66 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
Several of the numerical results contained in this table may be verified 
by. calculation. 
For, when two plates of different kinds are exposed together to the 
radiation of the source, their position relative to the entrance and the 
issue of the calorific rays does not affect the quantity of heat which passes 
through this system. This is easily proved by putting the first plate in 
the place of the second ; for the thermomultiplier, notwithstanding this 
change of order, continues to mark the same degree of its scale. Let 
us now take two plates and place them alternately in each of the two 
positions, for instance, the plate of alum and the chromate of potash. 
These two substances, exposed separately to 100 rays of heat emanating 
directly from the source, transmit 9 and 34 respectively. The quantities 
of heat that should fall on each of the two plates in order that 100 may 
emerge in each case is easily determined by these simple proportions: 
Ore sEOO! 22 LOO ss ne, 
B42 TOOw2: LOO: 202, 
which give 1111 for the alum and 294 for the chromate of potash. Now 
we know by experiment that chromate of potash exposed to 100 rays is- 
suing from alum transmits 57, and that alum exposed to 100 rays issuing 
from chromate of potash transmits 15. 
But the order of succession has no influence on the transmission of 
the pair: let us therefore reverse the system only in one case or the 
other. We shall then have the same plates exposed in the same man- 
ner to the two radiations of 1111 and 294. The quantities transmitted 
under both circumstances should accordingly be proportional to the 
incident quantities, as is actually proved within the limits of approxi- 
mation compatible with the nature of the experiments; for we have, 
672 WSs VIL 3.294. 
The table contains ten pairs which are submitted in both ways to the 
radiations of the source; there are in it consequently twenty numbers 
which should be in proportions analogous to the preceding. It is evi- 
dent too that these calculations require that the five plates emitting the 
100 rays which fall successively on the whole series of diathermanous 
bodies should be those that are indicated by the same names in the first 
column. I have accordingly taken care that this condition should be 
satisfied. 
The bodies submitted to the heat emerging from the screens present 
no longer the same order of transmission that they presented under the 
immediate action of the radiation of the lamp. The changes which 
take place have no apparent regularity whether we compare one series 
with another or consider only the different terms of the same series. 
Thus glass, Iceland spar, and rock crystal are more diathermanous to 
the heat emerging from the five screens than to that which comes di- 
