258 rETTKUSSON, os IFATKJ! AND ICE. 



importance but greater difficulty: are iJie indications of the nor- 

 mal iher mometers of Dr. GeissJer conformaUe to the real tempera- 

 iiires? The author first tried to decide this question by a 

 strict comparison of the Geissler thermometers with the air- 

 thermometer, described in the Journal fiir prakt. Chemie * 

 1882. The resnlt was the following: 



The niercury thermometers only agree exactly with the 

 air-thermometer a few degrees above and below zero, for 

 higher temperatures the indications of the two instruments 

 differ sliglitly, for lower temperatures the discrepancy gra- 

 dually but rapidly increases and at — 20'' C is considerable. 



If the indications of the air-thermometer are considered 

 to be the standard measure of temperature, the normal nier- 

 cury thermometers are far from being right, especially below 

 zero. It remained to be seen, if the Geissler thermometers 

 were correctly adjusted after the principle of the niercury ther- 

 mometers, i.e. if every centigrade degree oii tlieir scales corres- 

 ponded to an equal apparent dilatation of the niercury in the 

 bulb. I resolved to test this in the following way. 



The dilatometer, see page 262 and plate 20, was filled with 

 mercury, which was hollecl - in the reservoir. After cooling, the 

 instrument was placed in melting snow for several hours and 

 the excess of mercury alloAved to escape through >b,» until the 

 dilatometer was filled exactly to the stopcocks and weighed. 

 Afterwards the dilatometer was immersed in a mercury vessel 

 and the dilatation of the niercury in the instrument between 

 + 2°. 65 C and — 17°. is C determined in the ordinary way (see 

 page 264) by raeans of a scale-tube. The Geissler thermo- 

 meters Avere j)laced immediately beside the reservoir and the 

 temperatures read off with a katlietometer. The dilatometer 

 was thus transformed into a mercury thermometer of extra- 

 ordinär}^ size with a carefuUy calibrated stem (the scale-tube). 



» Journ. pr. Ch [2] Bd 25 p. 103. 



^ In spite of the fragile form of the instrument, this operation is not 

 verv hazardous. Care must be taken, that the quantities of mercury entering 

 through the capillarj' tube are heated beforehand to 200^ or 800° C. 



