258 Dr. A. M. Mayer on a precise Method of determining 



Barker I am also indebted for the iodide used in the experiments 

 I liere present. 



The first use I made of this substance was to track the heat 

 conducted by bars and plates of metal* ; and the sharpness of 

 the boundary of the colours instigated me to test the value of 

 this mode of experiment by applying it to a determination of the 

 elliptical contour of the isothermal of conduction in the principal 

 section of a quartz crystal. 



Senarmont, in his beautiful researches on this subject {Ann, 

 de Ch, et de Ph. S. 3. vols. xxi. & xxii.), has carefully determined 

 the ratio of the axes of this elliptical figure by coating a thin 

 longitudinal section of the crystal with wax and leading through 

 it a silver wire, by means of which heat was brought to the centre 

 of the plate, whence it was conducted outward, and its progress 

 and isothermal contour determined by the melting of the wax. 

 The following are Senarmont^s experiments on a plate 27 mil- 

 lims. square, whose sides were parallel and perpendicular to the 

 principal axis of the crystal : — 



Exp. 



Major axis. 



Minor axis. 



Ratios. 



1. 



. 12-50 



9-75 



1-28 



2. 



. 11-60 



8-50 



1-35 



3. 



. 10-00 



7-50 



1-33 



4. 



. . 12-00 



9-00 



1-33 



5. 



. 13-75 



10-00 



1-37 



6. 



. 18-00 



14-00 



1-29 



7. 



. 15-00 



1200 



1-25 



8. 



. 9-75 



7-50 



1-30 





Mean 



ratio . . 



. 1-31 



Senarmont, in the above experiments, used every precaution 

 to attain accurate results. He screened the plate from draughts 

 of air and from radiations, kept the plate horizontal and fre- 

 quently rotated it round its heated wire. After the ellipse had 

 become constant in its form, he allowed the plate to cool, and 

 then measured the axes of the ellipse by means of a micrometer. 



In the experiments which follow I used a quartz plate 27 mil- 

 lims. long, 22 millims. wide, and whose thickness was 1*2 millim. 

 Its centre of figure was pierced by a hole 1-25 millim. in dia- 

 meter, through which passed the vertical conical end of a silver 

 wire. The iodide was made into a paint with weak gum-water, 

 and in experiments 1, 2, 3, and 4 was applied to the surface of 

 the plate by a camePs hair pencil. In experiments 5, 6, 7, and 

 8 the better plan was adopted of flowing the iodide over the 



* The iodide is decomposed by contact with certain metals ; these should 

 be coated with a film of collodion, or electrotyped with copper, before ap- 

 plying the iodide, 



