A Glass Vapor-Density Balance J. H. SI3ZONS The Pennsylvania State College, State College, Pa.
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The beam can be adjusted for center of granty b y careful bending, and the balance can be adjusted by carefully pulling off or adding small pieces of glass. If this is done a t ordinary laboratory temperatures and pressures, the balance when assembled will balance in pure oxygen at about atmospheric pressure, when the vapor of boiling acetone is used in the jacket. The increase in temperature approximately com-
APOR-density balancea ai e very useful in connection with apparatus used for the study of gases. A very satisfactory design is shown in Figure 1. It is made entirely of Pyrex glass, has a vapor jacket to maintain constant temperature during measurements, and has been found to be as sensitiye as the presqure can be read on an ordinary mercury manometer.
-1.
B. C.
D. E.
T\-atei-cooled condeii>er Tia3 tiround j o i n t -upport t u b e Closed bulb
F. G.
Bearin. Cross ;od Counterpoi-e I. Spiral J. Liquid t r a p
H.
The beam is made of rod about 1 mm. in diameter, and is about 17 cm. long. At one end is a closed bulb, E , which is as thin as it can be made and yet have sufficient mechanical strength. It tapers toward the far end, so that it does not strikethe containing wall during swings. The bearing at F is made of thin rod which terminates in two sharp but fire-polished points. The cross rod, G, serves to stop the swings when its ends strike the mdl of the support tube and prevents the bulb with its greater surface from ~1~~ counterpoise,H,terminates nlaking contact the in a pointer. The support tube, D, about 35 mm. in outside diameter, makes a close but not tight fit in the inner tube, and is prevented from rotating by springs of flat metal, M , between it and the tube wall. The suppod is made by tearing a slot in the tube more than half way around and about 1 cm. from the end and shaping with carbon rods. It contains shallow depressions in which rest the bearing points. The inner tube, about 38 mm. in outside dianieter, terminates at one end in the large ground joint, C, and at the other in a short spiral, I , nhich extends through the end of the outer tube and becomes the connecting tube to whatever apparatus the balance is attached. The outer tube, about 51 mm. in outside diameter, is connected on the lower side to the flmk, L, by means of two tubes, one for vapor and a return tube, K , for liquid, and on the upper side to a water-cooled condenser, A . -1shallow trap, B , and drain return the condensed liquid without its flowing back into the jacket. J is a liquid trap.
K. L.
R e t u i n tulre Fla-h
.\I. S p r i n i -
pensates for the increase in density of oxygen Over air. It is the outer tube rvith a thin coating of asdesirable to bestos Paper for heat insulation except for places at which to observe the pointer and bearings. A telescope with cross hairs is convenient for observing the pointer. The center of gravity can be adjusted so that a difference of pressure of 1 of mercury at the balance point' ill cause the beam t o change from its ext,reme upper position to its extreme lower position. This gives a precision of readings cornparable Tvith the pre?ision Tritll TT]licha mercury manometer can be read, or in the region of joo-mm. pressure an accuracy of 0.2 per cent. This device has been found very useful in determining the molecular jveights of unknoT1-n gases, and in folloJving the of distillation of a mixture containing a homo~ogous series of Compounds having a number of isomers. Cuts can be made in the distillation at constant molecular weight rather than a t constant temperature. This balance has been found of months. to retain its calibration Over RECEIVED July 26, 1938.
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