368 
MESSES. W. E. WILSON AND P. L. GRAY 
chloride that it stood out with great distinctness against the dark metal in the field 
of the microscope. 
One observer, with his eye at the microscope, then switched on the current, and 
very slowly raised the temperature of the strip by turning the compressing screw of 
the carbon-resistance, until a sudden definite melting of the fragment took place ; at 
the same moment the second observer took the reading on the scale, which readiug 
then indicates the temperature 451° C. 
An exactly similar process was gone through, using a minute piece of chemically- 
pure gold (in weight about ^ of a milligramme), the melting-point of which we took 
as 1041° C. A curve was then drawn in which the abscissae are temperatures and 
the ordinates scale readings. One point on the curve is evidentl}" 0 on the scale 
at 15° C. (the temperature of the room). The other two points, viz., those 
corresponding to melting gold and melting AgCl, lie exactly on a straight line with 
this first point. That this coincidence was not mere chance is proved by the fact 
that we have calibrated three different strips—one in the first meldometer, in which 
the plane of the strip was horizontal, and two in the second instrunaent, with the 
plane of the strip vertical. The straightness of the line in each case is as perfect as 
it can be drawn wdth a straio-ht edge. 
o o 
The figures for the three strips are : 
