SPECIFIC GRAVITY. 



SPECIFIC GRAVITY. 



Inches English) is the unit of volume, and the gramme (15-482 

 grains) U the unit of weight. The gramme having been determined 

 by the weight of a cubic centimetre of distilled water of the tempera- 

 ture at which ita density is a maximum (39'"2 Fahr.). Thus the 

 weight of a cubic centimetre of any substance being expressed by any 

 number of grammes, * is the specific gravity of that substance. 



The numbers expretmng the specific weight* of substances are also 

 taken to represent their densities. Density, properly speaking, 

 denotes the degree of closeness of the particles of a substance to one 

 another; but this is evidently proportional to the number of particles 

 within a given volume of that substance; and since the weight of a 

 body is only the sum of the actions of gravity upon all its particles, it 

 follows that the densities of two substances under equal volumes will 

 be proportional to their specific gravities. It follows also that if two 

 substances have equal densities or specific gravities, their weights will 

 vary with their volumes ; and that the weights of bodies are to one 

 another in a ratio compounded of their specific gravities and volumes. 



Previously to describing the methods of finding the specific gravities 

 of substances, it will be proper to explain the construction of the 

 hydrostatical balance, which U the instrument employed for the 

 purpose. The beam of this balance rests, as usual, on the lower cir- 

 cumference of a circular perforation in both sides of the fork which 

 holds it, by a pin which is fixed in it perpendicularly to its length and 

 depth, at a small distance above the common centre of gravity of the 

 beam, scales, and weight The fork is suspended from the middle of 

 a horizontal bar, and this last is suspended from a spring at the top of 

 the pillar which supporte the machine. Care is taken that the two 

 arms of the beam are symmetrical, and that the points from whence 

 the scales are suspended are at equal distances from its centre of 

 gravity. Now let the substance which is to be weighed be put in one 

 of the scale dishes, and the number of grains necessary to keep it 

 in equilibrio be put in the other. If the weight of the substance 

 should be an exact number of groins, that weight is determined, but if 

 not, and it were required to ascertain the weight within one-hundredth 

 part of a grain (for example), the following contrivance may be adopted. 

 Suspend in a vertical position from the lower port of the scale con- 

 taining the substance to be weighed, o brass wire, whose volume and 

 weight have been previously determined, and let part of the length of 

 this wire enter into water which is contained in a vessel underneath 

 the scale. The scales with this wire thus attached to one of them 

 being previously put in equilibrio when the surface of the water is at a 

 certain mark on the wire, let the substance to be weighed be intro- 

 duced into the scale above the wire, and let weights be placed in 

 the opposite scale till one grain more would be found too great : 

 then gently raising the whole balance till, by the increase of the 

 weight on the side of the scale containing the substance, in conse- 

 quence of a greater portion of the wire being out of the water, an 

 equilibrium takes place. The wire being graduated so that 100 

 divisions correspond to a weight equal to one grain, the number of 

 graduations on it between the surface of the water and the fixed mark 

 before mentioned will enable the experimenter to determine the 

 number of hundredths of a grain by which the weight of the substance 

 in the scale exceeds the number of grains already placed in the opposite 

 Bade. 



If it be required to weigh a substance in water, or in any other 

 liquid, that substance may bo suspended in a vessel containing the 

 liquid by o horse-hair attached below the scale opposite to that from 

 whence the wire before mentioned was suspended ; and its weight 

 while immersed in the liquid may be found to the hundredth part of a 

 grain, as in the former case. The reason why horse-hair is employed 

 to suspend the substance in water is, that it has very nearly the same 

 specific gravity as that fluid. 



A solid body having greater specific gravity than water being thus 

 weighed both in air and water, may have ite specific gravity deter- 

 mined, that of the water being supposed to be known or assumed, by 

 the following proportion : 



The weight lost by immersion in water (that is, the weight of water 

 equal in volume to the volume of the solid [HYDROSTATICS]), 

 Is to the weight of the body in air, 

 As the specific gravity of the water is to that of the body. 



The weight of J I The ipecific 

 Or, an equal bulk > : I gravity of 

 of water. ) ( water =1-000 



( The weight ) ( The specific 

 : : ] of the body | : j gravity 

 ( in air. ) ( required. 



The specific gravity of a fluid is found by weighing any one and the 

 same solid body in air, in water, and also in the fluid, and observing 

 the two differences of weight. These differences are the weights o: 

 quantities of the two fluids which are equal in volume to the solk 

 body ; and they are to one another as the specific gravities of the two 

 fluids : hence that specific gravity which was required may be found. 

 [See also HYDROSTATICS.] It should be observed that the specific 

 gravity of the solid must be greater than that of either of the fluids 

 in order that the solid may sink when immersed in them. 



The specific gravity of a liquid may bo conveniently determined by 

 neans of a ijiteifr yramly ootUe, a light bottle containing exactly 



1000 grains of distilled water at 60, so that when the bottle is fillet 

 with the liquid whose specific gravity in to be determined, the weight 

 in grains of the liquid immediately determines the specific gravity 



required. There is usually a counterpoise for the weight of the empty 



i.-tt!... If Huch a bottle be counterpoised and then be filled with pure 



it will weigh only 792 grains, thus giving 0792 u the specific 



gravity of the alcohol. If the bottle were filled with sulphuric acid it 



would weigh 1845 grains, so that 1-84.1 wuM INJ the specific gravity 



of the acid. In some cases bottles containing only 100 or 200 grains 



may be used with advantage. 



