White — Melting Point Methods at High Temperatures. 489 



temper at are, and the operation is repeated as a control experi- 

 ment to see if there is anything in the situation of the crucibles 

 or the peculiarities of the thermoelements calculated to imitate 

 the thermal behavior produced by the presence of a residual 

 in one more than the other. An example of such a deter- 

 mination is given in a previous paper.* The magnitude of the 

 result there obtained with a 2 per cent excess of MgSiO, 

 indicates that at least *25 per cent could probably be recognized. 



Summary. 



1. Platinum resistance furnaces of simple construction pro- 

 vide complete control of the temperature for melting-point 

 work up to 1600° C. Where uniformity of temperature through- 

 out the working-chamber is important special modifications 

 are necessary. 



2. Small charges (2*5 grams) give very sharp melting points, 

 are economical of material, and permit of convenient manipula- 

 tion . 



3. A number of advantages result from the use of a second 

 thermoelement, which is made to give directly the furnace 

 temperature about the charge. The measurement and regula- 

 tion of the heat supply from the furnace is a factor of great 

 importance in accurate melting-point determination. 



4. Methods of treating and insulating thermoelements and 

 of avoiding the effects of contamination have been -developed, 

 suited to various conditions and kinds of work. 



5. The melting points of very viscous substances, showing 

 hysteresis, can be determined easily and effectively by very 

 slow heating, and occasional examination outside the furnace. 



6. The approximate determination of latent heats of fusion 

 directly from melting curves is possible by measurement of 

 the furnace temperature, but is encumbered by several hitherto 

 undetermined sources of error. The attainment of an accuracy 

 greater than L0 per cent (about 10 calories in many silicates) 

 accordingly requires special apparatus and procedure. The 

 smaller latent heats of inversion can usually be determined to 

 one or two calories with no other apparatus than the two 

 thermoelements. 



7. For determining faint or sluggish thermal effects, rapid 

 rates of heating and the utmost precision in furnace regulation 

 and temperature measurement are needed. 



8. 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 in- 

 volving the use of a neutral body. 



Geophysical Laboratory, Carnegie Institution of Washington. 

 Washington, D. C., July, 1909. 



*Diopside and its Relations to Calcium and Magnesium Metasilicates, this 

 Journal (4), xxvii, 11, 1909. • 



