HKTWKKN ONK ATMOSPHERE AND HALF AN ATMOSPHKKE OF PRESSURE. 355 



connected above, through F, with the pump or with the gas supply. The lower 

 chamber A communicates with the upper cli;iml-r I) of the right-hand manometer 

 and with an intermediate reservoir E, to which, as to the manometers, mercury can 

 be supplied from below. The lower chamber C of the right-hand manometer is 

 connected with the principal gas reservoirs H, H. It is here that the novelty enters. 

 In the 11)02 apparatus the two equal bulbs were superposed, being connected by a 

 narrow neck. For the doubled volume, both bulbs were occupied by gas ; but for the 

 single volume, only the upper one was available. For the comparison of the single 

 and double volume, a principal factor of the final result, preliminary gauging had to 

 be relied uj>on. In the new apparatus it was desired largely to increase the volumes, 

 and it was both more advantageous and more convenient to place the two bulbs H 

 side by side. The temperature conditions are thereby improved ; but what I wish to 

 emphasize at present is the elimination, thereby rendered possible, of dependence 

 upon preliminary gauging, for either bulb is now available for the single volume ; and 

 if both are symmetrically employed in each set of observations, the mean necessarily 

 corresponds to half the total volume, whether or not the two single volumes are 

 preri^-hi i-ifunl. The volumes are defined, as usual, by marks GG, II, upon the 

 associated tubes above and below. The use of the side-tube JK will be explained 

 presently. 



When, as shown, the mercury stands at the lower marks I, the double volume is in 

 use, and the pressure is such as will balance the mercury in one (the right-hand) 

 manometer. A vacuum is established in the upper chamber D, from which a way is 

 open through ABF to the pump. When the mercury is raised through one of the 

 bulbs to the upper mark G, the volume is halved and the pressure to be dealt with 

 is doubled. Gas sufficient to exert the single pressure (381 millinis.) must be supplied 

 to the intermediate chamber E, which is now isolated from the pump by the mercury 

 standing up in the curved tube AB. Both manometers can now be set and the 

 doubling of the pressure verified. 



The communication through to the pump is unobstructed, but on a side tube a 

 three-way tap is provided communicating on the one hand with the gas supply and 

 on the other with a vertical tube delivering under mercury, by means of which a 

 wash-out of the generating vessels can be effected when it is not convenient to 

 evacuate them. The six tubes of glass leading downwards from the gas reservoirs, 

 manometers, &c., are all well over a barometer-height in length, and are terminated 

 by suitable indiarubber hoses and reservoirs for the supply of mercury.* By this 

 precaution the internal pressure on the hoses is guaranteed to exceed the external 

 atmospheric pressure, and under this condition the use of iudiarubber seems to be 

 free from serious objection. If, however, the external pressure be allowed to be in 

 excess, there are soon signs of the percolation of air and probably of moisture. 



When settings are actually in progress, the mercury in the hoses is isolated from 

 * These reservoirs were protected from external moisture by tubes of chloride of calcium. 



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