S10 



CHEMISTRY. 



:AVITT. 



M possible, ha* been : Korthii piirpnwit must he 



carefully di-t i i led in glass vensclit, and should thon 1-e k.-pt 

 out of contact with tho at M ibsorplion 



of air, which wi>ul<l lessen its nwn >|vilic (gravity. 



i which the specific gravity of solid 

 - i t ii.cn. is thnt any body iniiiu-nuxl in i\ tluiil, 

 di<[>laci* a* much an is equal to the bulk of the solid, 

 ami that a ratio eziita between tho respective weights 

 of each in reference to that bulk. Thus, wo will sup- 

 pose equal bulks of gulil ami iron to IK- immersed in dis- 

 tilli-.l water. They will each displace exactly tlio same 

 ijuiiiitit i of water ; but if the equal bulks of water, iron, 

 and gold, be each weighed, they will bear a ratio to 

 each olVr of I, 7, ami lit n. . V>w, these ratios 

 an', in rnuiiil niiuilwrs, tin- actual specific gravities of 

 'iKstances named; and hence, if we can ascertain 

 tig l>ctwevn the weight of any body, and 

 tliat of tho bulk of water displaced by it when immersed 

 th rrin, wo have at once tho specific gravity. 



. this may be done in two ways. The first wo 

 mention will only illustrate the principle; whilst tho 

 Moord it that always employed. First, a vessel may 

 be exactly filled with distilled water, and a body, pre- 

 viously weighed, may be immersed completely in it. 

 Of course, as much water as is equal to the bulk of the 

 body will run over tho sides of the vessel. If all this 

 could be collect d :iucl weighed, then the weight of the 

 immersed solid, divided by that of the displaced fluid, 

 would be the specific gravity. But, practically, this is 

 impossible; because a considerable portion of the fluid 

 would be lost by adhering to the vessel as it runs over 

 the side. 



Secondly, the spec'inc gravity of any body may thus 

 be accurately found. Suspend the substance, by means 

 of a horse-hair or filament of silk, to the pan of a pair 

 of scales, and weigh it carefully. Then immerse the 

 body in a vessel containing distilled water, taking care 

 that all air-bubbles are removed, and that the substance 

 is entirely below the surface of the liquid. It will be 

 found th;it tho solid docs not weigh so much when in 

 the water as in the air. Balance it by means of 

 weights placed in the scale from which the substance 

 is suspended, and, having ascertained how much is 

 r -quired, divide the weight of the body as obtained in 

 the air, by the loss it sustained when weighed in water, 

 and the quotient will be the specific gravity. The loss 

 of weight represents that of the bulk of water, equal to 

 the bulk of the solid by which it was displaced. 



To illustrate this plan, the following instance will 

 suffice: A piece of lead weighed in the air just 45 

 grains. On being weighed in water it lost four grains. 

 The 46 grains is thus to be divided by the loss of four 

 grains; and the result, 11-25, is the specific gravity of 

 the lead. In this way the specific gravity of all sub- 

 stances weighing more than their own bulk of water, may 

 be readily taken. 



Tho practical value of this process is very great ; for 

 by consulting a table of specific gravities of bodies, we 

 can often have a guide to the proportion in which they 

 may exist in any substance containing two or more of 

 For instance, a piece of quartz may have a 

 greater specific gravity, in its mass, than any part of it 

 possesses. This indicates an admixture with some other 

 stiUtance. Supposing that to bo gold, a certain ratio 

 will exist between the two specific gravities ; that is, of 

 the mass and of the part ; and this ratio will increase 

 directly in proportion m tin- quantity of gold contained 

 n. When a rough approximation only is required 

 as to the value of a |Uirt7., this plan may lie 



readily employed ; and, in home instances, we have found, 

 on subsequent analysis, that the result thus obtained is 

 very near the truth, 'it . .m,.', tin- presence of other 

 foreign bodies will entirely vitiate tho re.ult. In tho 

 case of alloys, the method may bo also adopted, pro- 

 vi i d th.it i, m two metals are present. 



