XV111 



INTRODUCTION. 



a plate of caoutchouc. The bulb, however, is not surrounded 

 by a spiral, but by a metal shell, which resembles k k k in 

 fig. vii., and to this the projecting rod k' is fastened. If the 

 apparatus represented in fig. vn. is imagined to be made of 

 ebonite, and perforated in the centre, the dotted line will re- 

 present the position of the tube of the thermometer. Inas- 

 much as the object must in every case be placed on a covering 

 glass, two clips (e e, fig. vii.) are added to fix the glass. If the 



Fig. vi [. 



Slide capable of being heated, represented of natural size, kkkk, copper 

 ring and rim imbedded in the plate o o ; qq, heating rod ; e e, c'ips. 



plate is to be used as a- gas cell capable of being heated, the 

 object must be placed on the lower surface of the cover ; but if 

 only as a slide capable of being heated, it must be placed on 

 the upper surface, and requires then its own cover. In the 

 latter case the lower cover (g g, fig. vi. A. n.) is equivalent to the 

 ordinary slide, and only possesses the advantage of being a 

 thin plate, the temperature of which can be easily raised.* 

 The disadvantages of an ordinary gas cell appear prominently 

 in the cell capable of being heated. In no arrangement with 

 which we are at present acquainted does an equipoise between 

 the preparation and the atmosphere surrounding it occur. The 



* The mechanician, Heinitz, in Vienna, has constructed a gas cell capable 

 of being heated on the model of that just described, with a degree of ele- 

 gance that leaves scarcely anything to be desired. 



