ON RADIANT HEAT. 
343 
different illuminating powers. Now as in all cases there are several different 
species of heating rays emanating at the same time from the source, some 
luminiferous, and some not so, it is in no way a matter of surprise, or an ex¬ 
ceptional case, that the transmissive power of rock salt, or of any other sub¬ 
stance, should bear no proportion to the wen-, temperature of the source. 
M. Knoblauch, however, instituted an elaborate set of experiments, to 
ascertain whether any such relation could be maintained. 
The experiments were all conducted by means of the thermo-multiplier, 
which in this instance was constructed with especial precautions to eusure 
extreme accuracy and sensibility. 
M. Knoblauch's first series of experiments included as sources, alcohol 
flame, incandescent platinum, hydrogen flame, Argand lamp, of which the 
temperatures were in t he order of enumeration, the first being the highest. 
The effect of each on the tlicrmo multiplier was observed with the interven¬ 
tion of a series of screens, coloured glass, alum, mica, colourless glass, calc- 
spar, gypsum, &o. The transmitted effects varied of course with the different 
screens; but in every instance they were smallest with the first source, and 
increased in the order of enumeration, or in the inverse order of the tem¬ 
peratures. (Table I.) 
In this scries the real nature of the results is in fact evident from the 
distinctions above drawn. The effect is simply dependent on the light, or 
beating power of Species II., mixed no doubt to a certain extent in some cases 
with that of Species I.; which last probably does bear a close proportion to 
hz temperature of the source, but is in these instance? overruled by the effect 
of Species II. being very feebly, or not at all, transmitted by the screens. 
In this series also M. Knoblauch found that the transmission through 
rock salt was not exactly equal for all the sources, contrary to the assertion 
Of Mellon!: the difference, however, is very small. 
hi the second series the sources were a vessel of hot water of different 
temperatures, from 93° to 212°, the radiating side being in each instance 
covered successively with lamp-black, glass, wool, and in each instance the 
thermo-multiplier being placed at a greater distance, in the ratio of the in¬ 
creased temperature, so that the effects of direct radiation were equalized. 
each instance then u aeries of screens (the same as before) were inter¬ 
posed, each screen transmitting a different amount of heat, but the results 
with each temperature being found equal. (Table II.) 
In the third series the sources were hollow cylinders of iron and of copper 
closely surrounding the flame of a lamp, and heated by it to several tempera- 
tures, from284° T. up to u little below redness: in each instance the same 
equalization was effected as in Lhe last scries, and the same series of screens 
gave varied effects for each screen, but equal effects for each temperature, as 
ln the last series. (Table 111.) 
lhe fourth series included platinum in successive stages of heat: 1st, dark; 
-nd, just red; 3rd, yellow ; 4th, partly white. In each instance the series of 
screens was applied. In the ] at, 2nd and 4th stages of heat these gave uni- 
Ofui results, increasing with the heat; hut in the 3rd case, certain screens 
j? ve results less than in case 2, while another and smaller number only gave 
them (/renter. (Table IV.) 
he peculiar, and at first sight, apparently anomalous circumstance, that 
P atinum,at astage intermediate between red and white heat,transmits through 
tllC 8Cr< ‘ ens enjoyed rather less heat than when at the lower stage 
at ti' • ma J however be explained, if we suppose that the rays given off 
D mtcr| nediate stage are of such a wave-length as to be subject to a 
P u lar absorption by these particular screens. He then shows, from the 
