GRAVIMETER, the name given by M. Guyton to an instrument for measur ing specific gravities : he adopts this name rather than either aerometer or by ammeter, because these latter terms are grounded upon the supposition that a fluid is always the thing weighed: whereas with regard to solids, the liquid is the known term of comparison to which the un known weight is referred. Guyton's gravimeter is executed in glass, and is of a cylindric form, being that which re quires the smallest quantity of fluid, and is on that account preferable, except so far as it is necessary to deviate for the se. curity of a vertical position. It carries two basins, one of them superior, at the extremity of a thin stem, towards the middle of which the fixed point of immer sion is marked. The other or lower basin terminates in a point ; it contains the balls, and is attached to the cylinder by two branches. The moveable sus pension by means of a hook has the in convenience of shortening the lever which is to secure the vertical position. The cylinder is three fourths of an inch in diameter, and 6.85 inches in length. It carries in the upper basin an additional constant weight of five grammes, or one hundred and fifteen grains. These di mensions might be increased so as to ren der it capable of receiving a much more considerable weight ; but this is unne cessary. M. Guyton has added a piece which he calls the plonguer, because, in fact, it is placed in the lower basin when used, and is consequently entirely im mersed in the fluid. It is a bulb of glass loaded with a sufficient quantity of mer cury, in order that its total weight may be equal to the constant additional weight added to the weight of the volume of water displaced by this piece, It will be readily understood,that,the weight being determined at the same temperature at which the instrument was originally ad justed, it will sink to the same mark on the stem, whether it is loaded with a con stant additional weight in the upper basin, or whether the effect of this weight be produced by the additional piece in the lower dish. From this explanation there will be no difficulty in seeing how this instrument may be adapted to every case in practice. It may be used, 1. For solids. The only condition will be, that the abso lute weight of the body to be examined shall be rather less that the constant ad ditional weight, which in this instrument is about 115 grains. 2. For liquids of less specific gravity than water, the instru ment, without the additional weight a bove mentioned, weighs about four hun dred and fifty-nine-grains, in the dimen sions before laid down. It would be easy
to limit its weight to the utmost accuracy. We have therefore the range of one fifth of buoyancy, and consequently the means of ascertaining all the intermediate densities from water to the most highly rectified spirit of wine, which is known to bear in this respect the ratio of eight to ten with regard to water. 3. When liquids of greater specific gravity than water are to be tried, the constant weight being applied below by means of the ad ditional piece, which weighs about one hundred and thirty-eight grains, the in strument can receive in the upper basin more than four times the usual additional weight, without losing the equilibrium of its vertical position. In this state it is capable of sbewing the specific gravity of the most concentrated acids. 4. It pos sesses another property, namely, that it may be used as a balance to determine the absolute weight of such bodies as do not exceed its additional load. 5. Lastly, the purity of the water being known, it will indicate the degrees of rarefaction and condensation in proportion to its own bulk. To find the specific gravity of any solid by the gravimeter, observe this rule: " From the weight in the upper dish, when the instrument is properly immers ed in the unknown fluid, take the weight which is placed with the body in the same scale at the like adjustment. The re mainder is the absolute weight of the solid. Multiply this by the specific gra vity of the fluid,-and reserve the product. From the additional weight, when the body is placed in the lower basin, take the weight when it was placed in the up per. The remainder will be the loss of weight by immersion. Divide the reserv ed product by the loss by immersion, and the quotient will be the specific gravity of the solid with regard to distilled water at the standard temperature and pressure." To find the specific gravity of a fluid, proceed thus: " To the weight of the gravimeter add the weight required in the upper basin to sink it in the 'unknown fluid." Again, " To the weight of the gravimeter add the weight required in the same manner to sink it in distilled water. Divide the first sum by the latter, and the quotient will be the specific gra. vity of the fluid in question." See Spa Civic GRAVITY, HYDROSTATICS, and HY. DROMETER.