16 M. MELLONI ON THE FREE TRANSMISSION 
timetres in length, blackened on the inside; and at a certain distance 
from the mouths of these tubes are placed the stands destined to receive 
the screens. In strictness, a single tube and a single stand would be 
sufficient, and one of the sides of the pile might be closed by means of 
a small metallic cover ; but, when we have to operate on bodies differing 
in quality and thickness, it often happens that they differ in tempera- 
ture not only from one another but from the pile also. Then if we 
place but one screen before the apparatus, the calorific actions at the 
two sides are unequal, the index of the galvanometer moves away from 
zero, and we must wait for some time until the equilibrium of the tem- 
perature is established and the index returns to its original position. 
Now this inconvenience cannot occur when the pile is furnished with 
two tubes and two stands; for, by placing before each side of it a plate 
of the same quality and thickness, it is clear that, if care be previously 
taken to place the two in the same circumstances, they will have the 
same temperature, and will consequently emit the same quantity of heat 
on the two sides of the pile. The index of the galvanometer will re- 
main stationary, whatever may be the difference of temperature between 
the plates and the thermoscopic body, and we may therefore immediately 
proceed with the experiments. Hence, if we would save time, we should 
always have a pair of screens of each sort; and, as we have just observed, 
put both sides of the pile in the same state. 
In order to ascertain the influence exercised on free transmission, by 
the different circumstances relating to the surface, the volume, and the 
composition of the sereens, we must procure a constant source of heat. 
For this purpose, there is nothing better than a good lamp with a double 
current of air and a constant level. When this apparatus is well pre- 
pared and filled with oil freed from mucilage, by means of sulphuric acid, 
we obtain a flame which maintains an invariable temperature for more 
than two hours. Of this I have been able to satisfy myself by means of the 
thermomultiplier. Butin order to have things in this preparatory state, 
we must wait some moments until the pipe, the oil, and the glass funnel 
of the lamp shall have attained a maximum of temperature. This time, 
which varies a little with the construction of the lamp, is about ten or 
fifteen minutes. 
There may be some objections raised against the employment of an 
Argand lamp asacalorific source. It will be said, perhaps, that in this 
lamp the heat acts only through the glass funnel ; that the funnel itself 
becomes heated, and mixes its rays of nonluminous heat with the lumi- 
nous caloric of the flame; and lastly, that such a source of heat is 
neither uniform nor separated from the agent which usually accom- 
panies it in high temperatures. 
But I wish it to be particularly observed, that the only thing about 
which we are interested at present is, to know whether the state of the. 
