Thermodynamic Properties of Air. 305 



on the upper one, the cover can be made air-tight. Near the 

 centre of the upper brass cover there are four circular open- 

 ings. A fifth opening, with a short brass tube soldered in it, 

 is placed near the circumference ; it is connected by means of 

 a lead tube with the pneumatic pumps. 



The just mentioned four openings are intended to introduce 

 into the apparatus (1) the bulb s, which is to be cooled ; 

 (2) the bulbT of the hydrogen-thermometer ; (3) the electric 

 thermometer T'. The stems of these three pieces are held by 

 india-rubber stoppers cut into two halves and tightened by a 

 beeswax cement. Through the fourth opening enters in like 

 manner the delivering tube of the well-known apparatus of 

 Wroblewski and Olszewski for liquefying ethylene. 



The cold liquid ethylene flows down into the thin- walled 

 tall glass beaker C, surrounded by a wide glass tube B. 

 This tube rests on three elastic pieces of indiarubber w 7 hich 

 press its upper edge firmly against the brass plate P. This 

 double walling is intended to cool the surroundings of the 

 beaker C by cold ethylene vapour, which is forced to circu- 

 late as indicated by the arrows, until finally it is drawn out 

 by the pumps. 



Liquid ethylene is very liable to get superheated and to 

 evaporate in an explosive manner, especially when boiling 

 under diminished pressure. This property is a great incon- 

 venience when it is to be used to obtain constant tempera- 

 tures. After trying different contrivances to avoid it (air- 

 currents, Gernez's air-bubbles, &c), I contented myself with 

 the use of a large flask D inserted just before the pumps, in 

 order to diminish the fluctuations of pressure accompanying 

 the strokes of pistons. 



With the aid of this arrangement, it is possible to vary the 

 boiling temperature of ethylene through a range of some 

 forty degrees. Any desired temperature maybe obtained by 

 varying the number of air-pumps (I had three large Biancni 

 at my disposal), by inserting Babinet's stopcocks, or by vary- 

 ing the speed of the gas-motor driving the pumps. With 

 some practice it is possible to limit the range of fluctuations 

 of temperature in one series of experiments to no more than 

 2 or 3 degrees. 



§ 14. Construction of Isothermal Curves. — The temperature 

 6 of the bulb s in different experiments belonging to one 

 series was not exactly the same. To obtain strictly isothermal 

 values of the coefficient «, the following graphical method of 

 interpolation was used. All series of experiments having 

 been completed I constructed, on a large scale, a diagram 

 similar to that represented on PL I., using uncorrected values 



