6 PROCEEDINGS OP THE AMERICAN ACADEMY 



proportion between the two depending upon tlie relation between the 

 sizes of the tubes, the capacity of the aspirator, and the supply of gas 

 from the vessel to be exhausted. If a fluctuation occurs in either the 

 rate of the aspiration or the supply of gas to be exhausted, a greater 

 or less proportion of air will be taken in, with a corresponding but 

 very small change of the pressure within the apparatus. For the 

 successful working of the apparatus a proper proportioning of parts 

 to the work to be done is of course necessary, and this is mainly to 

 be accomplished by varying the size of the point to which the fine 

 tube is drawn out. The sensitiveness of the apparatus may be in- 

 creased by using a U tube, instead of a vertical straiglit tube with 

 cistern, and allowing the fine tube to extend into the open arm of 

 this tube. A stricture at some point of the U reduces the too sudden 

 fluctuations. Various other modifications will readily suggest them- 

 selves to persons using the apparatus. The regulator is of course 

 of service in other ways than mei'ely rendering uniform the action 

 of the aspirator. It is possible to maintain with it a constant ex- 

 haustion of any desired amount up to the full exhaustion which the 

 asj^irator can produce. when working under the smallest head of water 

 likely to occur; and in the form of a U tube, with various lengths of 

 tube DE, to be inserted at A and raised or lowered, it affords a 

 very convenient means of varying the exhaustion at will. 



Procedure in Measureaients. 



The first operation is thoroughly to dry the apparatus and fill it with 

 pure dry gas. This was accomplished by repeated exhaustion and 

 refilling of all necessary parts of the tubing, gauges, etc., none but 

 thoroughly diy gas being admitted. This operation was repeated on 

 each separate day of measurement so that any error from slight leak- 

 age or diffusion of either air or moisture should be avoided. On the 

 first introduction of the dry air or carbonic acid, the further precau- 

 tion was taken to run the apparatus under quite high exhaustion for 

 many hours consecutively ; and in all cases, on each day of measure- 

 ment, the apparatus was run for an hour or more before any readings 

 were taken. 



The procedure in measurement was as follows. The aspirator was 

 set in operation, /being full of the gas. The generator and purifiers 

 were connected at P, and the cock c opened, yremaining closed, so that 

 the supply of gas came from the generator; the temj^erature of DG 

 was regulated, the stirrer being in continuous operation ; the ice-bath 



