41 Allen, etc. — Blopside and its Relations 



Figs. 13, a and b. — Microscope equipped with electric resistance furnace. 

 With the exception of the base and the upper tube, which were taken from 

 a Fuess universal stage microscope, this thermal microscope was constructed 

 in the workshop of the Geophysical Laboratory. The microscope is fitted 

 with revolvable nicols and a low power 3-inch objective. The furnace rests 

 directly on the stage of the microscope and can be revolved through small 

 angles. Its different parts are shown in cress section in fig. 13& (one-half 

 actual size). The water jacket consists of three parts ; W 2 , base ; D, side 

 cylinder, and ¥,, cap. Each of these parts is complete in itself and by 

 means of rubber tubing (fig. 13a) the water is made to pass through W 2 , then 

 D and finally W } . At G in both Wi and W 2 glass plates are introduced 

 and allow light rays to be transmitted without permitting the heat to reach 

 the objective and lower condenser system. The circulating water is suffi- 

 cient to keep these plates cool. In the first experiments air bubbles from 

 the water collected between the plates and seriously disturbed the clearness 

 of vision. This difficulty was overcome by means of rubber plungers fast- 

 ened to Ai and A 2 (fig. 13) which could be passed back and forth in front of 

 the plates and the bubbles brushed aside. By means of the rod A 3 with its 

 attached brass plate, transmitted light from any part of the field can be shut 

 off and the effects of emitted light alone studied. The furnace itself con- 

 sists of a tube K (fig. 136) 7"5 cm long, 4 , 5 cm outside diameter and l cm inside 

 diameter, wound on the inside with fine platinum wire - 35 mm diameter. The 

 thermoelement wires are supported by the porcelain tube T, which rests 

 directly on the asbestos paper covering the upper plate of W 2 . The thermo- 

 element wires are introduced into the furnace at M (fig. 13a) and the furnace 

 wires on the opposite side of the microscope. The sides of the furnace, 

 F, are surrounded with magnesia powder and the ends capped with asbestos 

 paper, to prevent loss of heat from radiation so far as possible. 



Better results were obtained by another method, in which 

 single 5 water-clcar crystals of /3-MgSi0 3 (about *2 X '2 X l mm ) were 

 mounted in cedar oil on the universal stage and turned until the 

 clinopinacoid was normal to the line of vision and the twinning 

 planes appeared as sharp lines. After photographing in this 

 position (magnification 100 diameters) the crystal was placed 

 in a specially prepared platinum basket and heated in an 

 electric resistance furnace to a specified temperature, either 

 above or below that of the /3-MgSi0 3 inversion. After cool- 

 ing, the crystal was again photographed under precisely the 

 same conditions as before heating. The study of a long series 

 of negatives prepared in this way has brought out several 

 interesting points : In the inversion of a single crystal of 

 /3-MgSi0 3 to the a-form, no great volume change is involved 

 nor even a great redistribution of the molecules. This is 

 evident from the fact that after reversion from the a- to the 

 /6-MgSi0 3 the original crystal is intact and its faces still fairly 

 sharp. Twinning planes are still present though usually in 

 different positions, each lamella extending the entire length of 

 the costal as before heating. On such paramorphic change, 

 inversion into one form and reversion to the original, it might 

 be expected that, as in crystal aggregates formed by precipita- 

 tion, many crystal nuclei would be formed,* and that on 

 reversion each one of the new nuclei would produce at least 

 one separate individual of the original form, with the result 



*This actually happens on the inversion of wollastonite into pseudo- 

 wollastonite. This Journal, xxi, 107, 1000. 



