\TIVE .Si 



119 



:i'M Miuro weight* till it again restn with o at tho surface. The 

 weights added are eqnu : ihn w:iti-r ili-|.lu : 



! tin- is.-i-'iit f tho unbalance by thin, wo ascertain it* 

 :!< gravity. 



'I'hiiH 45i) grain* had to be placed on D to Mink the instrn- 

 mont to the mark ; when, however, u pi one of granite wan laid 



scale, 237 grains only W.T.- n-.|uin-.l. Th granite, 

 tii. !!'.. r.-. .. 113 Train*. < >,, I, ring transferred to the 



t ray, 79 graitm hud to ho added to o D. The specific 



y of tho granite wiui therefore *y,, or 2*696. 



want to UHO tii ra to find tho speoifio 



, <>f a liquid, we merely observe what weight in rnjuin-a 



it in th.- liquid to tho murk on tint Htom. To this we 



add tho weight of tho instrument, and thus find tin; weight of 



liquid equal to it in bulk. Jtut w know tho weight of 1111 equal 



bulk of wahT, for wo found before tho weight n <juir <\ to sink 



! rument to tho name depth in water; wo have, therefore, 

 to divido tho weight of tho liquid by that of tho equal bulk of 



and tho quotient is its specific gravity. 



!'! instance, an hydrometer weighed 600 groinfl, and, aa 

 above, 450 grains had to bo added to sink it in wator to tho 

 murk. When dipped into tho liquid to bo tested, 310 grain* 

 only wuri! required : what was its specific gravity? 



The weight of water equal in bulk to tho hydrometer is 600-J 

 450 or 1050 grains, that of an equal bulk of tho liquid, 600+310- 

 or 910. Tho specific gravity, therefore, is , or 0'866. 



hough it is found that water answers well as a standard 

 for the comparison of the specific gravities of solids and liquids, 

 gases are so very much lighter than it, that it is much more 

 convenient to take some other substance as a standard to com- 

 pare thorn with. A cubic foot of air at a temperature of 60.- 

 ;ind when tho barometer stands at 30, weighs about 536 grains, 

 or nearly 1J ounces, whilo a cubic foot of water weighs 1,000 

 ounee:- ; iio gravity of air, taking water as tho standard, 



would, therefore, bo 0-00122, and that of hydrogen pas less 

 than of this. Such numbers would be very difficult to re- 

 member, and awkward to work with; air, therefore, is fixed on 

 as the standard for tho specific gravity of all gases and vapours. 

 Tho temperature and pressure, however, very greatly alter the 

 bulk of any gas. It is, therefore, necessary to fix on some given 

 temperature for the standard. Now, as in the case of water, 

 60 is found to be a very convenient temperature, the pressure 

 indicated by tho barometer standing at 30 is also fixed on. 



It is not, however, always convenient to bring the gas to this 

 temperature ; we therefore ascertain the bulk at any temperature, 

 and from that calculate what its bulk would be at 60. It is 

 found that a gas expands about jJ of its volume at Fahren- 

 heit for each degree it rises in temperature ; that is, 460 cubic 

 inches at will measure 470 at 10, 20 at 60, and so on. 

 Now the weight of 100 cubic inches of air at 60 is 31-0117 

 grains, or that of a cubic foot, 536 grains. 12, then, we know 

 the weight of any bulk of gas, and the temperature at which it 

 is weighed, we can tell its specific gravity. Thus 80 cubic 

 inches of a gas at a temperature of 40 weigh 35 grains : what 

 is its specific gravity ? We first find what volume the 80 cubic 

 inches will occupy at 60. By tho rule just given the volnme 

 at 40 is to that at 60 as 500 is to 520. 



As 500 : 520 : : 80 : 83-2. 



^- eubio inches then .-it (50 ' weigh 35 grains. We can now, 

 by taking a second proportion, readily find the weight of 100 

 cubic inches. ^^ _ 100 : : 35 : 42 . OG7 



Now the same volume of air weighs 31*0117 grains. The spe- 

 cific gravity of tho gas is, therefore, ','"','-, or 1*35. 



\Y will now append a table of tho specific gravities of com- 

 mon substances. More extensive lists can be found in chemical 

 and other books. It may be well just to mention that some- 

 times 1000 instead of 1 is taken as the specific gravity of water 

 or air, the only difference, however, is that tho number is then 

 a whole number, instead of a decimal. 



f Bseh . . O-NW 

 | fir 



! Ebony.Ammriosa 1 331 

 Mulioguuy, 

 Spanish . 1-003 



SPECIFIC GRAVITIES OF SOLIDS. 



Gold . 

 Silver . 

 Copper . 

 Iron, cast 



,, wrought 

 Steel . 



19-250 

 10 -470 

 8-900 

 7-2-18 

 7-780 

 7-820 



Zinc 

 Brass . 

