GEJOPHYSICAI, LABORATORY. IO7 



(13) Melting-point methods at high temperatures. Walter P. White. Am. Journ. 

 Sci. (4), vol. 28, 474. 1909. 



Platinum resistance-furnaces of simple construction provide complete con- 

 trol of the temperature for melting-point work up to 1,600° C. Where uni- 

 formity of temperature throughout the working-chamber is important, special 

 modifications are necessary. Small charges (2.5 grams) give very sharp 

 melting-points, are economical of material, and permit of convenient manipu- 

 lation. 



A number of advantages result from the use of a second thermo-element, 

 which is made to give directly the furnace temperature about the charge. 

 The measurement and regulation of the heat supply from the furnace is a 

 factor of great importance in accurate melting-point determination. Methods 

 of treating and insulating thermo-elements and of avoiding the effects of con- 

 tamination have been developed, suited to various conditions and kinds of 

 work. 



The melting-points of very viscous substances, showing hysteresis, can be 

 determined easily and effectively by very slow heating and occasional exam- 

 ination outside the furnace. The approximate determination of latent heats 

 of fusion directly from melting-curves is possible by measurement of the fur- 

 nace temperature, but is encumbered by several hitherto undetermined sources 

 of error. The attainment of an accuracy greater than 10 per cent (about 10 

 calories in many silicates) accordingly requires special apparatus and proce- 

 dure. The smaller latent heats of inversion can often be determined to i or 2 

 calories, with no other apparatus than the two thermo-elements. For deter- 

 mining faint or sluggish thermal effects rapid rates of heating and the utmost 

 precision in furnace regulation and temperature measurement are needed. 

 The accurate location of eutectics by thermal means requires the detection of 

 small residues of the component in excess. This can be accomplished by a 

 special method involving the use of a neutral body. 



