CHAPTER XII. 



ADIABATIC EXPANSION OBSERVED WITH THE INTERFEROMETER. 



86. Introductory. In the preceding report 1 I tested a number of receivers 

 in which air was expanded adiabatically, by passing one of the component 

 beams of the displacement interferometer through the air contained. The 

 vessels then used were not very satisfactory, being, as a rule, not long or 

 capacious enough to insure trustworthy results. Moreover, the interferometer 

 did not at that time admit of the introduction of long or bulky apparatus, 

 whereas in the new form a length of almost 150 cm. is available. The main 

 purpose of the research will thus be to ascertain how long and thin a tube may 

 be made to be serviceable for expansion experiments. Furthermore, it seemed 

 worth while to repeat the work preliminarily with a large, staunch tank since 

 found in the laboratory. This was a heavy cylinder of cast brass, about 27.1 

 cm. (inside) and closed by plates of heavy glass, each 0.56 cm. thick and 20.3 

 cm. apart (inside), the whole containing a volume of air of about 11,713 

 cubic centimeters, to be increased to 12,800 cubic centimeters, because of the 

 efflux pipe. The expansion pipe was 2 inches in diameter and closed by a 

 -2.^/2- inch brass stopcock, with a plug practically floating in oil to prevent the 

 ingress of air from without. The glass plates were secured by iron bolts, a 

 layer of resinous cement (equal parts of beeswax and resin) between glass and 

 the flat end faces of the cylinder being introduced to prevent leakage. 



To expand the gas in the receiver, the 2 -inch pipe communicated with a 

 tall, galvanized iron boiler used as a vacuum chamber, 29.4 cm. in diameter 

 and 147 cm. high, thus containing a volume of 99,800 cubic centimeters, or 

 100,200 cubic centimeters with the influx pipe. It was in communication 

 with a large air-pump and provided with a mercury gage for the measurement 

 of the partial vacuum produced by the pump. The air flowing into the air- 

 chamber after exhaustion was dried in the usual way and the influx controlled 

 by a fine screw stopcock. There was a special opening for a thermometer. 

 Vacuum and air-chamber were rigidly connected by a brass union with a 

 rubber washer. There was no appreciable leakage so far as the atmosphere 

 without was concerned. The 2 -inch stopcock, however, was not quite tight 

 within, so that air passed very slowly from the air to the vacuum chamber, 

 in proportion as their pressures were different; but as the air-chamber is in 

 service, either at atmospheric pressure (the influx cock being open) or, after 

 exhaustion, at approximately the same pressure as the vacuum chamber, this 

 leakage was of no appreciable consequence. Otherwise the interference pat- 

 tern would not have been stationary. 



While this apparatus was not long enough to fully realize the advantages 

 of the method of displacement interferometry for the purposes in question, 



Carnegie Inst. Wash. Pub. 149, Part II, Chapter IX, 1912. 

 142 



