[ 399 ] 



X. 



ON THE MELTING POINTS OF MINERALS. 

 By RALPH CUSACK. 



(Plate Y.) 



[commtjnicated bt peof. g. f. fitzgerald, m.a., scd, f.k.s. f.t.c.d.] 

 [Eead NovEMBEK 9, 1896.] 



Hitherto mineralogists appear to have made no efforts towards obtain- 

 ing; tlie melting points of minerals in tlieir natural state, though the 

 subject is one full of interest, especially in the case of ejected igneous 

 rocks and lavas, which at the time of ejection were subject to no 

 great pressure from the surrounding strata. There is also the theoretic 

 interest attached to such minerals as are the only known representa- 

 tives of a particular molecular grouping. 



As will be seen from the determinations given further on, melting- 

 points afford in many cases an easy and very convenient means of 

 identifying minerals, and may be used for this purpose where only 

 minute quantities of the mineral can be obtained. 



The instrument used for the following determinations is Dr. Joly's 

 meldometer — an instrument fully described by him in a Paper 

 published by the Royal Irish Academy.^ The working of this instru- 

 ment depends entirely on the expansion of a platinum ribbon heated 

 by an electric current under suitable control. The instrument shown 

 in the accompanying figure'^ is the latest form of the meldometer, as 

 made by Messrs. Yeates & Son (see page 400). • It consists of a 

 rectangular piece of slate cut as shown, on which are affixed two 

 forceps, one of which is rigid, and the other free to rotate round a 

 vertical axis, the lower end of which axis dips into a trough of 

 mercury, to ensure good electrical contact. A small spiral spring 

 attached to the vertical axis of the movable forceps, which may be 

 seen at the left-hand side of a figure, serves to keep the platinum 

 ribbon stretched when it is fixed in position. Projecting from the far 

 end of this forceps is a flat steel spring, on the further end of which is 

 fixed a small gold plate with which the platinum point of the micro- 

 meter screw, when carried forward, makes contact, which contact 



1 Proc. K. I. Acad. vol. ii., Ser. 3, p. 38, PI. vi. 

 ^ Kindly lent by Yeates & Son. 



