30 M. MELLONI ON THE FREE TRANSMISSION 
Tas_e IV.—Gilass (coloured). Common thickness 1™85. 
Deviations of Rays 
the galvanometer. _ transmitted, 
TICEPWIOIEL  o.c. 2. cecroctes sce sne cotton venane 18°62 5o 
Yellowish red (flaked) ............s000008, x 18°58 53 
Purple red (flaked)":........0..inasmaneues 18:10 51 
Wari 26d ic ccc. oeSe0 8. c0 oo bu eee aeaaee 16°54 AT 
Teale, VIM Cts jsieicccte vnaals oe sawlaleuiesteteneeatal 16°08 45 
rae FOR. ccc n ces thienncesapaspulécbistectes' 15°49 44. 
Clear Mate aay carters tanainaleuiinie sage ovens 15°00 42 
Deep CHOW, .: -acomecte st dunc ecucces Wier entire’ 14°12 40 
Bris YEHOW seals ccvsc evo: delde dev ere 12:08 34 
Golden yellow (or) fii iiecceececcs teavevaceve 11°75 33 
Wiceproline ee cctis tetoe cee ecdeanenmtceadeds 1160, 33 
PUP ECEN sa etaesmnckiadvesaac ves eulehes S15 26 
Winn @eal Pre eie si siin)s oaisatines vocieeesietlectiea 8:20 23 
Wet eGR TOME oi. viracans'spareciws<leaansives 6°88 19 
It is sufficient to cast the eye rapidly over the second and third tables 
to be fully sensible of the truth of the proposition, that “the capacity 
which bodies possess of transmitting radiant heat is totally independent 
of their degree of transparency.” 
In fact, the liquid chloride of sulphur of a tolerably deep red brown 
transmits a considerably greater number of caloric rays than the fat 
oils of nut, the olive, and colza having a clearer tint; while these 
oils, although of a very decided yellow colour, are more permeable 
to radiant heat than several other liquids which are perfectly limpid, 
such as concentrated sulphuric and nitrie acid, zther, alcohol, and water. 
The case is the same with solid bodies, among which we see sulphate 
of lime, citric acid, and other very diaphanous substances allow a 
much smaller quantity of heat to pass than some other bodies coloured 
or translucid, such as emerald, agate, tourmaline, borax, adularia, and 
sulphate of barytes. 
But nothing is better calculated to demonstrate that transparency 
has little or no effect in the transmission of heat than a comparison 
of the effects obtained by the crystal of alum with those obtained by 
means of the smoky rock crystal. The table shows that, in respect to 
these substances as well as the others which we have just mentioned, 
the capacity to transmit radiant heat is inversely as the capacity of 
transmitting the rays of light. I was anxious to try how far this in- 
verse ratio of the calorific to the luminous transmissions might extend, 
by varying the degrees of thickness so as to give to the light all the ad- 
vantage and the whole of the loss to the caloric. We submitted to the 
test a plate of well-polished and perfectly transparent alum only one 
millimetre and a haif in thickness, and a smoky rock crystal the thick- 
ness of which in the direction of its polished faces was 86 millimetres. 
