368 



NATURE 



[/^jiQiisl 19, 18S0 



of Pascal, that when a liquid is put into a closed vessel, 

 and then subjected at any point to a pressure, this pres- 

 ^=ure is transmitted equally in all directions. If the vessel 

 be a strong one and provided with two movable pistons, 

 a large and a small one, the area of the large piston 

 being many times as great as that of the small one, any 

 pressure exerted upon the small piston to the liquid will 

 be transmitted equally over equal amounts of surface, 

 and hence the total pressure on the large piston will be 

 many times as great as the original force, just in pro- 

 portion as its area is greater than that of the small 

 piston. This is, in fact, the principle applied in the 

 hydraulic press of Bramah and in the hydraulic ma- 

 chinery of Sir W. Armstrong, by which heavy bridges, 

 dock-gates, and elevators are set in motion. The writer 

 of this article, when sore-pressed to devise an experi- 

 mental illustration of the principle of the hydraulic press, 

 contrived the following arrangement. The lid of a coffee- 

 pot was removed and a piece of sheet- indiarubber was 

 lied tightly over the open top. Into the spout a piece of 

 lead-pipe about six feet long was inserted, firmly fixed 

 with sealing-wax, and then turned up vertically. The 

 pot was filled with water, and a heavy book placed upon 

 the top. Water was poured into the lead tube until it was 

 filled up to the top. A column of water six feet high 

 affords a pressure of nearly three pounds per square inch, 

 and this, exerted over the whole area of the rubber- 

 covered top, gave a sufficient total pressure to raise the 

 heavy book. 



The air also possesses weight, and exerts a pressure 

 which may be upwards or downwards according to cir- 

 cumstances. Let a wine-glass or a tumbler be filled full 

 of water and a thin card laid upon the top of it, so that 

 bubbles of air arc excluded. Now invert the whole, 

 pressing the card lightly on to the glass during the 

 operation, to prevent accidents, and it will be found (see 

 Fig. 9) that the water will remain in the wine-glass, and 

 will not fall out. In fact the pressure of the air upwards 

 against the card is much more than sufiicient to counter- 

 balance the downward pressure of the water in the wine- 

 glass. 



Most of the experiments upon the pressure of the air 

 require, however, the aid of an air-pump for their per- 

 formance. With the air-pump a large variety of inter- 

 esting properties of the air can be demonstrated, which 

 otherwise cannot be shown. A few, however, do not 

 require the aid of this instrument. The effect of the 

 external pressure of the air in raising the level of a liquid 

 in a tube from which the air has been partially exhausted, 

 thereby reducing its pressure, can be shown by sucking 

 with the mouth at the top of a glass tube, the lower end 

 of which dips into the liquid in question. Thus it is 

 possible to suck up mercury to a height of fifteen inches 

 into a tube ; for the lungs are strong enough to reduce 

 the air in the tube to about half the ordinary pressure. 

 If a glass tube of sufiicient length were available it would 

 be possible to suck up water in it to a height of about 

 sixteen or seventeen feet ; for a column of that height 

 would be sufficient to counterbalance the difference 

 between the inside and outside pressures. 



The rising of a liquid into a space from which the air 

 has been partially removed may also be illustrated in the 

 following pretty way. Take a small bit of card and let 

 it float upon the surface of water in a shallow dish. Upon 

 it place a few shavings of wood and light them with a 

 match ; or place a small red-hot coal upon it, and on this 

 sprinkle a little brimstone to burn. Then quickly invert 

 over the blazing mass a wine-glass or a tumbler, as in 

 Fig. 10. As the shavings or the brimstone, as the case 

 may be, burn away, they withdraw the oxygen of the air 

 inclosed in the space above, until only the nitrogen (about 

 four-fifths of the whole) remains. The gases inside, 

 therefore, will not exert so great a pressure as before, 

 and consequently the pressure of the air outside will 



force the water to rise in the glass as the remaining 

 gases cool down to the temperature at which they were at 

 first. 



{To be continued.) 



ON THE ABSORPTION BANDS IN CERTAIN 

 COLOURLESS LIQUIDS 

 [Preliminary Notice] 

 TTAVING occasion to examine the absorption spectra 

 ••• ^ produced by considerable thicknesses of alcoholic 

 solutions of certain cobalt salts, we were led accidentally 

 to obcerve that alcohol alone gave a very distinct band, 

 and afterwards, on examining water, found that it also, 

 when a column of six feet was used, gave a very distinct 

 absorption band in the orange, a little on the less refran- 

 gible side of D. By graphical interpolation we find the 

 centre of this band to be about 600, and that the band 

 extends from 607 to 596. This position corresponds very 

 closely, if it be not identical, with Piazzi Smyth's rain 

 band,' and also with the band seen in 330 feet of high- 

 pressure steam by Janssen.- 



Fig. I represents this spectrum. It will be seen that 

 the absorption at the red end extends up to the line C, 

 and the end of the shadow is so sharp that it is probable 

 there is a band at this point also, but masked by the 

 general absorption. To convince ourselves that this band 

 belonged to water and not to any accidental impurity, we 

 experimented with different samples of water, using ordi- 

 nary tap- water, ordinary distilled water, also water which 

 had been made with much care absolutely pure ; in all 

 these samples this same band was visible, and as long as 

 the water was clear, as far as we could judge, it was of 

 the same degree of intensity, A column of water eight 

 feet long shows the band clearer than one only six feet ; 

 still greater lengths we have not yet tried. We next tried 

 the effect of increase of temperature on the water. For 

 this purpose the glass tube containing the water was 

 fitted into an air-bath, and the temperature was raised 

 from 20° to 60° without removing the tube from before 

 the spectroscope ; no change in the band, either in 

 position or intensity, as far as we could see, occurred. 

 Further, it seemed to us that it would be interesting to try 

 whether, on dissolving different colourless substances in 

 water, the band would be affected. We consequently 

 exammed saturated solutions of the following substances 

 in a tube 8 feet long : — ."Xmnionium chloride, ammonium 

 nitrate, ammonium carbonate, potassium nitrate, lead 

 nitrate, sodium chloride, and sugar. In all these cases 

 the band was as visible as in pure water, and no additional 

 band was seen. With a mixture of I volume of sulphuric 

 acid and 5 of water the band was unaffected, but if pure 

 commercial hydrochloric acid was examined in a 6-feet 

 tube the band was invisible, but with 8 feet a faint indi- 

 cation of it was seen. 



This absorption with water being so marked, we naturally 

 went on to try whether other so-called colourless liquids 

 gave, when depths of 6 or 8 feet of them were examined, 

 absorption bands, and at first really our difficulty was to 

 find any liquid which did not show clearly one or more 

 bands. 



The ordinary solution of ammonia gave a very clear 

 and marked spectrum (Fig. 2). It consists of four bands, 

 the centres of which are at 650, 630, 610, and 556. The 

 band at 650 is much the darkest, and the band at 630 is 

 remarkably sharp. Then with regard to the 610 band, it 

 is characterised by sharpness only on the least refrangible 

 side, but shades off gradually on the other side, the shade 

 extending as far as 596 ; this shade is probably due to 

 the water band ; and lastly, the band at 556 is by far the 



' Piazzi Smyth, " Edinburgh Astronomical Observations," vol. xiv. 

 ' In this and the following experiments a Desaga's spectroscope with a 

 single heavy glass prism was used, and the source of light was an ArganA 

 re expressed in the millionths of a metre. 



