Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/555

539 HTDEQMETEH 539 c FIG. 5. having been employed by the latter to construct his instruments for the French revenue. The point at which the instrument floated iu distilled water was marked 10 by Cartier, and 30 U on Carder s scale corresponded to 32 on Beaume s. Though constructed upon a very different principle from ordinary hydrometers, we may briefly refer here to Brewster s capillary hy drometer or staktometer, which is based upon the difference in the surface tension and density of pure water, and of mixtures of alcohol and water in varying proportions. If a small piece of paper be bent into such a shape as this, _/_, it maybe made to rest upon the surface of water without being immersed. If now a drop of alcohol be placed on one corner of the paper, it will rush violently away and generally spin round, somewhat resembling the action of pieces of camphor, indicating that the surface tension of the water is dimin ished by the presence of alcohol. If the water be very pure it is suffi cient to bring a drop of alcohol on the extremity of a pipette near to the paper, without touching either it or the water, and the vapour absorbed will produce the same effect. For other proofs of the same action see the article CAPILLARY ACTION. Now, if a drop of water be allowed to form at the extremity of a fine tube, it will go on increas ing until its weight overcomes the surface tension by which it clings to the tube, and then it will fall. Hence any impurity which diminishes the surface tension of the water will diminish the size of the drop (unless the density is proportionately diminished). Now, according to Quiiicke, the surface tension of pure water in con tact with air at 20 C. is 81 dynes per linear centimetre, while that of alcohol is only 25 5 dynes. Also, a small percentage of alcohol produces much more than a pro portional decrease in the surface tension when added, , to pure water. The capillary hydrometer consists Jji simply of a small pipette with a bulb in the middle of the stem, the pipette terminating in a very fine capillary point. The instrument being filled with distilled water, the number of drops required to empty the bulb and portions of the stem between two marks m andn (fig. 5) on the latter is carefully counted, and the experiments repeated at different temperatures. The pipette having been carefully dried, the process is repeated with pure alcohol or with proof spirits, and the strength of any admixture of water and spirits is determined from the , Corresponding number of drops, but the formula gene- Brewster s rally given is not based upon sound data. Sir David Btakto- Brewster found with one of these instruments that the meter, number of drops of pure water was 734, while of proof spirit, sp. gr. 920, the number of drops required was 2117. Perhaps the main object for which hydrometers have been con structed is the determination of the value of spirituous liquors, chiefly for revenue purposes. To this end an immense variety of hydrometers have been con structed, differing mainly in the character of their scales. In Speer s hydrometer the stem has the form of an octagonal prism, and upon each of the eight faces a scale is engraved, indicating the percentage strength of the spirit corresponding to the several divisions, of the scale, the eight scales being adapted respectively to the temperatures 35, 40, 45, 50, 55, 60, 65, and 70 F. Four small pins, which can be inserted into the counterpoise of the instrument, serve to adapt the instrument to the temperatures intermediate between those for which the scales are constructed. William Speer was supervisor and chief assayer of spirits in the port of Dublin. For a more complete ac count of this instrument see Tilloch s Phil. Mag., vol. xiv. p. 151. The hydrometer constructed by Mr Jones of Holborn, consists of a spheroidal bulb with a rect angular stem (fig. 6). Between the bulb and counterpoise is placed a thermometer, which serves to indicate the temperature of the liquid, and the instrument is provided with three weights which can be attached to the top of the stem. On the four sides of the stem AD are engraved four scales corresponding respectively to the unloaded instru ment, and to the instrument loaded with the re- U u spective weights. The instrument when unloaded FlP fi __T nT, A&amp;lt; .&amp;gt; a serves for the range from 74 to 47 above proof; , ? m ,? r when loaded with the first weight it indicates from J 46 to 13 over proof, with the second weight from 13 over proof to 29 under proof, and with the third from 29 under proof to pure water, the graduation corresponding to which is marked W at the bottom of the fourth scale. One side of the stein AD is shown in fig. 6, the other three in fig. 7. The thermometer is also provided with four scales corresponding to the scales above mentioned. Each scale has its zero in the middle corresponding to 60 F. If Fig. 7. the mercury in the thermometer stand above this zero the spirit must be reckoned weaker than the hydrometer indicates by the number on the thermometer scale level with the top of the mercury, while if the thermometer indicate a tem perature lower than the zero of the scale (60 F.) the spirit must be reckoned stronger by the scale reading. At the side of each of the four scales on the stem of the hydrometer is engraved a set of small numbers indicating the contraction in volume which would be experienced if the requisite amount of water (or spirit) were added to bring the sample tested to the proof strength. The hydrometer constructed by Mr Dicas of Liverpool is provided with a sliding scale which can be adjusted for different temperatures, and which also indicates the contraction in volume incident on bringing the spirit to proof strength. It is provided with thirty-six different weights which, with the ten divisions on the stem, form a scale from to 370. The employment of so many weights renders the instrument ill-adapted for practical work where speed is an object. It was adopted for the United States revenue by Act of Congress, August 10, 1790. Quin s universal hydrometer is described in the Transactions of the Society of Arts, vol. viii. p. 98. It is provided with a sliding rule to adapt it to different temperatures, and has four scales, one of which is graduated for spirits and the other three serve to show the strengths of worts. The peculiarity of the instrument con sists in the pyramidal form given to the stem, which renders the scale-divisions more nearly equal in length than they would be on a prismatic stem. Atkins s hydrometer, as originally constructed, is described in Nicholson s Journal, 8vo, vol. ii. p. 276. It is made of brass, and i8 provided with a spheroidal bulb whose axis is 2 inches in length, the conjugate diameter being 1| inches. The whole length of the in strument is 8 inches, the stem square of about inch side, and the weight about 400 grains. It is provided with four weights, marked 1, 2, 3, 4, and weighing respectively 20, 40, 61, and 84 grains, which can be attached to the shank of the instrument at C (fig. 8), and retained there by the fixed weight B. The scale engraved upon one face of the stem contains fifty -five divisions, the top and bottom being marked or zero, and the alternate intermediate divisions (of which there are twenty-six) being marked with the letters of the alphabet in order. The four weights are so adjusted that, if the instru ment floats with the stem emerging as far as the lower division with one of the weights attached, then replacing the weight by the next heavier causes the instrument to sink through the whole length of the scale to the upper division 0, and the first weight pro duces the same effect when applied to the naked instrument. The stem is thus virtu ally extended to five times its length, and the number of divisions increased practically to 272. &quot;When no weight is attached the in strument indicates densities from 806 to 843 ; with No. 1 it registers from 843 to 880, with No. 2 from 880 to 918, with No. 3 from 918 to 958, and with No. 4 from 958 to 1 000, the temperature being 55 F. It will thus be seen that the whole length of the stem corresponds to a difference ot density of about 04, and one division to about 00074, indicating a difference of little more than $ per cent, in the strength of any sample of spirits. The instrument is provided with a sliding rule, with scales corresponding to the several weights, which indicate the specific gravity corresponding to the several divisions of the hydrometer scale compared with water at 55 F. The slider upon the rule serves to adjust the scale for different temperatures, and then indicates the strength of the spirit in percentages over or under proof. The slider is also provided with scales, marked respectively Dicas and Clarke, which serve to show the readings which would have been obtained had the instru ments of those makers been employed. The line on the scale marked &quot;concentration&quot; indicates the diminution in volume con sequent upon reducing the sample to proof strength (if it is O. P. ) or upon reducing proof spirit to the strength of the sample (if FIG. 8. Atkins s Hydrometer.