OF RADIANT HEAT THROUGH DIFFERENT BODIES. 47 



babillty 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 i-A = ttit 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 -roTrdth 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 Avhich they lead. 



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 arcs described by 

 the magnetic needle of the galvanometer and the corresponding forces. Let us 

 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 J-; 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 ahva3's in a con- 

 stant ratio, and the forces of deviation are proportional to the degrees in those 

 arcs 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, 30 for the third, 

 40 for the fourth, &c. Now we know that the first force answei-s 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 G", 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 contf.ined in the tables of intensities alreaclymade, or de- 

 termine the elements necessary for the construction of nnv tables. 



