GENERAL METHODS OF INVESTIGATION, BY S. STBICKER. xiii 



warm the stage in its whole extent, but only its centre ; or, 

 what is still better, the glass cover placed on a slip of caout- 

 chouc. An amount of heat so small as this we may reasonably 

 expect to obtain from the interruption of even feeble currents 

 of electricity. It is well known that the heating of a wire, 

 introduced into the arc of a constant current, increases with the 

 diminution in diameter of the wire ; and indeed, according to 

 Biess, in the proportion of the bi-quadrate of the diameter. 

 For this purpose, therefore, we employ a proportionately thin 

 wire, attached to the centre of a glass plate, the ends being in 

 connection with the electrodes of a constant battery. When the 

 current is closed, the temperature of the centre of the glass 

 plate is raised. The attachment of the wire presents, however, 

 certain inconveniences ; and we possess in tin-foil a more appro- 

 priate means at our disposal. I am accustomed to cut the tin-foil 

 into the form represented by S in the adjoining figure, and then 



iir. TV. 



Slide adapted for being heated by means of electricity. Natural size. 



to glue it to a glass slide ; if now the extremities of the tin-foil 

 are introduced into the arc of a constant current, the end in view 

 is at once attained. A very convenient method of introducing 

 the slide into the current is to attach to one of Hartnack's 

 microscopes a couple of brass springs, by which the preparation 

 can be firmly clipped. These springs (D D, fig. v.), which are 

 attached to holes in the stage by means of brass pins, are pro- 

 vided also with india-rubber pins, by which means they are 

 isolated from the microscope. When they firmly clip the slide, 

 they at the same time press on the broad end of the tin-foil S. 

 It is then only requisite to attach a conducting wire at any 



