40 A. M. Mayer—Boundary of a Wave of Conducted Heat. 
were then obtained for each fixed temperature. Hach deflection 
given below is the mean of three experiments. 
_ Temp. Lamp-black. Iodide. Ratio of Deflections. Changes in Color. 
60° 18°75 = =—-:13°75 ree 
« § Cherry red, and turning in 
° 2.07% * pa > foo) 
ia gd a oa 1 spots to chocolate color. 
Dark red, with spots of 
0 . . 7 * > 
Rae aah! ides 3) Th oy ecslake coke, 
2 : ‘ , Whole surface of a deep 
70 24°0 16°87 r= 0 eo | 
72°: 260 17°62 pee 6 Deep purplish brown. 
75° 26°25 18°62 Ba we 
100° 45°0 30°5 ee an 5 3 e 
The last experiment, in which the temperature of the surface 
was 100°, gave deflections so far exceeding those produced before 
that I sought to pense them comparable by removing the hot 
water cube to a greater apt from the thermo-battery, 
when I obtained the following ra 
Temp. Lamp-black. Iodide. Ratio. 
100° 20° 13°41° L367 
The result was the same ratio as apa ee obtained. 
These 
? 
light, does not appear to have any action on its power of 
radiating the rays of heat of low intensity. I intend, however, 
to return to this investigation, provided with an apparatus 
giving the differential actions of two cubes, and having a - 
carefully calibrated galvanometer, and with this arrangement to 
test the reflecting as well as the radiating power of this and 
other iodides. 
Several applications of this iodide for showing elevations of 
temperature will naturally present themselves; for example, 
Foucault's experiment of the heating of a copper disc, when 
rotating in the magnetic field, can “be exhibited to a large 
audience by painting the dise with this iodide; on the dise 
attaining 70° C., the brilliant scarlet will ehange to a deep 
. brown, to regain {its former brilliant hue on coolin 
A more useful application may be made of this, or of several 
other more appropriate metallic compounds, by painting them 
on the pillow-blocks, and other parts of machines which are liable to 
injurious heating from friction. Thus the machinist can, from 
the colors of these paints, ascertain the temperature of these 
sometimes inaccessible parts of moving machines. 
May 20, 1872. 
