40 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
emitted by a source having a constant temperature. This condition 
could be complied with by means only of certain flames and boiling li- 
quids. I was therefore unable to vary the experiments so much as I 
should have desired.. The sources however which I have employed pre- 
sent the most remarkable phases of the héating and combustion of bo- 
dies. They are four in number; namely, the flame of oil without the 
interposition of glass, incandescent platina, copper heated to 390°, and 
boiling water. Thus I had two dwminous and two non-luminous sources. 
The first is furnished by a Locatelli lamp*; the second is a spiral of 
platina wire kept in a state of incandescence by means of a lamp fed with 
spirit of wine; the third is obtained by covering a flame of alcohol with 
a plate of copper, which soon acquires a fixed temperature whose mean 
value, as found by the method of immersion, is 390°Cent. (732° Fahr.) ; 
and the last source is merely a vessel of thin copper, blackened on the 
outside and filled with boiling water. 
The intensities of the radiations have been always ascertained by the 
thermomultiplier. The means necessary to be adopted in order to ob- 
tain with this instrument the measure of the immediate transmission 
having been stated in the Memoir already quoted, I think it needless to 
enter here into further detail as to the arrangement of the apparatus and 
the nature of the galvanometric indications. I shall only remind the 
reader that as this method requires that the operation should be per- 
formed under the influence of a radiation equivalent to 30° of my ther- 
momultiplier, the diaphanous substances, if placed at a suitable distance 
between the thermoelectric pile and the source of heat, cannot acquire 
a temperature sufficient to produce in the instrument any perceptible 
action. This is proved in three ways: first, by placing the screens on their 
stand after having exposed them to a calorific radiation of the same in- 
tensity as that to which they are exposed during the experiment; secondly, 
by substituting for the diaphanous body plates of blackened glass or 
metal, flakes of wood or stone, or sheets of paper; thirdly, by varying the 
nature and thickness of the medium (more or less transparent) through 
which the rays are to pass, from the thinnest plate of mica to pieces of 
rock crystal, glass, or Iceland spar several inches.in thickness. In the 
first case the index of the galvanometer remains unmoved, notwithstand- 
* The Locatelli lamp is merely a common lamp with one current of air and 
fed with oil. It has a wick of the shape of a quadrangular prism, which exactly 
fills the beak, but has no funnel. It gives a fine flame of constant tempera- 
ture. The Argand lamp produces a flame of much greater intensity. 
In the first series of experiments the main object was to determine the differ- 
ence between the calorific and the luminous transparency. We therefore pre- 
ferred the source which was least favourable to the establishment of the principal 
fact which it was then our purpose to verify. In the present experiments we 
proposed more particularly to examine the calorific transparency by itself. It 
was therefore necessary to operate upon rays that had not been forced to un- 
dergo a transmission previously to their being employed in the experiments. 