 Tin 



Platinum 

 Lead 

 OIuss, crown 



7-191 Glass, flint 

 8-39J Coul 



7-291 



11*350 



Diamond . 

 Beeswax . 

 Oak, seasoned 

 Elui 



ir:> (." 

 l :: .1 

 3521 

 0-900 

 0743 

 0-671 



1'jue. white 

 Cork . 

 I vary 

 Chalk . 



Qaaite. 



0210 



11*17 



-.*, 



Zlw* 



flaadfrtOM . 



LilMStOB* . Xt&J* 



M.rbl* . 

 llriok . 

 1'ortUud tOM 2-000* 



Wiit.-r . 

 Sea-water 

 Mercury 

 Muriatic add, 

 commercial . 



Air 

 Oxygen 



PBCmC OUAV1T1KH OV LIQUID*. 



1-000 



; j.. 



Nitric acid. 



..lalu.-r.-l.il 



Sulphuric acid, 



c -.tiim. -r.-ni 



<rita of win* 0-W 

 1*500. Ulool . . 1-OK 



1 -.. 



Alcohol, aUoluto <j~'j, 



Milk 



SPECIFIC OBAVITIES OP CASKS. 



Nitrogen . 0-072 i Marsh gas 

 Chlorine . 2*470 Cool gat 



1*106 



'. t.,- 



%* The different specimens of those narked (*) varjr considerably ; 

 tho numbers given are therefore approximations only. 



EXAMPLES. 



1. A piece of silver wire weighs 95*2 grains. The flu*k filled with 

 distilled water weighs 251-2 grains ; but when the wire u put in, it 

 weighs 337-21 grains. What is the specific gravity of the silver? 



2. A body weighs 871 grains in air and 357 in water : what ! its 

 specific gravity ? 



3. Required the weights of blocks of elm, limestone, and lead, each 

 measuring 10 inches x 6 x 3. 



4. An hydrometer, weighing 180 grains, requires grains to sink it 

 in water to the mark, when immersed in oil it only takes 40 grains. 

 What is the specific gravity of the oil ? 



5. Tho same instrument requires 31 grains to be added whem a 

 stone is laid on the upper tmy ; but when removed to the lower, 44'5 

 aro needed. Find tho specific gravity of the stone. 



6. A piece of motol weighed 15 ounces in air and 13-08 ounces in 

 water. It was then attached to a piece of wood, and the two together 

 weighed 18'3 ounces in oir and 11 '46 in water. Required the specific 

 gravity of tho wood. 



7. 48 pounds of copper are mixed with 27 pounds of zinc. Find the 

 specific gravity of the compound. 



RECREATIVE SCIENCE. XI. 



THE Tontoonstelling, or International Exhibition of Holland, 

 attracted many visitors to that part of the world, and they were 

 able to vouch for the accuracy of the following statement 

 with regard to tho use of mirrors as portions of the external 

 furniture belonging to tho houses of the merchant princes. 

 The Family Library states that " to almost every house in 

 Eotterdam (and, it may bo added, in Amsterdam), and some- 

 times to every window of a house, on the first floor, there 

 is fixed a single or double looking-glass, or reflector, by means 

 of which a person in tho room, sitting before the window, can 

 sec, by reflection, the whole length of the street, tho pas- 

 sengers, the trees, tho canal, and tho shipping. When two 

 of these reflectors are placed at right angles, and the right 

 angle points towards tho window, thus, A, a person within, 

 directing the eye to that angle, will see tho whole street, 

 both to tho right and to the left. In some of the towns of 

 England wo may now and then observe one of these reflecting- 

 glasses, which is generally supposed to, be intended to put the 

 inhabitant on his guard against unwelcome visitors, and on that 

 account they have been whimsically called ' dun-tier scope*.' In 

 Rotterdam they aro adopted for the amusement of the ladies." 



This utilitarian ago, disdaining to waste time in looking at 

 the reflection of the streets of London, or its innumerable oc- 

 cupants, adopts the same idea in another fashion, and endeavours 

 to illuminate dark and dingy back offices by reflecting in light, 

 although it is drawn from the half -obscured, soot-defiled beams of 

 tho sun which may be collected in a street of modern Babylon. 



There is a pretty little toy (Pigs. 1 and 2) called the " Magic 

 Designer," in which the reflecting surfaces of two small perpen- 

 dicular mirrors, at right angles te each other, are placed 

 inwards, and fixed in a semi-circular card-board box, which is 

 open at the top and bottom. The spectator looks through the 

 top, and places any small article that he wishes multiplied at 

 the bottom. Tho toy is, in fact, a simple modification of Sir 

 David Brewster's Kaleidoscope, invented and patented in the 

 year 1814. The patent, however, conld not be maintained, as 

 he had, unfortunately, shown the instrument to two or three 

 philosophical instrument-makers before securing the proper 

 legal documents ; and as the letter and not the spirit of the lw 



