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POPULAR SCIENCE REVIEW. 
The Elasticity of Rods of Calcareous Spar. — In the 11 Chemical News ” of 
Feb. 12, Herr G. Baumgarten gives an account of experiments undertaken 
to determine the elasticity of crystalline bodies, calcareous spars being 
selected for the sake of convenience. The results may be summed up as 
follows : — The bending of a rod is independent of the position of the lateral 
plane ; it depends on dimensions in the same manner as in non-crystalline 
bodies, being directly proportional to the cube of the length, and indirectly 
proportional to the cubes of the breadth and thickness. There is no symmetry 
towards the principal axis. The minimum bending is in the direction of 
the corners, and the maximum in the short rhombus diagonal. It appears 
that all the planes of a rhombohedral primary form are tangents of the 
superficies of deflection along a curve. The deflections are not proportional 
to the weights, and the increase of the deflections becomes smaller when the 
weights are greater. It appears as if the linear differential equations of 
elasticity lost their validity in case of crystals. 
On Specific Gravity. — In a late number of the “Proceedings of the 
Philadelphia Academy of Science,” Professor Leidy remarks, that in taking 
the specific gravity of minerals by means of the scales in weighing the 
substance in water, the usual plan is to suspend it from one side of the 
instrument by a delicate thread or hair. The attachment of the specimen 
was tedious and often difficult, especially in the case of small crystals and 
polished gems, from which the hair would slip, and could only be made to 
retain its position by causing it to stick with some adhesive matter. He 
dispensed with the thread or hair, and substituted on one side of the balance 
a double dish. The lower dish is perforated, and is kept suspended in a 
glass of water. After weighing the specimen in air in the upper dish, it is 
simply necessary to change its place to the lower dish to weigh it in 
water. 
Lights that ai'e Useful in Photography. — In the “ Comptes Rendus” 
(Jan. 25), MM. Riche and Bardy state that they have re-examined the 
bisulphide of carbon and nitric oxide light of Delachanal and Mermet, in 
the hope of finding means to obviate the danger of explosion, either by 
modifying the manner of operating, or by suppressing the use of sulphide 
of carbon, and of comparing the various flames which act upon the salts of 
silver. They recommend as the most efficacious, and as perfectly free from 
danger, the flame of sulphur burning in a jet of oxygen. 
Electro-chemical Resistance of Aluminium. — The “Chemical News” of 
Feb. 26 says that M. E. Ducretet has recently been experimenting on this 
subject. It seems that a voltameter with acidulated water received a slip 
of platinum and a slip of aluminium, placed in communication with the 
poles of a battery ; if the aluminium is the negative electrode, hydrogen is 
disengaged upon it, and the current has its ordinary intensity. When the 
direction of the current is reversed, there is no longer decomposition of 
water, and the intensity of the current becomes very feeble. The surface of 
the aluminium does not seem affected ; it is preserved by a thin layer of 
alumina. The author applies these results to the construction of a liquid 
rheotome, permitting the passage of the current only in one direction. 
