White — Melting Point Methods at High Temperatures. 477 



Fig. 1. 



<f 



tive and much more convenient than inclosing the whole fur- 

 nace in an air-tight bomb. 



II. The crucibles. — Two types of crucible installation have 

 been used for inversion and melting-point determinations. 

 The first is a relatively large crucible (5 cm in diameter) hold- 

 ing about 100 gr.* in which is inserted a porcelain tube for the 

 reception of the thermoelement. (See fig. 2, previous paper, 

 p. 455.) The porcelain tube is protected from the action of 

 silicate melts by a platinum jacket. This passes 

 through a platinum cover which shields the 

 charge somewhat from the cooling effect of the 

 furnace top. In case it is desired to stir or add 

 to the charge when hot, the cover may be omitted, 

 but it is much better to retain it even then and 

 to add a second tube, opening into the crucible 

 below through an appropriate hole in the cover. 

 To prevent the platinum jacket from dropping 

 off the porcelain tube when hot, notches are filed 

 in the sides of the tube into which the edges of 

 the jacket are bent. The jacket can afterward 

 be twisted off easily by the application of a little 

 force. 



The second type of crucible (fig. 1) holds 

 charges of about l cc . It is a platinum tubef 

 borne on a porcelain tube open at both ends, to 

 which it is fastened in the same manner as the 

 platinum jacket described above. The thermo- 

 element is passed down through and guided by 

 the porcelain tube and its bare end dips directly 

 in the charge. To insure proper centering of 

 the junction in the charge (1) two capillary insu- 

 lating tubes are used, one on each wire ; these 

 so nearly fill the larger tube as to be properly 

 guided by it ; (2) these tubes are brought as close 

 to the junction as possible without danger of their 

 touching the charge, thus leaving only 10 to 12 mm of the wire' 

 unsupported ; (3) the two capillaries and the wires are firmly 

 fastened together at the top, since a slipping of one wire up 

 or down relatively to the other will bend the junction to one 

 side ; (4) the wires are pushed into the melted charge very 



* 25 gr. charges have also been used in the past, but do not afford sufficient 

 depth of immersion for an inclosed element. 



f For substances which can easily be removed by melting them out, the 

 tube and crucible are preferably in one piece. For silicates, whose removal 

 usually requires hard pounding, it is more economical of platinum to make 

 the lower portion, which stands the wear, separate. When in use, it is held 

 in place by crimping its edges over the slightly flared end of the upper por- 

 tion (as illustrated in the figure). 



Am. Jour. Sci.— Fourth Series, Vol. XXVIII, No. 167.— November, 1909. 

 32 



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