34 REPORT—1840. 
To the continental philosophers belongs the first invention of 
the instrument, without whose aid none of these investigations 
could have been accomplished ; while all the earliest and most im- 
portant discoveries of the varying diathermancy of substances ; 
the knowledge of the singular constitution of rock-salt, (which 
has placed a new instrument in the hands of the experi- 
menter) ; and the capital fact, disclosed by means of it, the 7e- 
fraction of heat from dark sources; together with the very sin- 
gular phenomena of the changes in the nature of heat, by trans- 
mission through certain substances; the remarkable effect of 
smoked rock salt; the circularly polarizing power of quartz for 
heat;—all these important discoveries (besides others of minor 
value) are imperishably associated with the name of Melloni. 
Our own country as fairly and incontestably boasts, besides im- 
provements in the apparatus and methods, many important re- 
sults connected with the transmission of heat, accurate measures 
of its refraction, together with some indication of phenomena 
analogous to those of diffraction. In addition to these, the 
sole and undisputed credit of first unequivocally establishing the 
grand facts of the polarization of heat, even from non-luminous 
sources, by transmission through mica, through tourmaline and 
by reflexion ; together with the peculiar and invaluable property 
of mica split by sudden heating (a fact holding a parallel rank 
with that of the diathermancy of rock salt); the dipolarization 
of beat; its consequent double refraction and interference ; its 
circular and elliptic polarization; its length of wave, and the 
production of that wave by transverse vibrations ; the confirma- 
tion of the circular polarization by the rock-salt rhomb, and 
the peculiar effects of metallic reflexion; these constitute the 
unquestionable claims of Prof. Forbes. 
On the main point in controversy between these two philoso- 
phers, the equal or unequal polarizability of heat from different 
sources, I have endeavoured to place the facts and arguments 
clearly before the reader; but must confess my own conviction 
to be in favour of the wneqgual ratio of polarizability in the radi- 
ations from luminous and from obscure sources, while in some 
instances the apparently opposite results seem distinctly traced 
to known causes, and in others the equalization of the effects 
appears to depend on some of those modifications which the in- 
tervention of screens produces in the nature of the rays of heat. 
The very remarkable class of phenomena just referred to, is 
perhaps of all the recent discoveries that which seems most sin- 
gular and anomalous : that the same ray should acquire an entire 
change of property and nature by and in the act of simply pass- 
ing through certain media, seems little in accordance with any 
