118 



THE POPULAR EDUCATOJi. 



however, soluble, and therefore some other means must be 

 resorted to. 



We have first to find somo liquid in which the substance is 

 insoluble. Oil of turpentine will frequently be found to answer, 

 or, for many substances, alcohol may be used. 



Having chosen the liquid, we proceed, as before, to find the 

 weight of a volume of it equal in bulk to the substance, either 

 by weighing the solid in it and ascertaining the loss of weight, 

 or by means of the flask. We then ascertain the specific gra- 

 vity of the liquid, and from this, by the following equation, we 

 can ascertain the weight of a bulk of water equal to the solid. 

 As the specific gravity of the liquid is to that of water (which 

 is 1') so the weight of the equal bulk of liquid is to that of the 

 same bulk of water. 



We will illustrate this by an example recently set at an 

 examination. 



A piece of blue vitriol weighs 3 ounces in vacua and 1'86 in 

 oil of turpentine. What is its specific gravity, that of the 

 turpentine being 0'88 ? 



Since it weighs in vacua .... 

 And in oil of turpentine only . . . 



Ounces. 

 3-0 

 1-86 



It loses 



1-14 



This, then, is what an equal bulk of turpentine weighs. Now 

 the specific gravity of the turpentine is 0'88 ; or, in other words, 

 if any bulk of it weighs 88 ounces, an equal bulk of water will 

 weigh 1 00 ounces. How much, then, will water, equal in volume 

 to T14 ounces, weigh ? The following equation tells us : 

 As 88 : 1-14 : : 100 : 1'295. 



This, therefore, is the weight of a volume of water equal in bulk 

 to the vitriol, and hence the specific gravity is p-^ = 2'316. 



You will have noticed in the above example, that the weight 

 of the blue vitriol is stated to be 3 ounces in vacuo. All bodies 

 ought strictly to bo weighed in the absence of the air, as other- 

 wise we do not ascertain their true weight, but that amount 

 less the weight of the air they displace. In practice, however, 

 the difference is so slight, that it is disregarded, and a body is 

 always weighed in air, and this taken as its true weight. A 

 proof, however, that it does make a difference is seen in the 

 fact that if we fill a small balloon with gas, and add weights, 

 so that it can only just ascend, it will apparently have no weight 

 at all. The real fact being, as we shall see when we come to 

 treat of Pneumatics, that its weight is then just equal to that 

 of the air it displaces. 



There is still one class of substances whose specific gravity 

 we cannot ascertain by any of the ways hitherto described ; it 

 is those which are lighter than water. 



These, when immersed in water, seem to lose more than all 

 their weight, for they have a tendency to rise. We have, there- 

 fore, to proceed in a different way. A piece of metal is pro- 

 cured heavy enough to cause the substance, which we will 

 suppose to be a piece of wood, to sink when attached to it. 

 This sinker is weighed in air and then in water, so as to ascer- 

 tain the weight of water it displaces. It is then fastened to 

 the wood, and the two together weighed in air and in water ; we 

 thus ascertain the weight of water they together displace. 

 From this we deduct the amount which is displaced by the 

 metal, and thus find out the amount displaced by the wood 

 alone. Dividing the weight of the wood by this, we ascertain 



its specific gravity. 



Grams. 



For example, a large cork was found to weigh in air 74 '3 

 The brass sinker weighed iii air .... 853' 1 



Weight of the compound body 

 In water it weighed 



. 927-4 

 . 446-0 



Therefore a body of water equal to it in bulk weighed 481 '4 

 Now the sinker weighed in air . 853'1 



in water . 757 '0 



Weight of water equal to it in bulk . . . 96 - 1 



Therefore the weight of water equal in bulk to the 



cork is . . 385-3 



Its specific gravity, then, is K^T-T- = 0'192. 

 ooo'o 



The piece of cork in this case was porous and contained air, 

 and therefore its gravity appears much less than that of cork 



C k 



Fig. 1C. 



really is, it being usually set down in the table as '240. A 

 rough estimate of the specific gravity of such a sub- 

 stance may be formed by observing to how great a 

 depth it sinks in water. If three-fourths of it is im- 

 mersed, its specific gravity is 0'75. 



In the manufactures it is frequently required to 

 Fig. 15. know the strength of some solution, and this may be 

 found by ascertaining its specific gravity. The strength 

 of spirit is thus taken by the excise ; milk is also sometimes 

 tested this way- to find whether or not it is adulterated. In 

 such cases no very great degree of accuracy is required ; the 

 process, however, must be simple, so that it may be carried out 

 by a man without much special knowledge. Several piecos of 

 apparatus have accordingly been devised, which are known by 

 different names according to the special purposes for which 

 they are intended. Hollow glass beads are prepared, and 

 weighted so as to have different specific gravities, which are 

 plainly marked upon them. If we take a series of these, and 

 drop them successively into the liquid to be examined, we 

 shall find that some will sink, but we shall come at last to 

 one that just floats on the surface, and this shows the density. 

 For instance, if that marked '930 sinks, and '920 floats, we 

 know that the specific gravity of the liquid is between these 

 two. 



More usually, however, an instrument called an hydrometer, 

 is employed. It consists of a hollow 

 bulb of glass, A, carrying above it a 

 thin tube, with a scale marked on it, 

 or on a paper enclosed in it. A smaller 

 bulb is also blown beneath A, into 

 which mercury or shot are put, so as 

 to adjust the weight, and at the same 

 time cause tho instrument to float in 

 a vertical position. The liquid to be 

 examined is poured into a tall glass 

 jar, D, and tho hydrometer immersed ; 

 the specific gravity may then be read 

 off from tho stem. 



It is manifest that the denser the 

 liquid, the higher the instrument will 



fk.ut in it, the weight of the liquid displaced being always equal 

 to that of the hydrometer. If we want great delicacy, the 

 bulb must be made large and the tube small ; sometimes the 

 latter is removed and replaced by a graduated wire, and thus 

 great accuracy is obtained ; but the greater the accuracy, the 

 less the range. These instruments are usually supplied in 

 sets. Sometimes two are used, one for liquids lighter than water, 

 the other for those heavier ; but it is better to have more, each 

 one having a range of about '200 ; say. for instance, one from 

 600 to '800, another from '800 to TOGO, and so on. This in- 

 strument, when manufactured for testing 

 milk, is called a lactometer when for 

 ascertaining the strength of spirit, an 

 alcoholmeter, and by other names when 

 made for other purposes. The only dif- 

 ference, however, is in the graduation 

 and the range. 



A modification, called Nicholson's Hy- 

 drometer, is represented in the annexed 

 figure. It is constructed of metal instead 

 of glass, and the lower bulb is replaced 

 by a small tray, E F, weighted so as to 

 keep the instrument vertical, and the 

 tube, by a wire stem, carrying another 

 tray, CD. A small mark is made on the 

 stem at O. 



The instrument is first carefully 

 weighed in air ; this weight, being con- 

 stant, should be marked on it. The 

 weight which must be placed on c D to 

 sink the instrument to the mark o should 

 also be carefully noted. To ascertain the 

 specific gravity of a solid by it, the substance is first laid on 

 the upper tray, and weights added till the instrument sinks 

 to the mark o. The difference between this and the former 

 weight will give that of the solid in air. Now remove the 

 solid from c r>, and place it in the lower pan, E F. Some water 

 will be displaced, and the instrument will rise. We accordingly 



Fig. 17. 



