RICHARDS AND MATHEWS. — ELECTRICAL HEATING. 



523 



material, a narrow cistern was blown into the bottom of a common 

 stout distilling flask. Into this depression the heating coil was placed. 

 The coil consisted of about 40 centimeters of platinum wire having a 

 resistance of about 0.7 ohms. A 

 current of from ten to fifteen am- 

 peres was led to the resistance wire 

 from above by heavy copper wires 

 encased in glass tubes, into the ends 

 of which the ends of the platinum 

 wire were sealed, contact being made 

 by a drop of mercury. It is necessary 

 that these copper wires be heavy 

 (about 2.5-3.0 mm. in diameter), so 

 that they may not become heated 

 by the current and thus superheat 

 the vapor coming into contact with 

 the glass tubes encasing them. For 

 this reason it might be well to intro- 

 duce the electrical connection from 

 below, through the glass walls of the 

 cistern ; but obviously the present 

 arrangement can be most easily 

 made. It is necessary that the coil 

 and mercury contacts be entirely 

 covered by the liquid at all times. 

 The diagram illustrates the arrange- 

 ment. The coil was more compact 

 than that represented in the figure, 

 so that it was possible to distil all 

 but four or five milliliters without 

 uncovering the resistance. 



It is almost needless to call attention to the fact that short-circuit- 

 ing through the liquid may cause slight decomposition when electro- 

 lytes are thus heated ; hence the method is not well applicable to 

 liquids of this class. 



Because the bubbles of vapor arise only from the small area of the 

 hot resistance wire, ebullition proceeds quietly, and there is never any 

 tendency to "bump." This method of heating is therefore especially 

 applicable to fractional distillations under reduced pressure, where so 

 much trouble is usually experienced from the explosive formation of 

 vapor. Concentrated sulphuric acid, for example, boils as quietly 

 under greatly reduced pressure when so heated as does water or 