The density of a powder not soluble in water may be ascertained by 

 means of the specific gravity bottle, or if soluble in water spirite of 

 wine, oil of turpentine, or some other liquid, may be employed. But 

 suppose the powder to be sand, we weigh into the 1000 grain bottle 

 say 150 grains of the sand. If the sand had not displaced any of 

 he water the bottle, when filled up, would weigh 1150 grains; but 

 t is found to weigh only 1096 grains, so that the sand has displaced 

 "1 grains of water. Hence 



54 : I'OOO : : 150 : 2764 = the specific gravity of the sand. 



If the body whose specific gravity is to be found be a solid lighter 

 .linn water, there must be annexed to it, before it is weighed, a mass 

 of some material of known specific gravity, and such that the two 

 xxlies may together sink in the water. The compound mass, anil the 

 teavier body alone, must be weighed both in air and water ; also the 

 ighter body must be weighed in air. Let w be the weight of the com- 

 xmnd mass in air, and w' the weight of the same mass in water ; also 

 et w be the weight of the heavier body in air, and / the weight of 

 the same body in water : then w w' is the weight of water equal in 

 volume to the compound body, and auf is the weight of water equal 

 in volume to the heavier body. The difference between w w" and 

 to to' is the weight of water equal in volume to the lighter body; 

 therefore, by hydrostatics, that difference is to the specific gravity 

 of water as the weight of the lighter body is to its specific gravity. 



When the body is soluble in water, it may be weighed in air, and 

 also in some fluid whose specific gravity is known, and which is not 

 capable of dissolving it ; then its specific gravity may be found by the 

 first of the above-mentioned rules, substituting the weight lost in the 

 fluid for the weight lost in water. 



If the solid body imbibes water without being dissolved in it, let it 

 be weighed when perfectly dry, and call that weight w : again, we find 

 the weight of water displaced by the body when dry, and call it D ; 

 then D is to w as the specific gravity of water is to the specific gravity 

 of the body in its actual state. But D w is the weight of the water 

 displaced by the solid part only of the body ; therefore D w is to w 

 as the specific gravity Of water is to the specific gravity of the solid 

 part only of the body. 



It must be observed that the true weight of any body, or that which 

 the body would appear to have if weighed in vacuo, is greater than the 

 weight which it is observed to have when weighed in air, by the 

 weight of a volume of air equal to the difference between the volume 

 of the body and that of the object by which the weight is determined. 

 It should also be observed that the numbers expressing the specific 

 gravities of substances are strictly correct only on the parallel of 

 latitude passing through the place where the weight of the water 

 under the unit of volume is determined ; for the force of gravity, 

 and consequently the weight of any substance under a given 

 volume, increases in proceeding from the equator towards either pole 

 of the earth. 



In order to determine the specific gravity of the atmosphere, or of 

 any gas whatever, the air or gas must be weighed in a globular vessel 

 of glass, of sufficient magnitude to prevent the unavoidable errors of 

 the operation from sensibly affecting the results. 



In taking the specific gravities of gases, correction must be made 

 for temperature. It is known that for every degree of Fahrenheit's 

 scale there is an expansion equal to ss ', of the bulk occupied at 32 

 Fahr. If, for example, 9"2 cubic inches of gas at 70 were reduced to 

 60 the change in volume would be as follows: Since 70 82 = 38, 

 491 parts of a gaa at 82' would at 70 have increased in bulk 38 parts 

 or would hove become equal to 529 parts. Again 6032 = 28, so 

 that a gas which at 32 occupied 491 parts, would at 60 occupy a 

 space equal to 519 parts. The volume therefore of any gas at 70 

 would bear the same proportion to the bulk which it would occupy 

 at 60 as 529 does to 519. Hence, 529 : 519 : : 9-2 : 9-026 cubic 

 inches. A correction is also required for pressure; but in taking 

 the specific gravity of a gas, Kegnault has reduced the number of 

 corrections required by counterpoising the globe in which the gas is 

 to be weighed by a second globe of equal size. The film of moisture 

 adhering to the glass is equal in both globes, and 09 the bulk 

 of air displaced is equal in both coses, the calculation for ite 

 buoyancy is thus got rid of. A balance capable of weighing 

 2 Ibs., and turning with the 1 50th part of a grain when loaded, U 

 placed on a chest, furnished with folding doors, within which the 

 glass globes, each of the capacity of about 600 cubic inches, attached 

 to the scale pans, ore freely suspended. The counterpoise globe is 

 hermetically sealed, the other globe is furnished with a stop-cock, the 

 air is exhausted from it, and it is filled with the gas to be tried in 

 a pure and dry state. The globe is again exhausted so as to get rid 

 of the last portions of atmospheric air, when it is again filled with 

 the gas, which may be again pumped out and again filled. To get 

 rid of the correction for temperature the globe is placed in melting 



