where necessary, were wrapped with a thick layer of asbestos. 

 A considerable time had to l)e spent, to get all connections 

 conipletelv free from leakage ; but when this was accomplished the 

 indications of the manometer were so prompt as to be practically 

 instantaneous. After this no errors from this source needed to be 

 feared. 



The connection of the tubes occurred in the usual way, as with 

 high-pressure apparatus ; these connections appeared to remain gas- 

 tight, even after a longer use. 



§ 7. For the measurement of the maximum pressures to be observed, 

 originaliy a mercury-manometer of the type indicated by Scheel and 

 Heuse ^) was used. The instrument had been modilied in some details ; 

 but it appeared not to be suitable for our work, because of the 

 necessity of always reading two menisci, which was very troublesome 

 with a pressure varying continually up to the moment, when the 

 maxiïnum was reached. 



This instrument therefore, which is very well adapted for static 

 measurements, was only employed for the calibration of the manometer 

 finally constructed. This second instrument was built on the principle 

 of the manometer with two liquids. 



In the measurements of organic liquids, it was necessary to avoid 

 any contamination of the connecting-tubes with the vapour of the 

 manometer-liquids, so that oidy pure mercury could be used as one 

 of the liquids in the manometer. For the second liquid we chose 

 normal octane. This liquid is vei-y thin, behaves very well in contact 

 with glass-walls, and, if completely dry, appeared not to blacken 

 the mercury-surface, even after long exposure. The vapourtension at 

 20° C. is only 10.45 mm., the viscosity at 23° 0. is 0,0052 C.G.S., 

 the surface-tension at 25° C. is 21,3 Erg. pro cm'., and the expansion- 

 coefficient is 0,00118. After repeated fractional distillation, its boiling- 

 point was found to be 125° C. under 758 mm. pressure, and its 

 specific gravity at 25° C. was : (ho = 0,6985, i. e. about 19,38 times 

 less than that of mercury at the same temperature. This last relation 

 operates in the following way : 



Suppose the diameter of the wider tube (fig. 4) to be Z), that of the 

 capillary tube d and the mercury-meniscus to the right to be a m.m. 

 higher than to the left. Then the height of the octane-column to the left 



1) Scheel und Heuse, Ein heizbares Quecksilbermanometer fur Drucke bis 

 100 m.M. ; Mitt. .1US d. Phys. Techn. Reichs-Anstalt, Zeits. f. Instr. 30, (2). 45. 

 (1910). 



