14 in which liquids can without loss be brought to room-temperature (or even 25 C., 77° F.) for weighing (see Fig. 30). As seen in the illustration, the bottles may be made of such size as to hold any desired weight of distilled water. They are provided with a long, narrow tube stopper graduated into millimeters from 0 to 50 or 100, to which is attached a safety reservoir fitted with a ground-glass stopper. The capacity of the bottles is so adjusted that the prescribed weight of recently boiled distilled water will reach to the O mark, or a little above it, when the bottle and contents have been kept in a bath of melting ice at 0° C. (32° F.) for fifteen minutes or until the volume ceases to recede. The height to which this same weight of distilled water will rise in the graduated tube at any higher temperature can be readily ascertained by immersing the bottle and contents in a water-bath kept at the desired temperature until the column ceases to rise. By keeping a memorandum of the height of the column in 4-millimeter divisions to which the prescribed weight of distilled water will rise at any stated temperature, an equal volume of any other liquid at the same temperature may readily be obtained, accurate adjustment being made by means of very narrow strips of blotting-board passed down the bore of the graduated stem for the purpose of absorbing and removing minute quantities of liquid. Having found the weight of such a volume of any liquid, the specific gravity of that liquid, as compared with water at the same temperature, can be quickly ascertained by dividing the weight found by the prescribed weight of water. ; Since glass bottles contract appreciably for two years or more after they have been made, the graduations should be verified every six months or more until contraction has ceased, a memorandum of the changes being kept for reference when the bottle is to be used thus the point for the volume at 4° C. may have advanced 2 or 3 divisions of the scale, and similarly for any temperature volume. The bottles are always used in a bath of either warmed or cooled water, and when the volume does not change for five minutes, as indicated by the graduated scale, the contents of the bottle may be known to have assumed the temperature of the bath as ascertained by means of a delicate thermometer. A leaden collar is used to keep the bottles steady in the bath. Besides taking the specific gravity of liquids by means of a pycnometer, accurate results may also be obtained with the so-called loaded cylinder. Its use is based on the law formulated by Archimedes, a Greek philosopher, that all bodies immersed in a liquid are buoyed up with a force equal to the weight of the liquid displaced by them, and thus appear to lose weight. For instance, a piece of metal the size of 1 cubic inch, when immersed in water, will exert as much less pressure on the bottom of the container as will equal the weight of 1 cubic inch of water-a fraction over 252 grains -and hence will weigh 252 grains less in water than in air. Float ing bodies always displace their own weight of water irrespective of their volume, while immersed bodies always displace their own volume of water irrespective of their weight. All bodies, therefore, which weigh less than an equal volume of water are sure to float in that liquid, only so much of the body being immersed as corresponds in volume to a weight of water equal to the weight of the whole body; on the other hand, all bodies weighing more than an equal volume of water must sink in that liquid and be completely immersed, as the downward pressure of the body exceeds the upward pressure or buoyant force of an equal volume of water. FIG. 31. FIG. 32. The loaded cylinder, as shown in Fig. 31, consists of a glass tube partly filled with mercury, and sealed at the top, to which is affixed a hook for convenient suspension to a scale beam. Having weighed the cylinder in air and then in pure water, at any given temperature, the weight of an equal volume of water is ascertained by subtracting the weight in water from the weight in air; the cylinder is then weighed in any desired liquid at the same temperature as the water, and the loss in weight again noted, which is the weight of an equal volume of that liquid. The volume of the liquid to be tested, being equal to the volume of the cylinder, must be equal to the volume of water also, for things that are equal to the same thing are equal to each other; by dividing the weight of the given volume of the liquid by the weight of the same volume of water, the specific gravity of the liquid is obtained. Example: A loaded cylinder weighs in air 150 grains, and in water 120 grains, loss of weight in water 30 grains; immersed in sulphuric acid it weighs 96 grains, showing a loss of 54 grains; equal volumes of the acid and water weighing 54 and 30 cylinder. grains respectively, the specific gravity of the acid must be 1.800, for 5430=1.8. Loaded Glass or metal plummet. When only a small quantity of liquid is available for taking the specific gravity the loaded cylinder may be replaced by a small glass or platinum weight of the shape shown in Fig. 32; or Grauer's method may be followed. This consists in using a small pipette having a fine orifice at one