TIN-FOIL-LEAD-LEAF-SHEET-LEAD.

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ground, and are frequently more convenient than basins for the transference of jars containing gas, especially from shallow portions of fluid or in confined situations (754). They serve as covers to glasses and jars; as vessels for the evaporation and crystallization of small portions of fluids or solutions (575), and as insulators in electrical or electro-chemical arrangements. In the latter, drops of the fluids to be decomposed should be put upon them (1047), or the metallic vessel containing the fluid should be placed on the glass plate. They are often very valuable in testing minute quantities; the smallest quantity of a re-agent may be added to a drop of a solution placed on such a plate, and from the transparency of the plate, and the different positions in which the tested matter may be held, the appearances may be observed to the greatest advantage.

5. Uses of Leaf and Sheet-Metals.

1349. Tin foil is very useful in the laboratory as a conductor of electricity for the purpose of establishing a metallic communication between the different apparatus standing upon it, and for the purpose of forming metallic linings and coatings to those which, like the Leyden jar, require them. It may readily be cut with scissors or a knife, and is easily applied by means of paste and rubbing with the hand, as in the case of pasted paper. It is highly useful upon certain occasions of refrigeration, particularly when a solid substance requires immersion in the mixture. The metal leaf should be wrapped so closely round the substance to be frozen, as to prevent the penetration of the frigorific mixture; and being a good conductor of heat, the circumstances are then the most favourable for rapid and great diminution of temperature. It is often useful also when wrapped round tubes to darken a certain point, or to cool that part more rapidly. It is a very manageable leaf metal, but should not be handled carelessly, lest it become full of holes, when it is, of course, no longer a water-tight wrapper.

1350. Lead leaf has similar uses. Sheet lead is of considerable service in supplying counterpoises for the balance (39), being readily cut by a knife or scissors. It is of service

600

COPPER PLATE-ZINC LEAF-PLATINUM FOIL.

in heating and cooling bodies by contact; and from the facility with which it yields under the hammer or pressure, may easily be formed into dishes or basins, when those of metal are necessary for particular purposes; or into vessels for freezing, and into other temporary apparatus, when better cannot be had.

1351. Copper plate is of great service as an electromotor, and, in conjunction with the metal zinc, is of continual use in Voltaic electricity. Slips of it are required also in the laboratory for the precipitation of certain metals. It is soft when annealed, and is then easily bent into temporary metallic vessels. Copper foil has been already referred to as of the greatest service when wrapped round glass tubes (716), both in strengthening and conducting the heat uniformly over them. It is often used in the same manner, on a smaller scale, with tubes closed at one end and held by the hand. Copper leaf, as it is usually called, is a particular kind of brass, which, being extended very greatly, answers well for observing, in a general manner, the effect of agents upon a metal in a finely divided state.

1352. Zinc plate and leaf. Zinc rendered malleable, and rolled out into plate and leaf, is very useful in both forms. Plate or sheet zinc is a very powerful electromotor, and with sheet copper, as above mentioned, enters into the construction of Voltaic apparatus. A temporary instrument may be formed in a few minutes with these two metals and a few discs of flannel. Plates of sheet zinc are often required for the precipitation of metals. The foil, being thinner than the plate, answers for similar occasions when a smaller quantity of metal than that in the sheet is required, and, being lighter, has on that account partial superiority. From its thinness, also, it is highly advantageous in exhibiting chemical action, mechanical separation being carried in it to a considerable extent. It always comes from the rolling mills covered with a coat of oil, from which it should be freed by washing with a little soap or alkali, before it is used in chemical experiments.

1353. Platinum foil. Many of the services of this substance have already been referred to (204, 236, 1038, 1048,

GALVANIC PRECIPITATION-WINDSOR BRICKS.

601

1051). In all experiments in which it is used to support other substances at high temperatures, the fusible metals, or even their oxides, sulphurets, and other compounds, when mixed with carbonaceous matters, should be kept from contact with it; for the metal alloying with the platinum forms a fusible combination, and the foil is destroyed or rendered impure. It is very useful as an electromotor. A piece of zinc and a piece of platinum foil in contact, when put into a solution, will separate many metals from it if present: the metal passes to the platinum, and may afterwards easily be removed. An application of this kind, made by Dr Wollaston, has been already referred to (523, 1065). It is not difficult, upon occasions of necessity, so to fold up a piece of platinum foil as to make a vessel of it capable of retaining fluids, and in that state it may serve the purpose of a platinum crucible so far as to allow the performance of an analysis, which could not otherwise have been effected.

