Page:The American Cyclopædia (1879) Volume IX.djvu/132

 124 HYDROMETER revolving body, like the earth, if covered with a liquid, would have a form like that repre- sented in section by the dotted line, and it may be demonstrated that this form is that of a spheroid formed by an ellipse revolving about its minor axis. Its surface, to which that of the earth approaches, is called a level surface. HYDROMETER, or Areometer, an instrument for determining the specific gravity of liquids. It generally consists of some buoyant body, as hollow glass or copper, weighted at the bot- tom and supporting a graduated stem, or one having a definite mark. There are two kinds, those of constant and those of variable im- mersion. Those of constant immersion are made to sink in the tested liquid, whether dense or light, to the same depth, by balancing with weights. Those of variable immer- sion have no mov- able weights, but rise or fall accord- ing to the den- sity of the liquid. Nicholson's hy- drometer, fig. 1, is of the first kind. As usually con- structed, when this instrument is immersed in wa- ter it requires a weight of 1,000 grains to make it sink to a certain mark on the stem. According to the principle of Archi- Fio. 1. Nicholson's Hydrometer, medes (see HYDRO- MECHANICS), the weight of the instrument, together with the 1,000 grains which it sustains, is equal to the weight of the volume of water displaced. If the instrument is placed in a liquid lighter or heavi- er than water, and the weight changed until it sinks to the same depth, the specific gravity of the liquid will be indicated by the formula g = ^rj^, where W is the weight of the in- strument, and w that of the weights placed upon the pan. If w is less than 1,000 grains it will show that the liquid is lighter, and if it is more than 1,000 grains it will show that it is heavier than water. This instrument may also be used to find the specific gravity of solids, or as a delicate balance. For these purposes it has a small cup or wire cage suspended at the bottom to hold the body, which may be either heavier or lighter than water. To find the specific gravity of a solid, let it be first weighed in air, by placing upon the pan a piece of the substance which weighs less than 1,000 grains. Suppose the substance to be sulphur, and that 440 grains are required to be added to make the instrument sink to the mark on the stem, the weight of the sulphur is, evi- dently, 1,000440 = 560 grains. Now, what it loses if weighed in water will be the weight of an equal bulk of water, and this will be found by placing it in the cup or cage at the bottom, and adding sufficient weights to those in the pan at the top to bring the mark to the level of the water. If it requires the addi- tion of 275-2 grains, that amount will represent the weight of a volume of water equal to the sulphur; consequently the specific gravity of the sulphur will be $?.? = 2-03. If the body is lighter than water, it will of course require the addition of more than its weight to the pan, and for immersion it will require to be placed in the wire cage. Fahrenheit's hydro- meter differs from Nicholson's in being con- structed of glass, and having a constant weight of mercury in a bulb at the lower end. Its use is therefore restricted to the weighing of fluids. Of hydrometers of variable immersion, Baume's is the one most frequently used, and furnishes a good example of the class. Two instruments, of different forms, are represent- ed in figs. 2 and 3. They are made of glass ; their stems are hollow and lighter than the fluid in which they are immersed. Fig. 2 is called a salimeter, and is used for estimating the proportion of a salt or other substance in solution. It is graduated in the following manner : Being immersed in water at a tem- perature of 12 0., the point to which it sinks is marked ; it is then placed in a solution containing 15 parts of common salt to 85 of water, the density of which is about 1-116, and the point to which it sinks is marked 15, and the interval divided into 15 equal parts ; the graduation is then extended downward, generally terminating at 66, which corre- sponds to the density of sulphuric acid. When the instrument is to be used for liquids lighter than water, the zero is not placed at the point to which it sinks in pure water, but at a point to which it sinks in a solution contain- ing 10 parts of com- mon salt to 90 of wa- ter. The point to which it sinks in pure water was marked by Bau- m6 10, and the grad- uation was continued upward to the high- est point to which the stem might be immersed in the lightest liquid. Fig. 3 represents the in- strument for liquids lighter than water. The graduation of these hydrometers is arbitrary, and is an indication of the strength of the li- quid only after trial. Hare's hydrometer, a FIG. 2. Fio. 8. Salimeter. Alcoholimeter. Baume's Hydrometers.