400 WRIGHT: ELECTRICAL GONIOMETER FURNACE 



jacket connections have been made, the furnace can be carried 

 to any temperature up to 1150° and the crystal angles or the 

 refractive indices of a prism measured by the ordinary room tem- 

 perature methods which are in general use. As the crystal is 

 enclosed in the furnace a dark room is not necessary for the 

 measurements. 



As noted above the adjustment of the crystal or prism is done 

 practically by hand both by moving and tilting the crystal 

 slightly and by bending the platinum jaws as a whole. In the 

 present apparatus this procedure is often tedious and it is planned 

 to modify the present adjustment device so as to facilitate this 

 part of the procedure. For crystal angle measurements this ad- 

 justment is unnecessary because the theodolite principle is there 

 used and any direction in the crystal may serve as a pole, the 

 proper reduction of the observed position angles being made 

 later by routine calculation. The arrangement of the alundum 

 segments in the furnace for the measurement of crystal angles 

 is indicated in figure 3a, while that for measuring the refractive 

 indices of properly oriented crystal prisms by the minimum de- 

 viation method is shown in figure 3b. The methods of measure- 

 ment followed are the standard room temperature methods and 

 need not be described here. 



It may be of interest to note that recent preliminary measure- 

 ments in the goniometer furnace on a cleavage rhomb of calcite 

 indicate that the cleavage angle of calcite at 600°C is 75°52', 

 while at room temprature (30°) it is 74°55', a change in the cleav- 

 age angle of nearly one degree during a temperature rise of about 

 600°. At 700° the calcite crystal faces lose their lustre and be- 

 come white (formation of CaO) and are valueless for goniometric 

 work. The above change in cleavage angle indictes an average 

 increase of 1 minute in angle for every 10° temperature rise. It 

 would seem, therefore, that the practice of expressing crystal 

 angles to seconds of arc without giving the temperature at the 

 time of measurement can serve little jxirpose and is in fact illusory 

 as regards the actual accuracy implied. 



An extended series of measurements of the change of the op- 

 tical properties and crystallographic angles of the rock making 



