Fletcher — A Refined Method of obtaining Sublimates. 461 



reach nearly to tlie top of the box when bent at right angles at the hinge. 

 The forceps are provided with milled-head screws MM'. The hinges 

 permit of the use of varying lengths of carbon rod, placed at any 

 distance from either cover-plate, and, in the case of carbon, allow for 

 expansion. They are easilj^ manipulated, and simplify frequent changes of 

 the carbon rod. The sublimation chamber, which is closed above and below by 

 cover-plates of convenient size, is fitted with inlet and outlet tubes, so that the 

 heating process may be carried out in any desired atmosphere. Tlie cover- 

 plates may consist of glass, clear or opaque silica, biscuit ware, plaster of Paris, 

 or even white paper, and may be raised if necessary by circular washers. A 

 glass cover-plate is conveniently cooled by a drop of water on its upper surface. 



This apparatus, which, with the possible exception of the forceps, can be 

 readily constructed in the workshop, is sufficiently inexpensive to warrant its 

 general use by students. 



In general practice in tliis work a carbon rod was long since substituted for 

 the platinum ribbon of the meldometer, as platinum was found very unsatis- 

 factory, both on account of the ease with which it is attacked at high 

 temperatures, and owing to the limited temperature range consequent upon 

 its use. The powerful reducing action of the carbon at high temperatures is 

 also a sufficient advantage to recommend it as the most suitable substance 

 both in mineral work and in general chemical analysis. Small arc carbons 

 of the requisite diameter can easily be obtained, or a small cored carbon 

 arc rod having a diameter of 6 or 7 mm., with a soft core of 2 mm. 

 diameter, may be filed or sand-papered down on each side until after the 

 core has appeared.' The flat rod thus produced is easily broken along its 

 axis to form two flat carbon strips, having a cross-section about 2 mm. square, 

 which, having been freed from all traces of the core material, are broken 

 into convenient 3-cm. lengths. As it is desirable to localize the heat, a 

 lengtli of about half a cm. of the strip is reduced by a file to a very small 

 cross-section, in which a small hole is bored to hold the powder, and when 

 this part burns through, a fresh rod is substituted. This procedure is 

 economical, and prevents undue generation of heat within the sublimation 

 chamber. It is possible to raise the reduced portion of one of these strips, 

 which has a resistance at red heat of about half an ohm per cm., to nearly 

 the temperature of the carbon arc— about 3,600° 0. — with a current of less 

 than 20 amperes. At very high temperatures the strip slowly burns through. 

 For lower temperatures one may use the stamped graphite core of a lead- 

 pencil. This at high temperatures buckles and yields a white sublimate. 



' Tlie special oartona used in these experimeuts, having a cross-section 2 mm. square, can be 

 obtained from the "Le Carbone Company," London, E.G. 



8z2 



