346 



surface of (he investigated liquid, can be made in the following way. 

 The surface of the liquid is strongly illuminated, and now attenti- 

 vely the moment is watched, when the capillary tube, while screwed 

 downwards, will just make contact with its mirror-image, seen in 

 the surface of the liquid. At temperatures abov^e 500° C. ordinarily not, 

 and above lOOO'' C. never, a special illumination of the surface is 

 necessary : for the liquid radiates then sufficiently to make the obser- 

 vation of the moment of contact a very accurate one. If the tem- 

 perature however becomes 1400° or higher, it is often no longer 

 possible to discern the end of the capillary tube from the surround- 

 ings : in these cases the adjustment must be made by the aid of 

 the manometer, as is further below described in some details. 



This visual method involves, even after sufficient practice an 

 uncertainty of about 0.1 mm. in the case of organic liquids, which 

 corresponds to ca. 0,006 to 0,008 mm. of mercury in the determi- 

 nation of the maximumpressure H. The percentage error of the obser- 

 vation caused thereby, is about from 0,4 to 0,77o ; this uncertainty 

 in the adjustment of the capillary tube on the surface of the liquid, 

 really appeared to be the chief source of the errors, as has been said, 

 and is hardly or not to be avoided. However just at higher tempera- 

 tures this and in the case of molten salts, where H is very great, the 

 accuracy of the method is only slightly affected by this uncertainty. 



The other way of adjustment is this, that the capillary tube is 

 approached quite near to the surface of the liquid ; then carefully 

 a flow of nitrogen is applied, and then, while the capillary tube is 

 slowly lowered by means of the micrometer-screw, by observation 

 of the manometer, just the moment is determined in which a sudden 

 rise of pressure, caused by the contact of the tube with the surface 

 of the liquid, is seen. In this way the proposed aim was also often 

 attained ; but the uncertainty appeared to be here of the same order 

 of magnitude, as in the case of the visual method. Furthermore it 

 is necessary to ascertain that the small column of liquid, which 

 often remains hanging in the capillary tube, if turned upwards, 

 has no misleading influence on this observation ; first this column of 

 liquid needs to be blown out by the aid of a sufficiently strong flow 

 of nitrogen, before the contact with the liquid is made in the way 

 just described. 



§ 11. The manometer was originally calibrated by immediate 

 comparison with a mercury-manometer, which was read by means 

 of a cathetometer aud a divided scale. The paralaxis appeared to 

 be extremely small; the accurate adjustment of the mercury-surfaces 



