244? Dr. J. W. Draper's Remarks o?i the 



filled. The rim round the extremity a a, is then daubed with a 

 piece of burnt caoutchouc, and upon it is tied a thin piece of that 

 substance, with a fine but strong waxed thread. Over this is se- 

 cured a piece of stout silk or cotton cloth, for the purpose of 

 fortifying the elastic barrier. The wires d d e e, are next made 

 to communicate with the poles of an active voltaic battery, 

 and the condensation commences ; for the gas which is evolved 

 from these electrodes, rising to the top of the tube, accumu- 

 lates there, causing the column of water in the short leg to 

 rise and condense the atmospheric air above it. The mem- 

 brane though fortified gives way to a certain extent, becoming 

 convex outwards ; and as the accumulation of gas in the long 

 leg continues, the condensation of that in the short one in- 

 creases, as is indicated by the gauge cc. A very thin India- 

 rubber, of the diameter here indicated, will stand a pressure 

 of 6 to 20 atmospheres without rupture, if its silken support 

 is good ; and I have found that anointing the edges of the 

 rim with the burnt substance enables the operator to tie the 

 barrier on so that no leakage can occur between it and the 

 glass, under the severest pressures. When the gauge indicates 

 that the required degree of condensation is arrived at, the 

 connexion with the battery is broken, and the condensation 

 of course stops : the siphon being carried to the mercurial 

 trough, taking care to keep its position erect, its short limb 

 is depressed under the mercury and carried into ajar contain- 

 ing sulphurous acid. If, under these pressures, any of the 

 acid gas finds its way into the condensed air, its presence is 

 detected by the reddening of the blue litmus water. It is 

 necessary here to observe, that the indications of the tube 

 gauge do not give a correct estimate of the amount of con- 

 densation, but always represent them higher than they are, 

 according to Marriotte's law. It has long been known, that 

 the volume of gas dissolved in water depends in a great mea- 

 sure on the pressure exerted on it ; now it will be found, when 

 the operation is conducted in an instrument arranged as this, 

 that a certain proportion of the air in the gauge disappears 

 in this manner. Its zero point is therefore altered, and the 

 condensation appears higher than it really is. It may be re- 

 marked, in passing, that it is surprising to see to what an 

 extent the absorption of the oxygen and hydrogen is carried 

 in the longer leg, owing to their making their appearance in 

 the nascent form. To ascertain the true condensation, so 

 soon as the passage of the sulphurous acid or other gas has 

 taken place satisfactorily, the membrane is to be punctured 

 with a pin ; and when a pneumatic equilibrium is attained, the 

 height of the liquid in the gauge will mark the point where 

 the zero of the scale should be placed. 



