TLA 



TLA 



are two plates of iron on the cheeks of a 

 gun carriage, from the cape square to 

 the centre, through which the prize- 

 bolts go, and on which the handspike 

 rests when it poises up the breech of the 

 piece. Breast-plates are the two plates 

 on the face of the carriage, one on each 

 cheek. Train-plates are the two plates 

 on the cheeks, at the train of the car- 

 riage. Dulidge- plates are the six plates 

 on the wheel of a gun-carriage, where 

 the felloes are joined together, and serve 

 to strengthen the dulidges. 



PLATFORM, in the military art, an 

 elevation of earth, on which cannon is 

 placed, to fire on the enemy; such are 

 the mounts in the middle of curtins. On 

 the rampart there is always a platform, 

 where the cannon are mounted. It is 

 made by tlie heaping up of earth on the 

 ramparf, or by an arrangement of madri- 

 ers, rising insensibly, for the cannon to 

 roll on, either in a casemate, or on attack 

 in the out-works. 



^ All practitioners are agreed, that no 

 soot can be depended on, unless the 

 pief e can be placed on a solid platform ; 

 for if the platform shakes with the first 

 impulse of the powder, the piece must 

 likewise shake, which will alter its direc- 

 tion, and render the shot uncertain. 



PLATFORM, in architecture, is a row of 

 beams, which support the timber-work 

 of a roof, and lie on the top of the wall, 

 where the entablature ought to be raised. 

 This term is also used for a kind of ter- 

 race, or broad, smooth, open walk at the 

 top of a building, from whence a fair 

 prospect may be taken of the adjacent 

 country. Hence an edifice is said to be 

 covered with a platform, when it is flat 

 at top, and has no ridge. Most of the 

 oriental buildings are thus covered, as 

 were all tiiose of the ancients. 



PL.V1TNA, or PLATIXTTM, a metal, 

 which in most of its properties is equal to 

 gold, but in others it is very superior. It 

 v. as first ascertained to be a distinct, me- 

 tal by Schetfer, a Swedish chemist, in the 

 year 1752. By him it was named white 

 gold, because it resembled tins metal in 

 many of its properties. It immediately 

 became subject to the experiments of ail 

 the chemists in Europe, and obtained, 

 from its colour, the name of platina, sig- 

 nifying little silver, from the word plata, 

 which is Spanish for silver. Platina has 

 been found among the gold ores of South 

 America, and more particularly in the 

 mine of Santa Fe near Garth age r.rs, and 

 in the district of Choco in Peru. Phitina, 

 iu the state in which it reaches this coun- 



try, is contaminated by the presi 

 several other metals, as iridium, osmium, 

 rhodium, and palladium, and, in fact, it is 

 merely an ore of platina. It is in the 

 form of small grains or scales, of a whiter 

 colour than iron, and extremely heavy. 

 Various processes have been contrived 

 for its purification ; but the one, which is 

 the most simple and practicable, is de- 

 scribed in the ninth volume of Nichol- 

 son's Journal. Platina has the following 

 properties. It is a white metal, resem- 

 bling silver in colour, but greatly ex- 

 ceeding it, and indeed all other metals, in 

 specific gravity, being, when it is ham- 

 mered, twenty .three or twenty-four times 

 heavier than water. It is not oxydized 

 by the long continued and concurrent ac- 

 tion of heat and air. It has the property 

 of welding, which belongs to no other 

 metal but this and iron. It is not acted 

 on by any other acid than the nitre-mu- 

 riatic and oxygenized muriatic. The for- 

 mer is best adapted to effect this solu- 

 tion. Sixteen parts of the compound 

 acid are to be poured on one of the lami- 

 nated metal, and exposed to heat in a 

 glass vessel ; nitrous gas is disengaged, 

 and a reddish coloured solution is obtain- 

 ed, which gives a brown stain to the 

 skin. The muriate of platina has the 

 characteristic property of being precipi- 

 tated by a solution of muriate of ammonia. 

 By this character, platina is distinguished 

 from all other metals, and may be sepa- 

 rated when mingled with them in solu- 

 tion. The precipitate, thus obtained, is 

 decomposed by a strong heat, and leaves 

 pure platina. When pure potash is pour- 

 ed into the muriatic solution, a precipi- 

 tate ensues, which is not an oxide of pla- 

 tina, but a triple compound of that oxide 

 with the alkali and acid. With soda, also, 

 it forms a triple combination. 



Plalina is acted upon by fusion with 

 nitrate of potash, and also with pure fix- 

 ed alkalies. The most delicate test of 

 the presence of platina is muriate of tin. 

 A solution of platina, so dilute as to be 

 scarcely distinguishable from water, as- 

 sumes a bright red colour, on the addi- 

 tion of a single drop of the recent solu- 

 tion of tin. 



Platina has been discovered by Dr. 

 Wollaston to be a remarkably slow con- 

 ductor of caloric. When equal pieces of 

 silver, copper, and platina, were covered 

 with wax, and heated at one end, the wax 

 was melted 3| inches on the silver ; 2j 

 on the copper; and one inch only on the 

 platina. Its expansion by heat is consi- 

 derably less than that of steel ; which, 



