EVOLUTION OF GASES FROM HOMOGENEOUS LIQUIDS. 279 



Effect of Reduction of Pressure on the Rate of Evolution of Gases. 



The retardation or promotion of certain chemical decompositions and combinations 

 by increase or decrease of pressure has from time to time been the subject of various 

 inquiries ; of these, some will be alluded to in the sequel. As the evolution of gas 

 from a liquid might apparently be modified by reduction of the superincumbent 

 atmosphere to which that liquid is subjected, as preventing a possible storage of 

 gas in its initial stage of formation, the effect produced by such a reduction of 

 pressure was considered a subject worthy of particular investigation. 



The apparatus employed consisted mainly of three parts, a blown-out flask of stout 

 glass, a SPRENOEL pump with the usual gauge, and a tube serving for the collection of 

 known volumes of the gaseous product. Through the neck of the flask passed a 

 "["-piece of glass tubing ; the vertical part of this was closed by a rubber cork through 

 which passed the thermometer, while the horizontal part, of stout glass, enclosing a 

 capillary airway, was connected by thick-walled rubber tubing with the SPRENGEL 

 pump. The delivery tube of the pump, resting within a small crucible filled with 

 mercury, was slightly curved up in order to pass the gas into a stout piece of glass 

 tubing of rather larger diameter. To the upper end of this latter was sealed an 

 upright piece of capillary tubing, which served to deliver the gaseous product iuto 

 some form of collecting apparatus placed upon the working bench of the laboratory. 

 The mercury dropping from the pump overflowed from the crucible into a porcelain 

 dish, and was returned from time to time to the funnel, to which the rest of the 

 pump was connected by a stout piece of rubber tubing, which could be compressed to 

 a greater or less degree by means of a clamp. In order to study the rate of evolution 

 of gas at any desired pressure below that of the atmosphere, a small strip of paper was 

 gummed on the gauge, and the flow of mercury adjusted proportionally to the evolu- 

 tion of the gas, so that the level of the mercury within the gauge varied only a few 

 millimetres on either side of the paper strip marking the reduced pressure. After 

 a little practice this process was found to be easy. In order to test the tightness of 

 the various joints, which were all carefully lubricated, the whole apparatus was 

 exhausted to a few millimetres pressure ; if the mercury in the gauge did not rise 

 appreciably after the lapse of several hours, the whole being left generally over night, 

 the apparatus was considered sufficiently air-tight for observations lasting only a few 

 hours. 



Decomposition of Oxalic Acid into Carbonic Acid and Oxide. 



The method adopted for collecting known volumes of carbonic acid and oxide has 

 been described above. Experiments were at first made under a reduced pressure, 

 then the mixed gases allowed to fill up the vacuum, and observations commenced as 



