10) M. MELLONI ON THE FREE TRANSMISSION 
natural state when the radiation is stopped ; or that the heat, which is 
supposed to be diffused through the material points of the screen, is but 
common caloric obeying the known laws of equilibrium. In the first case 
we should be only attempting to explain the very cause of the transmis- 
sion, and the hypothesis, true or false, does not at all invalidate the fact 
which we are desirous to establish. In the second case, this heat, when it 
has reached the interior of the body, must take some time to issue from 
it; besides, this time must vary with the thickness of the screen, and its 
powers of conduction and emission. But let us intercept the calorific 
communication in our apparatus ; let us remove the diaphanous screen 
from its stand, and expose it for some moments to the free radiation of 
the lamp on the other side of the diaphragm: if the supposition be true, 
the internal molecules of the glass will instantaneously acquire some 
heat. In order to see whether this heat really exists, let us replace the 
screen on its stand before the pile, still leaving the calorific communi- 
cation with the lamp intercepted. The further-surface of the plate of 
glass will, according to the hypothesis, immediately begin to emit to- 
wards the pile that caloric which reaches it successively from within, 
and the index of the galvanometer must lose its equilibrium. But what- 
ever be the nature or the thickness of the screen with which this expe- 
riment is performed, we never obtain the slightest indication of a move- 
ment in the magnetic needle. It is therefore completely demonstrated that 
the deviations of the galvanometer exhibited in the experiments made 
with the diaphanous screens are not to be attributed, in the least degree, 
either to the external or the internal heat of the screen itself, but solely 
and exclusively to free transmission. Thus, whenever, in consequence 
of the radiant heat of the source being made to fall on a screen, a 
deviation of the galvanometer is perceptible, we may rest assured that 
the whole of the effect produced is to be ascribed to the rays of heat 
immediately transmitted through it, in the same manner as luminous 
rays. 
Before I conclude these preliminary considerations, it is necessary to 
remark, Ist, that galvanometers of very great sensibility, such as must 
be used for the thermomultiplier, do not directly indicate quantities 
less than half-degrees; 2ndly, that the ratios of the degrees of the gal- 
vanometer and the forces of deviation are unknown. But it is often 
useful to have the fractions below the half-degree, and in certain cir- 
cumstances it is absolutely indispensable to know the ratios of the seve- 
ral degrees of calorific action which move the magnetic needles to dif- 
ferent distances from their primitive position. 
To find the fractions sought, we have only to take the means of a 
certain number of observations. As to the ratio of the deviations and 
the forces, it is difficult and, in the present state of the science, perhaps 
impossible to determine it generally. But electric piles, such as those 
