Intelligence and Miscellaneous Articles. 467 



their whole length, the fracture being conic and acicular. I am 

 indebted to the kindness of M. Friedel for a fine specimen of this 

 kind. The tubes obtained by letting melted glass flow in threads 

 of more or less thickness into water possess in a high degree the 

 explosive properties of Rupert's drops. They have almost always 

 the form of cork-screws, on account of the extreme expansion of 

 the upper layers ; and dipping the extremity in fluorhydric acid is 

 sometimes sufficient to cause an instantaneous explosion, with the 

 same characters in the fracture. In fine, the lumps of glass which 

 remain at the extremities of the canes by means of which the tubes 

 are drawn have the form of large Eupert's drops, and are of consider- 

 able weight. When these are detached from the cane they are in 

 the condition of a drop of which the bulb has been sawn : during 

 their cooling in the air they break up, throwing off splinters with 

 violence ; and their fracture is identical with that of the small drops 

 broken at the big end. A fragment of one of these large drops, 

 which I had brought from the works of MM. Appert at La Yillette, 

 presented an interesting phenomenon: on slightly squeezing it 

 between my linger and thumb, a considerable disengagement of heat 

 was produced ; the temperature rose to about 40° C. This confirms 

 the results obtained by M. Dufour concerning the heat disengaged 

 during the explosion of Eupert's drops. 



The existence of layers cooled with unequal suddenness in the 

 thickness of the glass affords an explanation of the brittleness of 

 chilled glass. In fact we may suppose that, on account of the low 

 heat-conductivity of glass, a very thin layer at the surface is, from 

 whatever cause, cooled with sufficient suddenness to be in a very 

 different state of expansion from that of the layers beneath. The 

 least shaking, or the slightest change of temperature, will cause it 

 to break ; and the fissure will be propagated in the mass of the 

 glass : exactly the same thing takes place when a crack shows itself 

 on pottery the glaze of which has been ill-compounded. — Comptes 

 Rendus de VAcademie des Sciences, vol. lxxvi. pp. 346-349. 



ON A METHOD OF MEASURING INDUCED CURRENTS. 

 BY F. H. BIGELOW. 



If a Wheatstone's bridge be formed, in which the secondary coil 

 of the inductorium is the resistance E x to be measured, the relation 

 between the resistances will be expressed by the proportion E x : E., 

 =E 3 : E 4 , where E 3 and E 4 are in a fixed ratio. By passing a cur- 

 rent from an independent battery through the primary coil, the 

 strength of the current in E 2 will be increased or diminished on the 

 breaking or making of the inducing current. Therefore, to preserve 

 equilibrium in the bridge, E x must be changed, and we shall have 

 E^C : E 2 +#=E 3 : E 4 , in which x is a resistance equivalent to 

 the effect of the induced current. Let ~R V ± be found by trial 

 such that the addition or subtraction of the induced current will 



2 H 2 