Wh'-n the specific gravity is required of a body 



m water, i plan is adopted, with the 



addition, however, that the body must be sunk in the tluid 



by having attached to it a heavier substance, the weight 



of which in air, and loss when weighed in watei, have 

 | been first ascertained. The loss of weight of the 

 body i then to bo subtracted from their joint loss w lien 

 weighed in water, and the weight of the light body in 

 air is divided by the amount .-. obtained; UM quotient 

 is the specific gravity of the light body. 



The specific gravities of liquids may lie ascertain. 1 in 

 various ways; for all that i- I. i- tip- ratio of 



weight of equal bulks of the dilleivnt fluids. A 

 common plan is that of using a. d>le of con- 



taining exactly 1,000 grains of distilled \\aterat a tem- 

 perature of (MI Fah. The bottle is maiked at the 

 place at which the surface of the liquid reaches the 

 nock of the vessel, and the liquid whose specific gravity 

 i, is poured into it until it reaches that point. 

 : ;ht of the whole is then ascertained ; and that 

 of tho bottle having been deducted from the ti 

 balance is that of the liquid, and its spe 

 also. The plan is remarkably simple; and all liquids 

 may thus have their Hjwcitie gravity taken, v 

 they be lighter or heavier than their own bulk ot 

 Thus, a bottle holding 1,000 grains of water, will con- 

 tain 1,840 grains of sulphuric acid, 790 of pure alcohol, 

 . and so on ; and thus those weights give the reap' 

 specific gravities of the liquids named as 181 and "'> 

 compared to water, the standard, which equals I.IMI 



is, however, one point to which we must draw 

 attention ; and that is, the fact that all bodies expand 

 by heat. This may be neglected in reference to taking 

 the specific gravity of solids; but tho c.ise is .1 

 with liquids, having a rate of expansion much greater; 

 henco their specific gravities must either be taken at a 

 temperature exactly that of 60. or allowance must be 

 made for their expansion for higher, or cout 

 through lower temperatures. 



Another mode of taking liquid specific gravities is 

 that of the hydrometer, of which there are numerous 

 forms. It is constructed on the principle that a body 

 will rise in a liquid of greater specific gravity, ami sink 

 in one tho specific gravity of which is less than its own. 

 These instruments are generally c<:rtruci' 

 which is so weighted, that one portion of the - 

 in it shall be level with the surface of distilled water 

 when the hydrometer is immersed therein. If tho 

 liquid be heavier than water, then the stem of the hy- 

 drometer will rise, as would be the case if salt and 

 water were the subject of experiment. If the liquid IK.' 

 lighter, then the hydrometer would sink ; and so, by 

 reaching the scale at the level of the liquid, the specific 

 gravity is at once obtained. 



The following cut illustrates the appearance of one 

 Fig S4. form of tho instrument ; but, as we have already 

 remarked, they vary much in shape, size, and 

 according to the uses to which they are to be 

 applied. 



Occasionally, glass balls, which have been so 

 weighted as to swim in any part of a liquid of 

 the same specific gravity as their own, are used 

 This plan, however, requires a separate ball for 

 each possible specific gravity ; and hence its use 

 is confined within very narrow limits. There 

 are other modes of ascertaining liquid spcciiic 

 gravities, such as weighing solids in them, A< . 

 but the aliovo are tho best practical m> t 

 The specitic gravity of gases is taken 1<\ 

 similar means to tho.se we have deseiiU-d m 



uce to liquids. I'.nt special care 

 quired, because tho variations of a! 

 temperature and pressure have the cll'eet of 

 interfering with the accuracy of the result 

 tained. The rate of expansion in gases far 

 exceeds that of either liquids or solids ; and 

 hence, it is absolutely necessary that the tempe- 

 rature and pressure of the atmosphere should bo vcrv 

 carefully attended to. 



In practice, a glass globe, or other vessel, is first 

 weighed as filled with air; it is then exhausted of 

 air by moans of a good pump, and again weighed, so 

 as tliat the weight of the vessel itself may be known 



