158 



( 11 i; MI -STRV. 





Tensive a, . : ami others 



'"' for weighing 'gates ; but they arc unnecessary, and do 

 not i -A > 11 js a much more simple and cheap 



contrivance which we shall now explain. Let a Florence 

 flask, or one of those thin glass flasks that come to this 

 country from Italy tilled with oil, be well cleaned, and 

 fitted witii a cap and brass stop cock, both as light as 

 Tiii* flask, by means of an intermediate short 

 brass tube properly furnished with screws, is made to fit 

 to ajar standing over water, and furnished with a stop 

 cock at the upper end, and to another jar of the same 

 construction standing in the mercurial trough. Suppose 

 we want to take the specific gravity of a gas standing 

 orer watrr. We first weigh the ihsk and stop-cock with 

 as much accuracy as possible. We then exhaust it of air 

 and weigh it again, noting the lo-s of weight, which in- 

 dicates the quantity of air drawn out of the flask. Let 

 us suppose this loss of weight l-> amount to 10 groins. 

 We now screw the flask to the jar containing the gas to 

 be weighed, and turning the stop-cocks, the gas rushes 

 in and fills the flask. We must take care that no water 

 or mercury make its way into the flask along with the 

 gas. This is best guarded against, by filling the inter. 

 mediate brass tube with cotton. We now weigh the 

 flask a third time. If its weight be the same as at first, 

 then the gas is of the same specific gravity an common 

 air ; if it is lighter, the gas is lighter than common air ; 

 if it is heavier, the gas is heavier than common air. Sup- 

 pose, for example, that the flask wants 5 grains of its 

 original weight. We infer, that the weight of air is to 

 the weight of our gas as 10 to 5, or that its specific gra- 

 vity is one half of that of common air. By this easy 

 method we determine the specific gravity of any gas, ta- 

 king the specific gravity of air at 1.00; and knowing 

 :hal at the temperature of 60, and when the barometer 

 stands at SO inches, 100 cubic inches of air weigh 30.5 

 iins, we can easily deduce from our experiments the 

 absolute weight of our gas. No correction is necessary, 

 cither for temperature or for the height of the barome- 

 ter, because all gases undergo the same change in bulk 

 by heat and pressure. 



Bibnce for 9. One essential requisite for taking the specific gra- 

 vity of gases, is a good balance. The one which we cm- 

 ploy was made by Mr Cuthbertson of Glasgow, and 

 weighs easily to the hundredth part of a grain, so that 

 we are sure of our specific gravities of gases to two de- 

 cimal places, but no farther. As Mr Cuthbertson's beams 

 differ in several particulars from any others that we have 

 seen, we shall give a figure and short description of one 

 of them, and mention some of its excellencies and defects. 

 See Fig. 7. 



The beam ab is made of cast steel ; and, as will be seen 

 by the Figure, is very slender, though of considerable 

 length. To prevent it from warping or bending, the 

 two slender wires c, c are riveted into the ends of the 

 beam, and screwed tight to the upper end of the tongue 

 *>f the balance. These wires, or stays, constitute the 

 rhief difference between this and other balances. There 

 is nothing peculiar in the construction of the other parts. 

 The sensibility is owing chiefly to the great length of 

 the beam. The principal defect of this balance, is the 

 way in which it is suspended to the top of the glass case 

 by means of the hollow brass cylinder ri. A solid cy- 

 linder of brass, attached to the top of the balance, plays 

 in this hollow cylinder ; and by means of the string e, 

 and weight/, the balance if raited above the stand, or 



VLATC 



Kif . 7. 



let down to it at pleasure. This c.odc of tiupcniiun 

 gives such a tendency of shakr balance, that the A l''' 



process of weighing is very tedn . "" ~~ 



10. Many of the gases are of an in/I .n. ruble nature, Electrical 

 and it is by their combustion with oxygen that the infe- machine. 

 fences are drawn with respect to their nature and con- 

 stituents. The combustion of the gases requires a pe- 

 culiar apparatus which it will be requisite to describe. 

 A good electrical mnchine is the first u-quisitc. '1 '< 

 may be either a plate machine, according to the more mo- 

 dern and fashionable construction, or a o h.idrical one. 

 All that is wanted is, th:it it should give out strung 

 sparks when in proper order; for burning tlu- gases, SL 

 very strong glass tube is used, about four inche: long, and 

 .}ths inch wide, shut at one end, and open at the other. 

 This tube is graduated, and should hold about three cu- 

 bic inches. Two holes are drilled in it opposite to c.u 

 other near the top, and into these arc cemented two bra** 

 pins, which project into the tube, and approach nea. 

 enough to transmit a spark from one to the other exter- 

 nally. These pins terminate in small rings. Tno mix- 

 ture of gases being put into this tube, it is firmly fixed 

 down in a water or mercurial trough by a strong iron 

 screw. The brass pins being now connected wiui the 

 electrical machine, a spark is made to pass from the one 

 to the other, which sets the gas on fire. 



When this experiment is made over mercury, the sud- 

 den expansion produced by the combustion of the gas, 

 is apt to drive come of the mercury out of the trough. 

 To remedy this inconvenience, the French chemists usu- 

 ally put a stop-cock at the bottom of their detonating 

 tube, which they shut before passing the electric spark. 

 But this is a bad method, as the shock is very apt to 

 break the glass tube however strong, unless the quantity 

 of gas burnt be very small. Mr Pepys has contrived the P( 

 apparatus represented in Fig. 8, which answers very we!'. L XLII. 

 The detonating tube a is supported by a strong iron Kig. 8. 

 stand c, which has a strong spring near the foot, which 

 draws out after the manner of a steelyard when the 

 concussion is given. The inconvenience may be reme- 

 died also, by placing the detonating tube in a copper cy- 

 lindrical vessel about 3 inches deep, and tilled with mer- 

 cury. The depth of the mercury prevents the concus- 

 sion from being so violent, as to drive out and spill the 

 mercury. 



11. It may be worth while to describe here an appa- Henry's 

 ratus contrived by Dr Henry, for the slow rombustion ap;iMius 

 of inflammable gases. This apparatus is represented in '" 

 Fig. 9. can is a pneumatic trough filled with water. ^ 

 n n a large glass jar likewise containing water, c c is a g4es. 

 stand, to which the graduated air holder b is made fast, Kig. 9. 

 by means of the moveable .irmdd. oo is another air- 

 holder, fixed by the plugs vv into the glass jar n n. It 

 is furnished with the stop cocka q, r, and the bent tube 

 .v. which passes into the water cistern, and passes up a 

 certain way into the air holder b, terminating in the ir-,.n 

 burner /. The air holder b is closed at top by a brass 

 cap e, and stop. cock _/i the lower orifice of which is 

 tapped internally, for the purpose of receiving a small 

 screw at the end of the copper wire g. The gas to be 

 burnt is put into the air- holder oo, while the air- holder b 

 is filled with oxygen gas. By opening the stop cocks 

 q, r, and letting water run into the jar n n, the gas 

 is gradually forced through the tube ts into the air- 

 holder b. An electric spark sets it on fin at /, and it 

 continues to burn till the oxygen or the inflammable gas 



