FURNACE. 25 



are usually permanently corrected by half an hour's glowing in the open 

 air at full white heat. The glowing must be done by passing a suitable 

 current through from end to end and not with a Bunsen burner or 

 gas blast. 



In event of a serious accident involving an exposure of the element 

 which can not be corrected by glowing, cutting out the exposed por- 

 tion of the platinum wire and reconnecting will almost always restore 

 the normal readings. 



The new form of element seeks to avoid both these difficulties; it 

 offers the advantage of an exposed junction without exposing the 

 platinum wire, and by making the platin-rhodium cap project but 

 little above the surface of the melting charge it avoids excessive loss 

 of heat by conduction away from the hot junction. In fact, in this 

 latter particular the new form enjoys a distinct advantage over the 

 usual form of heavily protected element. It has the disadvantage of 

 being more frail to handle, but there is little danger of anything more 

 serious happening than the breaking of the porcelain tube, which is 

 readily replaced. 



These elements are calibrated in metal baths like the others by 

 inclosing in a porcelain protecting tube. 



It need only be added that all the more important temperatures 

 throughout this work were separately determined with three different 

 elements. One of these was always from the Reichsanstalt set (pro- 

 tected) and one usually an unprotected element of the new form. 

 No systematic differences between the readings of the two tvpes 

 have ever been found. 



FURNACE. 



The furnace, in plan, differed but little from that in use for melting- 

 point determinations at the Reichsanstalt. In the working out, two 

 important changes were introduced, in order to enable it to reach the 

 higher temperatures of the mineral melting points. A more refrac- 

 tory and better insulating material (a mixture of magnesite and 

 corundum) was substituted for fire clay in the hotter parts and the 

 coil was wound on the inside of the oven tube instead of outside. The 

 latter involved some little mechanical ingenuity and skill in winding, 

 but the gain in economy and in the rapidity with which changes could 

 be effected or constant conditions established more than repaid any 

 additional labor in preparation. 



A diagram of the furnace in section is shown in fig. 3. It could be 

 used for any temperature up to 1600 C. without any difficulty or 

 especial precautions and could be regulated to maintain a constant 

 temperature at a particular point for long periods of time. 



