into twenty parts, and every other division numbered, from 0 to 10. A series of nine weights are furnished with the instrument, numbered from 10 to 90; these are to be added to the weight at the bottom to cause the hydrometer to sink, so that a reading may be had on the graduated scale; this reading added to the number on the weight employed, gives a figure which indicates the strength of the spirit by referring to a table which accompanies the instrument. Jones's hydrometer is similar to Sikes's, but by many is regarded as an improvement on it. Dica's hydrometer belongs to the same class. Twaddell's hydrometer is frequently employed in England, and technical works often quote the degrees of this scale. It is used for liquids heavier than water, and is graduated so that the number of the degree, multiplied by 5 and added to 1000, gives the specific gravity: thus, 20° Twaddell indicates the specific gravity of 1100 or 1.100; 50° Twaddell, 1250 or 1.250. Beck's hydrometer is rarely used or referred to: in this scale O corresponds to the specific gravity 1.00, and 30 to that of 0.850; the scale is extended equally above and below 0. For tables, see Bayley's Chemist's Pocket-Book, p. 178. Zanetti's hydrometers have a scale which requires the addition of a cipher to the number of the degree to show the specific gravity. 2. Hydrometers in which the depth of immersion is constant, but the weight subject to change. FIG. 53. Fahrenheit's hydrometer was one of the first instruments of this class to come into general use. Robert Boyle described, however, in 1675, his "New Essay Instrument," and Fahrenheit's hydrometer was very similar to it in principle; it had but a single mark on the stem, which was surmounted by a small scale-pan; weights were placed in the pan to cause the hydrometer to sink to the mark. Now, as this mark indicated the point at which the instrument would float in water at a given temperature when certain weights were placed on the pan, it follows that when it was immersed in a liquid of different specific gravity the weights would have to be changed to float the instrument to the fixed mark; the ratio which this weight bore to the weight used for water gave the specific gravity. Nicholson's hydrometer is similar in principle to Fahrenheit's, but is modified so that it can be used for taking the specific gravity of heavy or light solids. Fig. 53 is an illustration of one of the most convenient forms of the instrument; it is usually made of brass; there is a single mark on the stem and a scale-pan on the summit. To the lower extremity of the hydrometer two conical cups are attached; their apexes are joined so as to resemble an hour-glass; the lowest cone has several apertures at the top, to permit Nicholson's hydrometer. of the escape of air when the instrument is immersed. The weight of the hydrometer is usually so adjusted that a 1000-grain weight is needed on the scale-pan to float it to the mark on the stem. Now, to take the specific gravity of a piece of zinc weighing less than 1000 grains, the 1000-grain weight is removed from the pan and the piece of zinc substituted for it, weights are added until the instrument floats at the mark on the stem, and it is found that an addition of 655 grains has been necessary it is evident that the difference between 1000 and 655 gives the weight in air of the zinc, 345 grains. The zinc is now placed in the upper conical cup and weights are again placed upon the scale-pan, and it is found that the zinc has lost in weight 50 grains by immersion in water; the specific gravity is obtained by applying the well-known rule, divide the weight of the body by the loss of weight in water : =6.9, sp. gr. of zinc. The lower cup is used for taking the specific gravity of bodies lighter than water, and is very convenient, the weight of the hydrometer keeping the light body submerged when the lower cup is placed over it: the specific gravity is obtained in the same manner as in the case of bodies heavier than water. One of the advantages of Nicholson's hydrometer is that it can be used in place of a balance for weighing small quantities, as shown above. 345 50 = METHODS OF TAKING THE SPECIFIC GRAVITY OF SMALL QUANTITIES OF LIQUIDS. Mohr's Apparatus.-The illustration of this apparatus (see Fig. 54) represents an improved form, yet it is quite possible for a pharmacist to construct one for himself that will answer practical purposes. It will be noticed that one end of the beam is divided into ten equal spaces, and a small glass thermometer is suspended from the extremity by a slender platinum wire, whilst the opposite scale-pan is so adjusted that it exactly counterbalances the thermometer. When the thermometer is immersed in pure water at 15° C. (59° F.), a brass wire weight is placed upon the hook at the end of the beam, and this restores the equilibrium. Now, it is apparent that if a lighter liquid, like alcohol at 15° C. (59° F.), is substituted for the water, the equilibrium cannot be maintained, and the thermometer will sink: the brass wire weight is then to be moved along the beam towards the central knife-edge until the balance is nearly restored, and this point will be found at 8, which gives the first decimal figure; still further to approach equilibrium, a wire weight, one-tenth the weight of the larger one, is pushed along the beam until it rests at the 2-mark, which gives the second decimal figure; whilst thoroughly to restore the balance the smallest weight (still onetenth smaller) is placed at 5, and thus the third decimal figure is obtained, and the specific gravity of the alcohol is shown to be 0.825. Specific gravities of liquids heavier than water are obtained in the same way, except that the large brass-wire weight is left hanging on the hook at the end of the beam and additional weights are placed upon the beam until equilibrium is restored. A prescription balance could be easily converted into a Mohr's appa ratus, and the thermometer replaced by a glass stopper suspended by a horse-hair. The thermometer in the improved form of apparatus merely serves to indicate the temperature and act as a convenient weight: in the homemade apparatus especial care must be exercised in adjusting the wire-hook weight so as exactly to immerse the stopper in water at the proper temperature. Gannal's Method.-Gannal suggested a very convenient modification of this method of taking the specific gravity of a liquid. A piece of glass, "densimètre hydrostatique," having the shape of an olive, has a volume of 10 cubic centimetres. This is suspended from the hook at the end of the beam of a balance by a horse-hair (see Fig. 41), and weights are added to the opposite scalepan until the balance is restored; it is then immersed in the liquid, and the metric weight required to restore FIG. 54. จ Mohr's specific-gravity apparatus. FIG. 55. the equilibrium gives the specific gravity without a calculation. Specific-Gravity Pipette.-Grauer recommends the use of a small pipette having a fine orifice at the lower end, and at the upper end a short piece of rubber tube closed by a pinchcock; a mark is scratched on the pipette to show where a convenient weight of water rises to (1 C.c.); enough of the liquid to be tested is sucked through the tube to rise to the mark, it is then closed; the weight of the liquid indicates its specific gravity. Rousseau's Densimeter.-This ingenious instrument is constructed upon the plan of a hydrometer (see Fig. 55). The stem from B to C is divided into 20 equal parts; the cupshaped tube upon the summit of the stem holds exactly 1 cubic centimetre. When the densimeter is floated in water at the proper temperature, it sinks to the point C at the bottom of the stem; when the cup is filled with water to the cubiccentimetre mark, it causes the instrument to sink to the point B; this space, B C, being then divided into 20 equal parts, it follows that each division corresponds to of a gramme, or 0.05 Gm. If one cubic centimetre of oil of rose were placed in the cup, it would sink the densimeter to 17.2 divisions of the scale; then 17.2 × 0.05–0.860, sp. gr. of oil of rose. Rousseau's den. simeter. |