CLARK. — SURFACE TENSION BETWEEN LIQUIDS AND VAPORS. 373 



The pistons were made of tool steel hardened and ground as were the 

 cylinders in which they move. The fit is exceedingly good, there being 

 very little leakage of oil and scarcely any friction. It was feared that the 

 manometer would not give accurate results for pressures below ten atmos- 

 pheres, but the results are surprisingly good. It was compared with an 

 open mercury manometer and the error for readings as low as one 

 atmosphere was less than 1 per cent. 



The glass tube in which the pressures are read is 4 mm. in diameter 

 internally, and the scale is made of ordinary laboratory wooden meter 

 sticks. A displacement of one millimeter represents about 2.5 cm. on 

 an open manometer. The pump of the manometer is smaller than that 

 described by Amagat, and is used merely to control the position of the 

 pistons. The vertical motion of the pistons is aided by a twist given to 

 them by means of a lever, as described by Amagat. The manometer 

 has given the best of satisfaction and has proved itself a very valuable 

 adjunct to the laboratory. I am indebted to Professor Barus for the 

 drawings, the castings, and for many valuable suggestions. 



Since there are unavoidable minute leaks, and since when the manome- 

 ter operates there is an appreciable motion of the pistons and necessarily 

 an increase of volume in the experimental tubes, it is necessary to provide 

 some means of compensation. This is accomplished by a screw forced 

 into a steel oil chamber which is connected with the mercury chamber 

 under the heater (Figure 8) and with the manometer. This oil chamber 

 is simply a piece of steel shafting bored out to a diameter of seven- 

 eighths of an inch, and a steel screw three-fourths of an inch, forced in by a 

 hand wheel ten inches in diameter. The pitch of the screw is sixteen, 

 and the thread is packed with leather washers which had been previously 

 boiled in marine glue. Professor Barus has shown the efficacy of marine 

 glue for such purposes. The packing thus formed is screwed down with 

 the gland of the stuffing-box. 



Between the compressor just described and the mercury chamber of 

 Figure 8 is another reservoir for mercury. This is simply a chamber 

 filled with mercury and oil, connecting at the bottom with the mercury 

 reservoir of Figure 8 and at the top with the compressor. This obviates 

 soiling the mercury which comes in contact with the glass experimental 

 tube, and the mercury chamber, too, is kept clean. 



Between the two mercury chambers and also between the compressor 

 and the manometer are steel cocks which deserve mention. These are 

 made as shown in Figure 10. The especial feature is the hardened steel 

 conical stem which rotates on the stem proper. This cone is ground to 



