NATURAL PHILOSOPHY. 129 



drawings. Mr. Petschler believes that no chemical combination takes 

 place between the salt and the gelatine, but that the latter acts sim- 

 ply as a medium. The gelatine, when firm, retains a certain quantity 

 of water ; but when the moisture is driven off by heat, the crystalli- 

 zation is suspended. There is great similarity in appearance, and 

 there is, possibly, some connection in cause between these arborescent 

 crystallizations and the ramified form in which the salts of some 

 metals are found naturally in agate, slate, and even trap-rock, where 

 the oxide of manganese is frequently found to have assumed similar 

 forms. Mr. Mosley has suggested that the arborescent appearances 

 might, perhaps, arise from the density of the solution, from the re- 

 sistance of the gelatine to allow of crystallization in the usual rhombic 

 form, and possibly to the subtle electrical or galvanic action supposed 

 to be excited during crystallization. He has stated that some years 

 ago he obtained from a solution of bichromate of potash tree-like 

 forms with spreading branches and pendent rhomboids, which, under 

 the polariscope, appeared like a tree with gems of rich colors for 

 fruit. 



CURIOUS EFFECTS OF VIS INERTIJ3. 



M. Tardiret states that if a perfectly smooth and polished plate of 

 glass, ivory, or metal, is caused to rotate with great velocity in -a hori- 

 zontal plane, it does not communicate its own motion to a highly- 

 finished ball which may be placed upon it. 



ENLOXGATED PROJECTILES FOR RIFLED FIRE-ARMS. 



In a paper recently read before the British Association -by Mr. T. 

 Aston, the author, after alluding to the improvements that have been 

 made in war projectiles, which have resulted in the elongated form, 

 proceeded to notice the advantages which it possessed over the old 

 spherical shape. The elongated projectile, presenting to the resisting 

 atmosphere a sectional area considerably less than the spherical of the 

 same weight, is less retarded in its progress through the air. It fol- 

 lows, therefore, that although the spherical projectile, with a similar 

 charge of gunpowder, is more easily set in motion, and has a greater 

 initial velocity than the elongated form, and to that extent has at the 

 outset an advantage, the elongated form is much better able to over- 

 come the resistance of the atmosphere, and, owing to its superiority of 

 momentum, preserves its progressive power for a much longer period, 

 at the same time it is less disturbed by the varying conditions of the 

 elastic medium through which it is propelled. In short, it has a 

 longer and truer flight. The essential condition to the efficiency of the 

 long projectile is, that it shall move onward with its point foremost ; 

 if it turns over in its path, it presents a large surface to the action of 

 the air, its flight at once becomes irregular and is rapidly retarded. 

 The action of the common spinning-top suggests at once the idea that 

 the best mode of making the elongated projectile move steadily 

 through the air with its point foremost is to give it rotation round its 

 axis of progression. The rapid revolution of the body causes its inhe- 

 rent inequalities to be rapidly carried round a constant axis in regu- 

 lar order, and a kind of balance is thereby established, which give? 



