temporarily produced in Isotropic Bodies. 255 



ment) and 22, although their heights are in the proportion 

 of 1 to 2. 



2. The weight is independent of the length of the piece. 



This law is demonstrated by the equality of weights applied 

 to the pieces 1 and 2, 21 and 23, 31 and 32, &c., and it is con- 

 firmed by the direct experiment of verification which follows. 

 After having deterniined for the piece 10 the value of P, I cut 

 the piece in two by a saw perpendicular to its length ; two pieces 

 were thus obtained of the same transverse dimensions as the 

 entire piece, but the lengths of which were 10*5 millims. and 22 

 millims. j each of these two parts, tested in both directions, 

 required, to give the same tint as the entire piece of 35 millims., 

 the application of the same charge as we have found for the 

 latter. 



3. For the same substance , the charges are proportional to this 

 width of the pieces. 



We have verified this law by means of the charge P ^ x , which 



2 



has been determined for all the pieces ; in fact, the fifteenth 



column of our table contains the values of the ratio -— 1-, and these 



values are constant for the same substance, excepting such dif- 

 ferences as may be attributed to slight variations in their density 

 or structure. 



But the pieces 10 and 11 present a remarkable anomaly; both 

 of them require greater charges when the smallest of their trans- 

 verse horizontal dimensions is placed in the sense which we 

 denominate as width. They furnish, therefore, for the ratio, 

 values which differ among themselves, according to the direction 

 in which the pressure has been applied. 



It must be remarked that these two parallelopipeds, as alsd 

 the piece 12, have been obtained by a process csWedirefoulementj 

 and which consists in compressing the glass while it is still in 

 the condition of a paste, so as to make it take the form of the 

 heated mould. The annealing is not always sufficient to cause 

 the traces of this operation to disappear; hence it is that 

 this glass, perfectly homogeneous in appearance, and which in 

 polarized light shows only the gray cross which is attributed to 

 its temper, is already, and in a permanent manner, compressed 

 in one direction and dilated in the two others. 



Knowing the direction in which the pasty mass has been 

 compressed, I have been able to convince myself, by these expe- 

 riments and by others not included in the table, that the great- 

 est charge always corresponds to the pressure apphed in the 



