XIV INTRODUCTION. 



point of each spring (E E, fig v.), and the circuit will be closed 

 by the tin-foil. A second strip of tin-foil, of the same breadth 

 as that attached to the slide (6, fig. IV.), is wound round the bulb 

 of a thermometer, and introduced into the circuit at any con- 

 venient point. This furnishes the means of correctly estimating 



Fig. v. 



Foot and stage of one of Hartnack's microscopes. 



the temperature attained by the centre of the slide, when all the 

 secondary conditions are uniform. These latter can, however, 

 be estimated by comparison, and the due employment of a 

 thermometer, a proceeding that is always requisite, whatever 

 may be the mode of heating employed. In order to accomplish 

 this, at the point where the object is situate, a fatty substance, 

 the melting-point of which is known, should be placed, and the 

 reading of the mercury should be taken at the moment that the 

 fat begins to melt. The quantity of fat that is introduced 

 should be very small, and should be in the field of the micro- 

 scope. It will be found most expedient to cut a little disk out 

 of the fat, to cover it dexterously, to watch it with a lens, and 

 to calculate it accordingly. 



I also apply one of Meidinger's chains with amalgamated 

 zinc plates. A chain of this kind works with very great 



