Sulphuric and nitric acids in 

 the cold attack it slowly, but hydrochloric acid, 

 especially if warm, dissolves it readily. The purer 

 the metal the less easily is it attacked by acids, but 

 alkalies have a very energetic action on the metal, 

 evolving hydrogen and forming aluminates. 



The question as to its behaviour towards 

 organic acids, such as are to be found in wine 

 and beer, has been carefully studied on behalf of 

 the German army ; but it would seem that they 

 have no effect upon the metal when it is free 

 from impurities and sodium chloride. It easily 

 alloys with other metals, and many of its alloys 

 are of greater utility than the metal itself. 

 Amongst these alloys must be especially men- 

 tioned the aluminium copper alloy (which, under 

 the name of alummium bronze, has been in use 

 for a great number of years), and the newer 

 alloys, woUaminium and romauium, which owe 

 their remarkable physical properties to the pre- 

 sence of tungsten. The mechanical properties 

 ot aluminium are very remarkable. Its modulus 

 of elasticity is ten thousand ; its range of elas- 

 ticity high, a bar being capable of being stretched 

 one two-hundrtdihs of its length before breaking. 

 Its tensile strength is about twelve tons per 

 square inch, and the tenacity of a wire of 

 0-145 millimetres section has been found to be 

 about twenty-six pounds. The length of a bar capable of 

 supporting its own weight is 23-040 feet, i.e., equal to that 

 of ordinary steel and more than double that of bronze. 

 {To be continued.) 



WAVES.-IV. 

 SHIP WAVES, AND THE SOLITAEY WAVE. 



By Vaughan Cornish, M.Sc. 



OUR knowledge of ship waves and of the solitary 

 wave is largely based upon the researches of 

 William Froude, .John Scott Eussell, and Lord 

 Kelvin, and their treatment of the subject forms 

 the groundwork of the present article. 

 When a ship moves through the water, pressure is 

 greatest at bow and stern and least at the sides. The 

 surface of the water accordingly rises at bow and stern 

 and sinks down at the sides, so that the difi'erences of 

 level balance the ditl'erences of pressure created by the 

 motion of the ship. Thus a moving ship is accompanied 

 by waves. Three principal factors determine the form 

 and character of these waves, namely : first, the motion of 

 the ship ; secondly, the motion of each wave formed by the 

 ship ; and, thirdly, the transmission of energy by these 

 waves. As explained in the last article, the energy is 

 transmitted at half the velocity of the wave, consequently 

 a ijroup of waves is formed. In the front of the group the 

 waves continually die out as the energy falls behind and 

 forsakes them ; and, conversely, in the rear of the group, 

 new waves are formed. Consequently, the rear of the 

 procession of waves falls further and further behind the 

 ship, and, as the ship is constantly adding new waves to 

 the front of the procession, the vessel is accompanied by 

 a lengthening track. As the energy of the wave motion 

 proceeds at only half the pace of the waves (which is also 



FiQ. 1. — Steamer on Lake Como, showing Thwart-ship and Echelon Waves. 



the pace of the ship), the rear of the procession moves at 

 one half the rate of the vessel. We have here two distinct 

 velocities : that of the tail, which is the group velocity, and 

 that of the head, which is the wave velocity. If we could 