end, and at the upper end a short piece of rubber tubing closed by a pinchcock; a mark is made on the glass stem, showing the height to which a convenient quantity of water rises (say 1.0 Gm. or 1.0 Cc.), and enough of the liquid to be tested is drawn up through the tube to the mark previously made, the tube is closed, and the whole then weighed; the weight of the liquid in grammes expresses the specific gravity with sufficient accuracy for all practical purposes, as water increases its volume from 4° to 100° C. only to the extent of 0.012, or about 4. The principle of the loaded cylinder has been utilized in the construction of the Mohr specific gravity balance, of which the Westphal modification is a most desirable improvement (see Fig. 33). The specific gravity of a liquid can be quickly taken at any temperature between 7° and 30° C., since the loaded cylinder has been replaced by a short glass thermometer, which is suspended from the end of the beam by a thin platinum wire; the adjustment having been made at 15° C., a slight variation will be observed for any higher or lower temperature. The small thermometer has a range of twenty-three degrees on the centigrade scale, and, when suspended in air from the longer arm of the beam, establishes perfect equilibrium; when completely immersed in distilled water at 15° C. it displaces its own volume of the water and is buoyed up by a force equal to the weight of the water displaced-equilibrium of the beam being re-established by attaching the necessary counterpoise, which is called 1.000: at 7.5° C. the necessary weight was found to be 1.001, while at 27° C. it was 0.998. As seen in the illustration, the longer arm of the beam is accurately divided into ten even spaces, and the weights, or riders, used to counterbalance the thermometer when immersed in any liquid, are made of brass and aluminum ; they are so constructed that each smaller rider is of exactly the value of the next larger, the largest rider and the counterpoise used to balance the thermometer in water, however, being of the same weight or value. Without the necessity for calculation, if the temperature of the liquid be at 15° C., the specific gravity of the liquid can be at once read off, after the equilibrium of the beam has been established; for instance, in testing alcohol at 15° C., the counterpoise necessary to balance the beam in water will be found too heavy if attached at the same point in alcohol, hence it is removed and the largest rider is placed in the first, or, if necessary, in the second notch on the beam, where it may appear a little too light, and then the smaller riders are added as may be necessary to balance the beam perfectly. The value of each of the two larger riders, when suspended from the end of the beam, is considered as 1.000, while the three smaller riders are valued at 0.100, 0.010, and 0.001 respectively; when removed to the top of the beam the value of each rider is reduced by for every notch. If one of the large riders be placed at the notch marked 8, a second rider at 2, and a smaller rider at 1, the specific gravity of the alcohol must be read as 0.821. In the case of chloroform and all other liquids specifically heavier than water, the large counterpoise is suspended from the end of the beam, and the other riders are placed in the notches as may be necessary; thus chloroform may require all four riders on the beam, the largest at 4, the second at 8, and the smaller two at 9, which would be read as 1.4899 specific gravity. Whenever two riders of different weight are required in the same notch on the beam, the lighter of the two is suspended from the hook of the heavier, as shown in Fig. 34; thus the specific gravity of liquids can be read with accuracy to four decimal places. The Mohr or Westphal balance cannot be used, however, if only very small quantities of liquid are available, as sufficient liquid is required to immerse the glass thermometer completely. Specific gravity beads, also known as Lovi's beads, are small, sealed, pear-shaped glass bulbs of various specific weights, which have been carefully ascertained and are marked on them; these beads will float indifferently in any liquid having the same specific gravity, and may be used in adjusting liquids to a fixed specific gravity by dilution or evaporation. If a bead marked 0.93 be placed in a jar of alcohol, it will sink-unless the liquid happens to be official diluted alcohol-but will slowly rise upon the addition of water, until a sufficient quantity has been added to increase the specific gravity of the mixture to that indicated on the bead, when it will float about midway in the liquid. Results obtained with specific gravity beads are never so accurate as with other methods. Hydrometers, or areometers, are instruments intended to indicate either the density or the specific gravity of liquids, and in some cases also the percentage by volume or weight of certain liquids. They consist of a glass tube having a bulb blown at one end, a little above which the tube is usually expanded cylindrically for a short distance, and then terminates in a long stem in which is securely fast H The bulb is filled with instrument to assume a Hydrometers, like all ened a graduated paper scale (see Fig. 35). |