GENERAL METHODS OF INVESTIGATION, BY S. STRICKER. xvii 



tubes and the bulb lie in a groove made in an india-rubber slide. 

 A coil of very fine copper or platinum wire is wound round the 

 mercurial bulb a, and the ends are made to lie on the broad 

 metal plate p p. The springs which conduct the current through 

 the instrument press upon these plates. 



Fig. vi. A n. is a longitudinal section of the stage in full work- 

 ing order ; g g is the little glass cover, upon or to the under sur- 

 face of which the object to be examined is attached. The cover is 

 in contact, not only with the surface of the slide, but also with 

 the coil of wire surrounding the bulb of the thermometer, the 

 transverse section of which is seen at a a. When the circuit is 

 closed, the wire becomes heated, and acts on the one hand 

 upon the mercury, and on the other upon the cover. The hard 

 caoutchouc is a bad conductor of heat, and hence the cover 

 receives the greater part of the heat. The figure renders it 

 apparent, also, how the slide can be at the same time used as a 

 gas chamber. 



Where only the centre of the slide, or the cover, is desired 

 to be heated, the flame of a candle or gas jet may be con- 

 veniently employed as a source of heat. 



For this purpose a copper ring and rod of the form kkkJc fig. vii. 

 are so inserted into the glass slide o o, that they do not pro- 

 ject beyond its surface; when it is required to be heated, 

 the rod q, with its coil, is slipped over the free end K K, and 

 to the extremity q the flame, which should be as small as 

 possible, is applied. If the rod is of about the thickness of a 

 large knitting-needle, it can be made of sufficient length to 

 obviate any inconvenience to the observer from the flame. The 

 centre c of the slide must be accurately arranged for a par- 

 ticular object glass, flame and focus. If a very small one be 

 employed, we may reckon upon tolerable uniformity of tem- 

 perature being maintained, though of course this mode has no 

 pretensions to scientific accuracy. If, however, the general 

 effect of an increase of temperature within certain limits is all 

 that is required, it is sufficiently useful. The facility with 

 which it can be made renders it valuable for large laboratories. 

 I have constructed another slide with a thermometer at- 

 tached, on the same principle of heating. The thermometer is 

 fashioned, as in fig. vi., in the form of an arch ; and is imbedded in 



C 



