IV. Specific gravity of a body lighter than water. — In taking the specific gravity of cork, for example, which is proverbially lighter than water, it is necessary to sink it by means of a piece of lead, copper, iron, or some other heavy body. First weigh the heavy body and the united body separately, both in air and water, and ascertain how much each loses in the water, by subtracting their weight in water from their weight in air, and deducting the less of these remainders from the greater. Then, as This remainder Is to the weight of the light body in air, So is the specific gravity of water To the specific gravity of the body. V. Specific gravity of a body soluble in water.— When the substance to be examined is soluble in water, it is necessary to employ another fluid in which it is insoluble. Alcohol, oil of turpentine, pyroxylic spirit, &c. will generally answer this purpose. We first take the specific gravity of the fluid; we then take the specific gravity of the body, by means of the fluid, and have this proportion : Example. As the specific gravity of water Is to the specific gravity of the fluid, The specific gravity of a body was found in the mass, in alcohol of the specific gravity 8393, to be 1.962: we •8393 x 1.962 have, then, 1.6467= specific gravity in water. VI. Specific gravity of a gas. - In describing the method of taking the specific gravities of solids and liquids, it has been already stated that pure water is taken as the standard of comparison or unity. But in reference to gases the standard is different. Common or atmospheric air is usually employed for this purpose. In taking the specific gravity of a gas there is much greater difficulty in obtaining very accurate results than in any of the previously described problems, in consequence of the influ ence of various atmospheric causes. The experiment must be made in a room where there are no draughts; in short, the air must be perfectly still, and quite equable in temperature. The vessel to be employed for weighing the gases should be blown as light as possible, but should be strong enough to bear the pressure of the atmosphere when exhausted. This may be ensured by boiling the flask in oil. The first object is to ascertain, as in the case of the specific gravity of a liquid (for we must consider air merely as a rarer fluid than water) the weight of the glass flask or vessel. 8 This is effected by screwing it on the air pump; and having ascertained that the junctions are perfectly air-tight, we pump out the air contained in it, and weigh it in this condition. We then connect it with a vessel containing air which has been standing over water, in order to saturate it with vapour, and after waiting for a few minutes till it cools, it is also weighed. Lastly, we pump out the common air, and fill it with the gas to be examined, which also should be saturated with vapour, and weigh it. Fig. 10. represents the mode of filling the flask. We have then the proportion : As the difference between the flask, with and without air, Is to the difference of the flask without air and We have then the proportion 5.71 3 16:1: 553 = specific gravity of the gas. In experimenting with great delicacy, the gases should be dried before being introduced into the flask. This is effected by adjusting a tube filled with chloride of calcium between the source of the gas and the empty flask, so that the gas traversing the column of chloride is completely dried. To prevent the flask from being affected by changes of temperature, it may be suspended in a box retained at a regulated temperature. Further to equilibriate the influence of the atmospheric pressure, Regnault has adopted the method of counterpoising the flask used with another of the same capacity, instead of using merely weights. CHAPTER II. ON THE INTERNAL OR CHEMICAL PROPERTIES OF BODIES. As a knowledge of the results to be obtained by the use of the blow-pipe can only be acquired by a good deal of practice, the student should provide himself with a blow-pipe (which may be had very cheap when made of tin), and practise frequently, so as to be enabled to use the subsequent tables. The cheapest form 9 1 2 In of blow-pipe is No. 1, introduced charcoal to be used should be well burned and free from fissures; a small cavity being made by means of the point of a knife, to retain the substance and fluxes. Action of the blow-pipe upon bodies. It has been already stated that, by the influence of heat we are enabled to obtain much knowledge of the nature of many bodies. But the remarks already offered only refer to comparatively low degrees of heat, and such as can be communicated by a gas or spirit-lamp, when the substance is placed on a spatula held over it. When greater degrees of heat are to be applied it is necessary to use the blow-pipe, whether of glass or metal, bent at a right angle, and employed so to propel the flame of the lamp upon the body to be examined, as to enable the operator to observe distinctly the slightest alteration produced upon the body by the continued high temperature. When we propel a flame by means of a blow-pipe, there are two portions of the flame which produce different effects, and therefore require particular attention. The flame of a common candle consists of an external red, and an interior blue or dark, cone. A flame is simply a hollow cone of light, consisting of gases, which merely burn at the surface, where they are in contact with the oxygen of the atmosphere. The point a is obviously in contact with the air, while the point b is farther removed from it. But when we blow through a flame the circumstances are altered : b in the first figure corresponds with b in the second, as in both 10 unburned gases exist. In the second figure a is the oxidizing and b the re ducing flame. To reduce an oxide to the state of metal, that is, to cause the oxygen of the oxide to combine with the flame, we place the body in the reducing flame. To oxidize In exposing a body to the a metal, so as to cause the metal to combine with oxygen, we place the substance in the oxidizing flame. action of the blow-pipe we first ascertain the effect (per se) by itself, simply by holding the body in the flame by means of a convenient instrument, either a thin platinum wire twisted round the substance or a pair of forceps tipped with platinum. Action upon bodies per se. In making this experiment a long thin fragment of the body is fixed between the points of a pair of forceps, and held in the reducing flame. This method is particularly applicable to minerals, many of which have peculiarities that assist us in deciding on their species. A certain set of minerals, amounting to about thirty-six in number, are characterised by their frothing before the blowpipe, and hence they have been termed zeolites (from (ew, I boil). They are well typified by Thomsonite. They contain from ten to twenty per cent. of water, and when heated they part with the water, and, boiling up during the process, either fuse into a white bead or fall to powder. (For other characters of minerals, see Mineralogy, vol. i.) I. Colours produced by bodies per se. with the exception of platinum, palladium, iridium, rhodium, and osmium, are almost all capable of being melted before the blowpipe on charcoal. Fusible oxides. · The oxides are mostly infusible, with the following exceptions: Volatile metals.—The following metals sublime, leaving a powder on the charcoal : Mercury and osmium volatilize, without producing a sublimate. Reducible bodies.-Most metallic oxides are reduced to the metallic state on charcoal. The exceptions are, the alkalies, earths, cerium, uranium, manganese, iron, vanadium, chromium, tungsten, columbium, titanium. IV. Blowpipe action with carbonate of soda. a bead on platinum wire in oxidating flame. Colourless glass Bodies fusing into Manganese oxides Green Silica Antimony, oxide and acids Tellurium, oxide and acid Chromium oxide, Orange in oxi Red, and then dating, green in reducing Molybdic acid Milky Tungstic acid Yellowish Titanic acid and opaque when cold |