OF RADIANT HEAT TITROUGH DIFFERENT BODIES. 15 
for the forces which produce the deviations 35° and 16°, the first (46°7) 
will be found in the table, but the second, being under 20°, will have 
the same value as the arc; that is to say, 16. When we want to find 
the forces which correspond to fractions of a degree, we have only to 
ascertain the proportional part of the degree in question ; for, in the 
interval between one degree and another, the curve visibly coincides 
with the tangent. If, for example, we wish to know the force that cor- 
responds to the deviation 31°7, it will be sufficient to take at first the 
difference between 37-4 and 39°6 (the intensities of the forces belong- 
ing to 31° and 32°); this difference being 2-2, we shall find the value 
(a) of the force corresponding to seven tenths of the degree contained 
between 30° and 32° by this proportion, 
Poe Orng. 20 ee eae eo, 
Adding this to the number 37-4, which represents the force correspond- 
ing to 31°, we shall have 38°9 as the value sought. 
Of the Polish, the Thickness, and the Nature of the Screens. 
The suggestions which we have offered as to the manner of measuring 
the quantity of caloric instantaneously transmitted by diaphanous bodies, 
and as to the precautions to be taken during the experiments, leave us 
scarcely anything more to say on this subject. Nevertheless it may not 
be amiss to mention some particulars relative to the construction of the 
apparatus before we proceed to the exposition of the results. 
The pile employed in these researches is of the form of a quadrangu- 
lar prism; its two ends are plane surfaces, each measuring 4°24 centi- 
metres; it consists of 27 pairs and a half, or 5 elements of bismuth and 
antimony, 32 millimetres long, 2°5 broad, and 1 in thickness. It was 
not without considerable difficulty that we succeeded in combining and 
soldering together these minute bars. The facility with which liquid 
antimony oxidizes, the difference between its fusibility and that of the 
bismuth, and the extreme fragility of the two metals, presented so many 
obstacles, that it cost many an effort to overcome them. But a pile of 
very small dimensions was indispensable in the investigation of the laws 
of immediate transmission through rare liquids and crystallized solids. 
This was, therefore, to be obtained, or the experiments to be aban- 
doned. By this conviction we have been induced to persevere in spite 
of repeated disappointments, and by redoubling our patience have at 
last succeeded. 
The electric pile is passed into a ring formed of a thin square flake 
of copper internally lined with pasteboard and having a screw which 
serves to fix it on the stand, so that the axis naturally takes that hori- 
zontal position which it is to keep during the greatest part of the ex- 
periments. To each side of the ring there is fitted a tube of six cen- 
