TIME OF DESCENT, the technical term for the time employed by a material particle in falling down an arc of a curve under the action I gravity, the mode of obtaining which is explained in VELOCITY. When any number of curves are drawn from a given point, and another I curve is so drawn as to cut off from every one of them an arc which is described by a falling particle in one given time, that curve is called eautoehronous or a tautochron. But when a curve is such as the cycloid, namely, that a particle, wherever placed, will fall to the lowest point in the same time, such a curve is also called tautochronous by various writers, and isochronous by others. Our only object in insert ing this article has been to note this confusion of language.
TIN (Sn). One of the first known metals. Its early history is intimately associated with that of our own country, for three thousand years ago the Phcenicians regularly visited the southern shores of this island, and carried on smelting operations for tin.
The alchemists called tin Jove, or Jupiter, and gave it the sign 14, indicating that they thought it to be one half silver and the other half acrimony.
Tho chief ore of tin is the binoxide. [TIN, in NAT. HIST. Div.) It is roasted to peroxidise iron, and volatilise arsenic and sulphur ; washed, to remove sulphate of copper and the oxide of iron ; reduced to the metallic state by charcoal ; and refined by fractional fusion.
The equivalent of tin is 59.
Tin has a high metallic lustre ; its colour is eilver-white with a tinge of yellow ; in tenacity it is exceeded by several of the metals, but is very ductile, and, as tinfoil, is met with in plates varying from 1-250th to 1-1000th of an inch in thickness. It suffers but little change on ex posure to the atmosphere, and is unaffected by moisture. Tin is inelastic but very flexible, and when bent produces a peculiar crackling noise, termed the "cry" of tin ; the latter effect, as well as the heat rendered sensible during flexion, is due to the mechanical alteration in position, and consequent friction, of the particles of the metal. When rubbed, tin imparts to the fingers a peculiar and somewhat persistent odour. It is a good conductor of heat and electricity; has a specific
gravity of 1'292; melts when heated to 442° Fahr., and at a higher temperature burns with a brilliant white light. Tin, like iron, remains in a passive state when plunged into very strong nitric acid; on diluting with water, however, the metal is violently acted on, much nitrous vapour is evolved, and hydrated binoxide of tin is formed. Hydrochloric acid dissolves it with evolution of hydrogen ; aqua regia converts it into bieldoride; cold sulphuric acid does not affect it, but when hot converts it into sulphate, sulphurous acid escaping. The fused alkalies also oxidise tin, hydrogen being given off. At ordinary temperatures tin is not brittle, but when heated to near the fusing point it may be powdered with facility : the operation is most con veniently performed by pouring the melted metal into a wooden box and well shaking for a few minutes.
Oxygen and Tin combine in three proportions, forming Protoxide of Tin (SnO) cannot be procured perfectly pure by direct action : the beet method of preparing it is to evaporate a solution of protochlorkle of tin carefully to dryness, and then triturate it in a mortar with excess of crystallised carbonate of soda, which decom poses the chloride, and leaves the protoxide of tin. When this has been washed and dried carefully, it is of a fine bluish-black colour, is very soluble in hydrochloric acid, and when heated in the air takes fire, burns, and is converted into peroxide. The density of protoxido of tin is 6136 ; it is soluble in solution of potash and soda, but not in ammonia, nor do the alkaline carbonates dissolve it. The alkaline solutions of this oxide gradually deposit metallic tin, and peroxide remains in solution. Its salts very quickly absorb oxygen from the air, and form compounds which readily yield oxygen ; and it is on this account that it converts the sesquioxide of iron into protoside, and procipitates silver, mercury, and platinum in their nictallic state. With gold, a purple compound is formed, known by the mane of the purple powder of eissius.