Be SESE tee PO ay 
NATURE 
273 
q RADIANT HEAT 
i Lop ROSSE has shown* that the diathermacy of flame 
' cannot be determined by the method described by Mr. W. 
Mattieu Williams, in his communication to NATURE, vol. vi. 
p. 506. Referring to the discrepancy which the reader may re- 
member that Mr. Williams pointed out, Lord Rosse says: ‘‘ The 
explanation of the discrepancy seems to be that the radiant heat 
from a flame, like that from any other body, varies as the inver-:e 
square of the distance and therefore the total effect is propor- 
5 I I I . ee I 
tional to + a + rib + &e., not | + 7 + 7 
d, d@", &c., are the distances of the flames from the thermometer ; 
in which latter case the order of lighting the jets would answer 
the desired object.” 
Mr. Williams, in order to meet the objection raised, presents 
calculations based on the assumption that the mean distance 
between the seventeen small flames and the thermometer is 
14in. The space from centre to centre of the flames being } in., 
the distance between the thermometer and the centre of the 
nearest flame will be 12in. Evidently Mr. Williams is not 
aware that a thermometer placed at a distance of 12 in. from his 
cluster of diminutive flames, cannot indicate 53° C. (127°4° F.), 
unless the surrounding atmosphere be heated to a temperature 
exceeding 123° F. Had he ascertained, experimentally, that the 
radiation of a cluster of such feeble flames, at the distance of 
I2in. from the thermometer, imparts a temperature less than 
+ &c,, where 
-3°C. above that of the atmosphere, he would no doubt have 
admitted the correctness of Lord Rosse’s explanation of the sup- 
posed discrepancy, as freely as he admits the correctness of the 
theory on which that explanation is based. 
The subject having attracted much attention, the writer has 
deemed it necessary to institute a series of experiments in order 
to test the accuracy of the table of temperatures which Mr. 
Williams desires Lord Rosse to accept on the ground that ‘‘ the 
maximum error is less than ;'; of a degree, and the mean error 
lies between that and ~;}-z; of a degree.” 
Before entering on an examination of the result of the experi- 
ments adverted to, it will be necessary to call attention to the 
fact that, the transverse sectional area of a flame produced by 
17 jets arranged in a straight line, jin. apart, consuming 5 cubic 
feet of gas an hour, corresponds with the area of a civcle of 
0°87 in. diameter. The distance between the nearest point of 
the flame and the thermometer being 12 in., it will be found on 
calculation that the mean angle subtended by the heat rays pro- 
jected towards the thermometer will be 4° 9’. Agreeably to the 
laws of radiation, the temperature produced at a given point by 
the heat rays projected from a deep radiating body, depends on 
the mean angle subtended. It may be shown, therefore, that in 
order to produce the differential temperature of 53° — 19° = 34°C. 
(61°2° F.), observed by Mr. Williams, the intensity of the tlame 
must far exceed that capable of being developed by’any chemical 
means. A brief explanation of this important’ subject will be 
proper in this place. The accompanying illustration represents 
a pyrometer constructed by the writer for measuring with desir- 
able precision intensities of all degrees. The pyrometer consists 
of a conical vessel communicating with a cylindrical chamber, 
as shown in the sectional elevation, both being surrounded by 
an external vessel through which a current of water, of uniform 
temperature, is circulated. The thermometer for registering the 
temperature produced by the radiation of any incandescent body, 
is inserted in the cylindrical chamber at a fixed distance from the 
opening of the conical vessel ; hence the angle subtended by the 
heat rays projected by the radiating body towards the bulb of 
the thermometer, will remain constaut whatever be the distance 
of that body from the instrument. Of course the radiation must 
be large enough to cover the entire field contained within tie 
radial lines drawn from the bulb through the circumference of 
the opening of the conical vessel. We have already adverted to the 
fact that the temperature transmitted to the thermometer depends 
on the subtended angle. Consequently, by ascertaining practi- 
cally what degree of temperature is imparted to the thermometer 
of the pyrometer, by a radiator of vow intensity, we can 
determine the intensity of any other radiator by merely observing 
the temperature it imparts to that thermometer. It should 
be stated that in the pyrometers which have been constructed, 
the diameter of the opening of the conical vessel is }th of the 
distance from the thermometer ; hence the angle subtended is 
very nearly 11° 26’ Experimen{s m-de with an incandescent 
_* Naturg, vol. vii. p. 28. 
block of cast-iron, arranged as represented in the illustration, 
have shown that when the temperature of the block is 1930” 
above that of the surrounding atmosphere, the thermometer of 
the pyrometer indicates a temperature 20°7° higher than the 
circulating water in the casing. Applying the pyrometer in a 
similar manner, for ascertaining the temperature of a mass of 
overheated fused cast-iron, the indication of the thermometer has 
been found to reach 31°4° above that of the water in the en- 
closure. Now, the temperature of radiators is proportional to 
the radiant heat which they transmit, provided the subtended 
angles be alike; hen e it follows that the temperature of the 
fused metal must be 214 * 1930 = 19a 2940° above that of the 
20°7 
water in the casing surrounding the conical vessel. Adding 
the latter temperature, viz. 60°, we ascertain that the 
actual temperature of the fused metal will be very nearly 
3o0o°. The result of several experiments shows that this tem- 
perature corresponds with that determined by the practical ex- 
pedient resorted to during the investigation, of melting wrought- 
lion in the fused mass, It should be mentioned that in 
Pyrometer ‘or various intensities, 
constructing the scale of the new pyrometer, the radiant power 
correspording with given sulitended angles and given tempera- 
tures, has been ascertained wit critical nicety by experiments 
conducted within a vacuum. For the purpose of demonstrating 
that the distances assumed by Mr. Williams in his reply to Lord 
Rosse are greatly exaggerated, a reference to the amount of 
radiant heat transmitted to the thermometer by the fused metal 
will no'doubt be most convincing. We have before stated that 
the heat rays projected towards the thermometer from a flame 
produced by seventeen gas-jets subtend a mean angle of only 
4° 9’, while the heat rays projected from the fused metal towards 
the thermometer of the pyrometer, subtend an angle of 11° 26’, 
The areas corresponding with these angles being 4157 : 11°43? 
= 1: 7°6, it follows that, unless the intensity of the gas flame is 
3000° x 7°6=22800° F., it cannot transmit to the thermometer 
the same temperature as the fused metal, viz. 31°4° F. But 
Mr. Williams records an increment of temperature of 53°—19°= 
§ 61°2 x 22800 
34° C. (61°2° F.) ; hence —Ti 
temperature of his flame, if twelve inches be a true distance. 
Having thus proved by demonstration that the distances 
assumed by Mr. Williams are exaggerated, let us now briefly 
examine the result of the experiments which have been made 
with an apparatus constructed agreeably to the description con- 
| tained in his original communication. In carrying out these 
| experiments, it was deemed necessary, before inserting the re- 
cording thermometer in-a box as directed, to expose the same to 
the radiant heat of the flame, freely suspended in the atmosphere 
=44723° F., must be the 
