on the contrary, which rises upwards, and may be driven 
_ by winds to and fro, unites with air. I will call the first 
kind, for distinction sake, Radiant Heat.” Thus arose 
_ the termwe stillemploy. The whole passage reads as if 
it were written almost at the present day ; and the lucid 
_ style of the last sentence cannot fail to strike the reader. 
_ This is the more remarkable if we contrast it with the 
current ideas of the time, or even with Scheele’s own 
description of the heat of contact; for a little further 
on he states, “This heat is a peculiar acid, which has 
admitted a certain quantity of phlogiston in its mixture.” 
Soon after this Pictet made his well-known experiments 
on the reflection of heat. In these he confirms the fact 
of the reflection and convergence of obscure rays, and 
discovers that the velocity of radiant heat is beyond the 
_ reach of experiment. To him is also due our first know- 
ledge of the apparent reflection of cold, a fact explained 
some ten years later (1792) by Prevost, according to his 
famous theory of exchanges. 
The experiments on radiation published in England at 
the close of the last and the early part of the present 
century, will be familiar to most of our readers. It will 
be remembered that Sir William Herschel established 
the refraction of heat, and the difference in the quality 
of solar and terrestrial heat ; that he confirmed Leslie’s 
experiments on the heating power of different parts of 
__ the solar spectrum, and first discovered that the maximum 
heat was beyond the visible red (experiments subsequently 
verified by Sir H. Englefield) ; that he also determined 
the transcalency of various kinds and colours of glass, 
both to white light and to the light of the spectrum. 
Both Rumford’s and Leslie’s inquiries into the Nature 
and Propagation of Heat quickly added to this know- 
ledge ; to Leslie belonging the capital discovery of the 
reciprocity of radiation and absorption. The accounts 
in our modern text-books render a further allusion to 
these experiments unnecessary. 
A quarter of acentury now lapsed ; the attention of the 
scientific world being diverted by the electro-chemical 
discoveries ofthat period. One of the products of the new 
activity thus aroused was the discovery of thermo-electricity 
by Prof. Seebeck in 1822. Some ten years afterwards, 
Nobili constructed the well-known thermo-electric pile. 
Associating this instrument with a galvanometer, Melloni 
at once turned Seebeck’s discovery into a thermoscope of 
surpassing delicacy. The fruit of one man’s work thus 
soon became the seed of new and more vigorous inves- 
tigation. And so prolific was this seed in Melloni’s 
hands, that the blackened ‘face of a thermo-pile is at 
present considered the indispensable pre-requisite in every 
exploration in “the domain of radiant heat.” For six 
years Melloni pushed on with his researches ; determin- 
ing the amount of heat transmitted through innumerable 
solids and liquids—their relative diathermancy, as he ex- 
pressed it—and using these determinations to investigate 
the quality of heat emitted from various sources. But 
the discovery with which his name will always be asso- 
ciated is that each material possesses a selective absorp- 
tion, a veritable heat-tint, to which he gave the name of 
thermochrosis ; thus confirming and explaining a similar 
fact previously noticed by De La Roche. Hence it was 
that Melloni called the volume “La Thermochrose,” in 
which he grouped together the investigations that he 
NATURE 
67 
had published, in the “Annales de Chimie,’ and the 
Comptes Rendus, between the years 1833 and 1839. The 
appearance of these researches was characterised by M. 
Biot as “un nouveau champ de découvertes, que M. 
Melloni a exploité avec un sagacité une addresse et une 
patience inimaginables ;” the subsequent verdict of phy- 
sicists has not lessened this high opinion. 
The interest awakened by Melloni’s inquiries was no 
doubt the main cause of the rapid additions to our know- 
ledge of the phenomena of radiation and absorption, that 
followed. Among others, Forbes, Dulong and Petit, De 
la Provostaye and Desains, Knoblauch, Jamin, Masson 
and Courtépée, Miiller and Balfour Stewart, signalised 
themselves by the value of their investigations in this 
department of natural knowledge. But the whole of these 
inquiries were directed to the behaviour of solids or liquids, 
or the analysis of radiation itself. The influence of 
gases and vapours on radiant heat was not entertained. 
Melloni, indeed, thought such attenuated bodies could not 
come within the reach of experiment ; for he had ascer- 
tained that a column of air some 20 ft. long exerted prac- 
tically no absorption on the radiation from his source. 
Pouillet and Forbes, however, showed that the heat of 
the solar rays are largely absorbed by our atmosphere ; 
and Franz believed (though erroneously) that he dis- 
covered a considerable absorption of heat by the air con- 
tained in a tube only 3 ft. long. 
Briefly speaking, this was the state of our knowledge 
in this branch of Physics when Dr. Tyndall approached 
the subject in 1859. After having wrought for twelve 
years, Prof. Tyndall has now gathered into the volume 
before us the important results his unremitting labour has 
won, A summary of these results must be left to another 
article. W. F. BARRETT 
LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his correspondents, No notice is taken of anonymous 
communications. ] 
On the supposed new Marine Animal from 
Barraud’s Inlet 
As some interest seems to be excited as to the nature of the 
animal that forms the long calcareous axis that has been received 
from Barraud’s Inlet, I forward you a copy of a short notice I 
read on the subject before the California Academy of Sciences, 
July 17, 1871. also enclose a piece of the stem with some of 
the soft parts still adhering to it, as it is possible its examination 
by competent observers will determine if I correctly referred it 
to the sponges. 
“ An examination of the specimens received from Barraud’s 
Inlet enables me to refer them to the Protozoa class, Spon- 
gid or sponges. Although apparently nothing but the 
calcareous stem has reached us, yet on some specimens I 
found one end of the stem covered with a horny substance, 
which, when moistened and examined under the microscope, 
presents the character of a true sponge, being formed of a tough 
sarcode arranged in the form of irregularly reticulated tubes, the 
sides of which are studded with minute pores. The arrangement 
of the sarcode round the axis is not circular, but has somewhat 
the appearance of a Maltese cross. The central axis is formed 
by calcified layers of a tough chitinous substance. In the 
specimens we have received the greatest thickness is about { of 
an inch, but the longitudinal fissures found in many of the stems 
would indicate that they had shrunk. They contain about 80 
per cent. of carbonate of lime. At each end the stem is tapered 
off. The top terminates in a fine hair-like prolongation of 
uncalcified chitinous substance. The lower part of the stem, 
which in our specimens is the only part covered with sarcode, 
