402 Dr. H. Herwig on the Expansion 



water-bath, in order by violent boiling of the liquid to expel the 

 air from the leg of the tube and fill it exclusively with a certain 

 quantity of the vapour. When the wished-for degree of filling 

 is attained, this leg is closed by melting the end with a pointed 

 flame. As the boiling takes place under atmospheric pressure, 

 when we wish the half-leg filled with superheated vapour of 

 feeble pressure we have to keep only a corresponding fraction of 

 the interior of the leg free from mercury during the boiling, conse- 

 quently to put it in an inclined position, and to close the end as 

 soon as the small space left free is filled with pure saturated vapour 

 under atmospheric pressure. If, on the contrary, a greater pres- 

 sure of vapour is required, a larger space is to be taken for the 

 vapour during the boiling; consequently the tube must be in- 

 clined in the opposite direction ; moreover the tube can be closed 

 before the liquid is vaporized. In every case it is easy to secure 

 the filling of the leg with the exact quantity desired of the vapour 

 of the liquid in question, and free from air; of this we may 

 convince ourselves by letting the tube cool, when the vapour 

 becomes liquid again, and the entire half of the tube must be 

 occupied exclusively by mercury and a small quantity of the 

 liquid. This was always perfectly accomplished in my expe- 

 riments. 



The other leg is then, after the intervention of a drying-appa- 

 paratus, attached to the air-pump ; and when, after several ex- 

 haustions, it has been filled with dry air of the requisite density, 

 it also is hermetically sealed. 



We have thus a completely closed tube which, starting from 

 a certain temperature, contains at all higher temperatures super- 

 heated vapour on one side, and dry air on the other, of each a 

 fixed quantity. Both vapour and air then exhibit their pressure- 

 difference for each temperature through the difference of level of 

 the mercury between them in the lower half of the tube, and 

 therewith occupy each a determinate space. To ascertain with 

 exactness these spaces, I drew some strokes with a diamond upon 

 the tube, and then placed it, supported on a strong brass foot, 

 in a large water- bath, the fore and hind sides of which were 

 formed by plane-parallel looking-glass plates. The bath stood 

 upon a table fixed to the wail, in front o£ a window of the labo- 

 ratory, and was heated by gas-flames. A uniform temperature 

 in the bath was secured by means of a stirrer with a double 

 frame, which was moved up and down from 20 to 30 times in a 

 minute by a small steam-engine. 



The observations were conducted as follows. I first produced 

 in the bath the highest temperature that was to be attained by 

 the tube during the experiments, in order to make sure of ex- 

 pelling from the upper layers of the mercury any particles of 



