PHYSICAL PROPER! IES OF CLAYS. 31 



Attempts have been made to express the 

 fusibiliy of clays nuniercally, and this number has 

 been called the refractory quotient by Bishop* and 

 the fusibility factor by Wheeler. In both cases, the 

 figure is obtained by using the non-fluxing elements 

 of the clay for the numerator, and the fluxing impuri- 

 ties as a denominator; and in the case of the second 

 formula, the fineness of the grain was also taken into 

 consideration. As this mode of expressing the fusi- 

 bility has not come into general use, the reference is 

 simply given here. 



On the other hand, it is customary to express the 

 fusibility of the clay in degrees of temperature, and 

 this temperature is measured by one or another form 

 of pyrometer, whose principle depends on -the fusion 

 of alloys or single metals; thermo-electricity; fusion 

 of an artificial mixture; spectro photometry; expan- 

 sion of gases or solids; etc. Many of these are only 

 applicable at lower temperatures, others are largely 

 influenced by the personal equation, and only two or 

 three of the most important will therefore be mention- 

 ed here. 



THE THERMO-ELECTRIC PYROMETER. 



Le Chatelier's Thermoelectric pyrometer depends 

 on the measurement of a current generated by the 

 heating of a thermo-pile. The latter consists of two 

 wires, one of platium, the other an alloy, 90 per cent. 

 platinum and 10 per cent of rhodium, twisted together 

 at their free ends for a distance of about an inch, wlrle 

 the next foot or two of their lenth is enclosed in a fire 

 clay tube so that when the couple is inserted into the 



*Die Feuerfesten Thone, p. 71, 1876. 



English and Mining Journal, March 10, 1894. 



