OF RADIANT HEAT THROUGH DIFFERENT BODIES. 47 
bability of error is diminished in the same degree*. Let us now sub: 
stitute for the plate of glass a flake of alum, sugar, or ice; we shall find 
that the needle of the galvanometer is perfectly at rest: if there is any 
heat transmitted, it is therefore not more than 4.375 = zi of the whole 
radiation. Thus it is true that the transmission of these three substances 
reduced to plates of 2™™-6 in thickness and exposed to the radiation of a 
body heated to 390° is null or less than z}sdth part of the whole in- 
cident heat. It is by operations analogous to this that I have been able 
to ascertain the limits of the values of the zeros of transmission. 
Now that we know the degree of exactness to which the measures 
contained in our table have been carried, we may proceed to state the 
consequences to which theywead. c 
Let us, for the moment, not notice the results obtained with the rock 
salt. The order of the transmissions has no relation to the degree of 
transparency, as we have already determined in our first series of expe- 
riments. It is not strictly the same when we change the calorific source ; 
but each substance exposed to the successive action of the four radia- 
tions presents a like order of decrease in respect to the quantities which 
it transmits from each of the sources; that is to say, that all the sub- 
stances transmit quantities of heat which are feeble in proportion as the 
temperature of the radiating source is low. There are several cases 
in which the transmissions are nothing; but these cases do not make 
* This mode of estimating the energy of the calorific radiations enables us to 
determine without difficulty the ratios existing between the ares described by 
the magnetic needle of the galvanometer and the corresponding forces. Letus 
suppose the calorific source removed sufficiently far from the pile to produce 
but a feeble deviation of the galvanometer; one of 10°, for example. In the 
passage of the calorific rays let there be interposed a plate which transmits a 
certain fraction of the incident heat. We shall suppose this fraction to be 1; the 
needle will descend to 2°. By bringing the source near, the deviation produced 
through the plate will be increased. Let us stop, when the needle shall have 
reached 4°, 6°, 8°, &c. successively; the calorific source will then emit upon 
the pile twice, thrice, or four times as much heat as before; for the transmission 
through the same plate exposed to a constant source of heat is always in a con- 
stant ratio, and the forces of deviation are proportional to the degrees in those 
ares that are very near zero. Let the force which causes the galvanometer to 
describe the first degree of the scale be represented as 1, we shall then have 10 
for the first force or quantity of incident heat, 20 for the second, 80 forthe third, 
40 for the fourth, &c. Now we know that the first force answers to 10°. In 
order to determine the deviation produced by the force 20 we have only to re- 
move the plate when the galvanometer points to 4°; the calorific rays will then 
fall immediately on the pile, the angle of deviation will increase, and if the pro- 
portionality of the degrees to the forces continues through the whole extent of 
the arc of the first 20 degrees we shall see the index stop at 20°: at all events 
we shall have the corresponding indication. By repeating the same operation 
when the galvanometer points to 6°, 8°, we shall obtain the quantities sought, 
that is to say, the degrees answering to the forces 20, 30, 40, &c. Thus we 
may verify the results contained in the tables of intensities already made, or de- 
termine the elements necessary for the construction of new tables. -- 
