and Polarization of Heat. 143 
Another series on a different day gave the following quantities 
per cent. 91, 82, 94. Mean of the whole 86.4 : 100. 
21. Having obtained these decisive results, I proceeded to 
operate with other sources of heat, and with different tourma- 
lines.. Anxious to avoid the interposition of glass, I had a pair 
of tourmalines of large size cut without any support. But the 
best kind will not bear this, and they polarized imperfectly. On- 
ly fifteen-sixteenths (approximately) ofthe light in the bright po- 
sition was stopped in the dark, whilst with the tourmalines A and 
B every vestige of the brightest gas fame was excluded. With 
these tourmalines (which may be called C and D) I verified the 
general conclusions. I was unable to get sufficient effect from 
non-luminous heat to verify the law in that case. 
22. I had two very fine tourmalines cut and mounted on ex- 
tremely thin. glass. These we may eall E and F. With them 
I was enabled to extend and verify the law of polarization even 
to the case of non-luminous heated brass, (whose temperature 
when warmed by alcohol, M. Mrxionz estimates at 390° cent. 
= 734° Fahr.) And it is worthy of observation’ that among 
twenty-nine pairs of comparative observations, made with three 
sets of tourmalines, and heated from the following sources, ar- 
gand lamp, simple oil lamp, platinum rendered incandescent by 
alcohol, and non-luminous hot brass, there was only one which did 
not give positive indications of polarization. The effect, how- 
ever, with non-luminous heat is extremely feeble, and the per- 
centage very small, because it is with great difficulty that we can 
obtain results at all with the interposition of two plates of glass, 
and two of tourmaline (however thin), and a large portion of heat 
which reaches the pile is derived from conduction, and therefore 
diminishes the proportion of polarization. 
_ 23. It is very important to observe, that in this and all simi- 
lar cases, the effect of conduction or the secondary radiation of heat 
Srom screens always tends to disguise, and never to produce, the dif- 
Serences of which we are in search; that is, so long as the means 
of alternate observations are taken in the way we have described. 
