of certain solid Homogeneous Bodies. 451 
millims. long), the sensible flexions produced originally by the 
mirrors and the levers supported by the cylinders, the necessity 
of soldering the latter in the middle, and lastly the complication 
of the calculations necessary to reduce the observations, are so 
many sources of inconvenience and error. 
The following are, however, the results of these experiments : 
M. Kirchhoff found for yellow brass the value 0-387, for tem- 
pered steel 0-294: these numbers are, it will be seen, decidedly 
greater than 1, while } is very nearly equal to their mean value. 
M. Kirchhoff passes somewhat lightly over the former of these 
results, while he attaches great importance to the second: tem- 
pered steel appears to him to be a body eminently isotropic, 
while yellow brass is neither sufficiently homogeneous nor en- 
tirely free from the secondary effect before spoken of. 
We have already done justice to the latter observation, which 
moreover applies as well to steel as it does to brass, since this 
secondary effect has in fact been observed in neither. As far as 
isotropism is concerned, it is very gratuitous to attribute that 
property peculiarly to a tempered body: the action that tem- 
pered glass exercises on polarized light abundantly proves this ; 
and indeed if the least homogeneous among non-crystallized 
bodies were required, it would undoubtedly be on a tempered 
substance that the choice ought to fall. 
i am far, then, I repeat, from affirming that this ratio ought 
not to be somewhat less than 4 in the case of homogeneous steel ; 
but the present experiment does not seem to me to be conclusive 
on the subject. 
The experiment, on the contrary, made on yellow brass is the 
first in which my results have been attempted to be verified on 
one of the substances [ myself employed. The number 0°387 
is, it is true, greater than 4; but I shall prove, in a memoir on 
flexion I am about to produce, that the denominator of the frac- 
tion which represents in M. Kirchhoff’s results the ratio of the 
torsion to the flexion, is too little, and that this fraction, properly 
corrected, approaches much more nearly to the value $. 
In short, setting aside for the moment the experiment of M. 
Clapeyron, no fact has hitherto been advanced to show that the 
required relation is different in different bodies. The experi- 
ments that have been made, moreover, refer only to a small 
number of bodies; they have been made by means of methods 
all more or less indirect; and the cubical compressibility has 
never itself been the subject of any direct experiment ; so that we 
know not whether the proportion supposed to exist between the 
pressures and the diminutions of volume really does exist for 
changes of pressure, however small. This research will be the 
subject of the memoir which I shall shortly have the honour of 
submitting to the Academy. 
262 
\ 