6. Uses of Soft or Windsor brick.

1354. This brick is easily cut by a jagged knife or saw into numerous useful forms. It assists by juxtaposition in building up small charcoal furnaces, allowing of the formation of apertures through the brick itself. It is easily shaped into stoppers for furnace apertures (161), or into covers, plugs, and supports for crucibles; or into wedges, to be applied where necessary about furnaces or rigid apparatus. Its softness, in which it much surpasses ordinary brick or stone, is a considerable advantage, especially when it is used in contact with glass. These bricks are also very useful as supports upon the tables for hot apparatus. Furnaces, red-hot crucibles, or heated iron plates, may be supported on them very well and steadily, the heat transmitted through the brick being insufficient to do injury (197). They were first brought into notice for these uses I believe by Mr C. Aikin.

7. Conduction of heat.

1355. Heat may frequently be conducted by means of a solid mass of metal, to places into which it could not so conveniently have been introduced by other means, and the chem

602

CONDUCTION OF HEAT-FURNACE guarded.

ist will sometimes find his operations facilitated by such a contrivance. Sir Everard Home wished to coagulate the blood within an aneurismal tumour without disturbing the tumour or the neighbouring parts. This he effected by passing a needle through the place, and then heating it on the exterior; the heat conducted to the fluid within was sufficient to cause its coagulation. A similar instance is now very common in the structure of certain lamps; in which cocoa-nut oil in a reservoir is preserved in the fluid state by a metallic rod, one end of which enters the oil, whilst the other projects over a flame several inches off.

1356. In operations with a tube and a spirit lamp, particular parts of the former may be heated and cooled very conveniently by means of the conducting power of metals. The manner in which uniformity of temperature may be insured at the lower part of a tube when heated in the flame, by enveloping it in copper foil, has been already described (1351). If on the contrary it be required to cool a particular portion of the upper part of a tube or other vessel (102), for the purpose of more effectually condensing the vapour at that place, the tube may be wrapped round with metal foil, which is to be placed in contact with thicker metal, as sheet lead, or cooled by touching it with wetted paper. The envelope of foil may be moistened and cooled, when, if the glass itself were similarly treated, it would immediately fly to pieces. A heated basin, which, with its contents, requires to be cooled rapidly, may be placed in water, or upon the mercury of the pneumatic trough: the heat will be rapidly abstracted. When the end of a tube in which a substance has been submitted to heat will ultimately require to be broken for examination, it may be cooled and broken at the same moment, by plunging it whilst hot into the mercury of the trough.

1357. Bricks, tiles, and other bad conductors, are very usefully interposed at times between the laboratory table and hot crucible furnaces, to prevent the passage of heat to the former in an injurious degree (197, 388); and in many other situations a bad conductor is of service in a similar

HEAT-REFLECTING-RECEIVING-RADIATING.

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way, by retaining the heat within its original or intended limits.

1358. A very useful indication of the conducting power possessed by different substances for heat, is obtained by putting them in contact with the upper lip, or that part of the cheek which is near to the mouth. These places are highly sensible to changes of temperature; and as the substance which conducts best abstracts the largest quantity of heat in a given time, it will of course feel the coldest, i. e. supposing all the substances are at the same temperature, and several degrees below the temperature of the skin. In this way many differences of conducting power may be observed. Another method is to hold one end or side of a piece of the substance between the fingers, and to apply the other to the flame; the difference between wires of silver and platinum is thus easily distinguished, and also that between various stones, diamond, and glass. The substance which best conducts heat will first feel hot to the fingers.

8. Uses of reflective and receptive powers.

1359. Clean polished metallic surfaces receive heat by means of radiation (i. e. from a hot body not in contact with them, but at a distance) with great difficulty; and if made hot, it is with equal difficulty that they throw off their heat by radiation into space or to other bodies. Hence if a bright metallic vessel be placed before a strong fire, it will receive heat but slowly; or if it be filled with any hot substance and set in a cool place, it will be a long time before it will become cold by mere radiation. On the contrary, if the surface of the vessel be covered with a thin coat of lamp-black, varnish, paper, or any substance not metallic, its power of receiving and sending off radiant heat is greatly increased. Such a vessel will soon become hot before a fire, or if heated, will soon cool to common temperatures by radiation. These are facts which the student will gain from the most elementary treatises on chemistry, and which may frequently be applied with facility to useful purposes. If a crucible furnace (158) require to be placed so near the sides of the pneumatic trough, or any other piece of apparatus, as to en