HYDROMETER. The best method of weighing equal quantities of corrosive vo latile fluids, to determine their specific gravities, appears to consist in inclosing them in a bottle with a conical stopper, in the side of which stopper a fine mark is cut with a file. The fluid being poured into the bottle, it is easy to put in the stopper, because the redundant fluid es capes through the notch, or mark, and may be carefully wiped off. Equal bulks of water and other fluids are by this means weighed to a great degree of accuracy, care being taken to keep the temperature as equal as possible, by avoiding any con tact ofthe bottle with the hand or other wise. The bottle itself shews, with much precision, by a rise or fall of the liquid in the notch of the stopper, whether any such change has taken place. See Gasv ITY, specific.
But as the operation of weighing re quires considerable attention and steadi. ness, and also a good balance, the float ing instrument, called the hydrometer, has aiwaysbeen essteemed by philosophers, as well as men of business. It consists of a hollow ball, either of metal or glass, ca pable of floating in any known liquid ; from the one side of the ball proceeds a stem, which terminates in a weight, and from the side diametrically opposite pro ceeds another stem, most commonly of an equal thickness throughout. The weight is so proportioned, that the instru ment may float with the last mentioned stem upright. In the less accurate hy drometer this stem is graduated, and serves to show the density of the fluid, by the depth to which it sinks, as the heavier fluids will buoy up the instrument more than such as are lighter. In this way, however, it is clear, that the stem must be comparatively thick, in order to possess any extensive range ; for the weight of vitriolic ether is not equal to three-fourths of the same bulk of water, and therefore such an hydrometer, in tended to exhibit the comparative densi ties of these fluids, must have its stem equal in bulk to more than one-fourth of the whole instrument. If this bulk be given chiefly in thickness, the smaller dif ferences of density will not be percepti ble, and it cannot, with any convenience, be given in length.
To remedy this imperfection, various contrivances have been proposed, for the most part grounded on the consideration, that a change in the ballast, or weight employed to sink the ball, would so far rffiange the instrument, that the same short range of gradations on a slender stem, which were employed to exhibit the densities of ardent spirits, might be employed in experiments upon water. Some have adjusted weights to be screw ed upon the lower stem, and others, with more neatness and accuracy, have adjust ed them to be slipped upon the extremi ty of the upper stem. But the method of
Fahrenheit appears to be on all accounts the simplest and most accurate.
The hydrometer of Fahrenheit consists of a hollow ball, with a counterpoise be low, and a very slender stem above, ter Minating in a small dish. The middle, or half length of the stem, is distinguished by a fine line across. In this instrument every division of the stem is rejected, and it is immersed in all experiments to the middle of the stem, by placing proper weights in the little dish above. Then, ss the part immersed is constantly of the same magnitude, and the whole weight of the hydrometeris known, this last weight, added to the weights in the dish, will be equal to the weight of fluid displaced by the instrument, as all writers on hydro statics prove. And accordingly the spe cific gravities for the common form of the tables will be had by the proportion. As the whole weight of the hydrorrieter and its load, when adjusted in distilled water, is to the number 1,000, &c. so is the whole weight, when adjusted in any other fluid, to the number expressing its specific gra vity.
In order to show the degree of accu racy an instrument of this kind is capable of, it may in the first place be observed, that the greatest impediment to its sensi bility arises from the attraction or repul sion between the surface of the fluid and that of the stem. If the instrument be carefully wiped with a clean soft linen cloth, the metallic surface will be disposed to attract or repel the fluid. So that if it possess a tendency to descend, there will be a cavity surrounding the stem; or it; on the contrary, its tendency be to rise, the fluid will stand round the stem in a small protuberance. The ope rator must assist this tendency by apply ing the pincers, with which he takes up his weights to the rim of the dish. It is very easy to know when the surface of the fluid is truly flat, by observing the reflec ed image of the window, or any other fit object seen near the stem in the fluid. In this way the adjustment of the weights in the dish may, without difficulty, be brought to the fiftieth part of a grain. If, therefore, the instrument displace one thousand grains of water, the result will be very true to four places of figures, or even to five. This will be as exact as most scales are capable of affording Some writers have spoken of the ad justment of an hydrometer of this kind, so that it shall at some certain tempera ture displace one thousand grains of water, as if this were a great difficulty. It is true, indeed, that the performance of a piece of workmanship of this nature would require both skill and judgment on the part of the artist ; but it is by no means necessary.