ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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a slip of caoutcliouc. An amount of heat so small as this we may reason- 

 ably expect to obtain from the interruption of even feeble currents of elec- 

 tricity. 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. 

 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 tempera- 

 ture 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 

 appropriate means at our disposal. The tin-foil should be cut into the 

 form represented by S in fig. 51, and then glued to a glass slide; the 



Fig. 51. 



extremities of the tin-foil being introduced into the arc of a constant current* 

 A second strip of tin-foil of the same breadth as that attached to the slide 6, 

 is wound round the bulb of a thermometer and introduced into the circuit 

 at any convenient point. This furnishes the means of correctly estimating 

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

 conditions are uniform. These latter can, however, be estimated by com- 

 parison 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, a fatty substance, the melting point of which is known, 

 should be placed at the point whcsre the object is situate, and the reading of 

 the mercury should be taken at the moment that the fat begins to melt. 



As the temperature diminishes as the square of the strength of the 

 current, this decrease can to a certain extent be covered by diminishing the 

 transverse section of the tin-foil, so that if a weak current be in use the 

 strip of tin-foil must be made proportionately narrow. 



In order to exercise a direct control over the temperature of the cover- 

 glass, a thermometer should be attached to the slide itself. In fig. 52, a is 



Fig. 52. 



the bulb of the thermometer, the dotted line h indicating the direction of 

 the tube. Both the tubes and the bulb lie in a groove made in a hard 

 caoutchouc slide. A coil of very fine copper or platinum wire e is wound 



