638 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 15, No. 10

4

3

e 2 b I e Y a

'0 Y

_I k

2

1

0 20 40 SO PRESSURE. CM. Hg IN SYSTEM

FIGURE 2. MAXIMUM RATES OF GAS FLOW THROUGR SmEREn-Gws DISKS

As can he seen from these relations, the limiting pressure dif- ferential for nonpassage of mercury varies inversely as I, while the flow of gas varies as the cube or 4th power of r. It follows,

therefore, that a disk with low resistance to flow will also have a law resistance to passage of mercury, which destroys the regu- lating function of the disk.

For example, the medium disk passed mercury at ahout 1.5 atmospheres pressure differential. Using the following values in Equation 1

y = 485 dynes per cm. p , - p , = 1.5 X 10' dynes per sq. cm.

C O S 8 = -1

r is calculated to he about 6.6 microns. up to 2 stmospheres without passing mercury. would estimate I to he of the order of 3 microns.

The fine disk waa tested From this one

While this disk can be used at greater preasure differentials, the gas flow, as shown by Equations 2 and 3 and by Figure 2, is considerably reduced. To obviate this difficulty valve b was in- serted in parallel with the disk. This valve can he adjusted to the point where the flow of gas to the disk is of a magnitude capable of being handled by the disk and the regulating property of the disk functions.

Because of its ready availability a fritted-glass disk WES used. A much better scheme would involve the use of a long, cylindrical cup of fritted glass, whereby the mercury would find ita level by covering or uncovering the cylinder for passage of pas. This was done by Wanshrough-Jones (S), who used a clay tube closed a t one end for regulating flow of gases at very low pressure dif- ferences. In this case, however, the presetting of a desired pres- sure would he more difficult since d, Figure 1, would no longer he a flxed point and height h would he a varinhle. Pyrex brand sintered-glass disks manufactured by the Corning Glass Co. were used.

Literature Cited (1) Galdwell. M. J.. and Barham, H. N., IND. ENB. Ca~aa.. ANAL.

(2) Lewis, F. M., Ibid.. 13,418 (1941). (3) WansbroughJones, 0. H., Pwc. Roy. Soc., A , 127.530 (1930).

ED., 14.485 (1942).

Pne~raam by ~ermisaion of the director, Bureau of Minea. United States Department of Interior.

Modified Hershberg Melting-Point Apparatus MORRIS M. GRAFF, Southern Regional Resear-L T -L---+--r ?inn Rnh.r+ P 1 - RlvA Nsu n.lPPns 1.-

CCURATELY controlled temperature and heating of n A . mg-point baths have long been a problem in the determ tion of melting points up to ahout 150' C. Where the temp ture is controlled by heating resistance units, one is apt to counter a lag or overshoot in the temperature when contar made or broken.

The Herahberg precision melting-point apparatus' has t modified, so that the melting-point temperatures of low-me1 solids may he maintained to within 0.1' C. over 8. longer perio time (Figure 1). Heat is applied through radiation from an frared drying bulb of 105 to 120 volts and 250 watts. When lamp is turned on or off, heat is applied or discontinued ins taneously, thus avoiding any heat Isg or overshoot. The bu placed within a metal container sufficiently ventilated to allom heat dissipation and is focused on the heating chamber of melting-point bath, The chamber consists of a flittened por of the Thiele tube made by sealing two 6cm. Pyrex petri di into the tube. By using the infrared bulb specified the limi temperature is ahout 150' C. A higher temperature coulc attained by using two heating units, one on either side of heating chamber, or by using lamps of higher wattage. heating is eontrolled by means of a variable-voltage transfon

1 Hershberg, E. B.. IN". EN.. Cam., An*=. Eo., 8, 312 (19361. FIGURE 1