Intelligence and Miscellaneous Articles. 239 
cleavage*. The paper wherein this explanation was given never came 
to my hands; and I do not remember that you explained the thing 
to me when we had personal intercourse, The principle alluded to 
appears to me to be the true cause of the phenomenon. , 
If a plastic mass is exposed to pressure, the particles turn until 
they are in such a position as to offer the greatest resistance to the 
pressure brought to bear upon them. But that direction wherein 
they offer the greatest resistance to pressure is also that where the 
cohesion is least. Consequently cleavage ought to take place ina 
direction perpendicular to the pressure exerted. 
Am I right or wrong? Of course I could say more; but why 
should I carry water to the well? 
H. Desvus. 
ON THE CALCIUM SPECTRUM. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
In acommunication published in the last Number of the Philoso- 
phical Magazine, we pointed out the appearance of a well-defined blue 
line in the spectrum produced by igniting the evaporated residue of 
a deep-well water. From the circumstance of this line, which is situ- 
ated somewhat more towards the violet end of the spectrum than the 
line Sr 6, not being referred to in the paper of Profs. Kirchhoff and 
Bunsen, nor indicated in their beautiful representations of the spectra 
of the alkaligenous metals, we were induced to attribute its pro- 
duction to the probable existence of a previously unrecognized mem- 
ber of the calctum group of metals. Further experiments, however, 
have satisfied us that the blue line in question really belongs to the 
calcium spectrum. On finding this to be the case, we communicated 
with Professor Bunsen, who in return informed us that Professor 
Kirchhoff and himself had observed this line, but, not thinking it 
sufficiently bright to be suitable for the recognition of calcium, had 
not made reference to it in their paper. It can, however, be seen 
with a degree of brilliancy at least equal to that of many of the lines 
represented, when perfectly pure chloride of calcium is ignited in a 
somewhat darkened room. 
We remain, Gentlemen, 
Your obedient Servants, 
F, W. and A. Dupr&. 
ON THE LUNAR TABLES AND THE INEQUALITIES OF LONG PERIOD 
DUE TO THE ACTION OF VENUS. BY M. DE PONTECOULANT. 
In the Number of the Comptes Rendus of the labours of the Aca- 
demy of the 12th of November last, M. Delaunay has inserted a 
memoir in which he gives an account of the researches in which he 
has been engaged, concerning the two lunar disturbances of long 
* Fibrous iron was one of my illustrations.—J, T. 
