C. GENEKAL PHYSICS. 169 



cased in plaster of Paris, covering a little more than half 

 of their thickness, and the thin end be left protruding, the 

 moment that the neck is reached by the fluohydric acid, and 

 the drop is disaggregated with or without explosion, the frag- 

 ments will be found to remain very nearly in their place, and 

 grouped into a series of conic assemblages, encased one with- 

 in the other, and having their summits toward the thin end. 

 If, on the other hand, instead of using fluohydric acid, -we 

 saw into the big end, after the explosion takes place the small 

 conical arrangement is perceived, but the summit of the cone 

 is in an opposite direction. And, again, if the drop is sawed 

 in the middle, we have on each side of the incision opposite 

 conical arrangements. Similar phenomena are presented b}'- 

 thin glass tubes, which are chilled by cooling in the air at 

 the moment of their fabrication. 



De Luynes also confirms the results attained by Dufour 

 concerning the heat disengaged during the explosion of the 

 Rupert's drops. He even finds that occasionally a specimen 

 presents itself, such that on squeezing it between the finger 

 and thumb a considerable disengagement of heat is produced. 

 The explanation of all these phenomena, De Luynes thinks, 

 does not require that we assume the glass drops, when entire, 

 to be held in a condition other than that which arises from 

 unequal expansion, resulting from the difl*erence in the cooling 

 of the interior and exterior portions of the Rupert's drop. He 

 considers the drop as formed by the superposition of layers 

 of glass unequally chilled and expanded, yet cemented, as it 

 were, to one another. The exterior layers, because of the re- 

 sistance of the interior ones, can only yield to this force of 

 elasticity when through any cause wiiatever the interior lay- 

 ers of the drop are at the same moment set free to return 

 to their normal state of expansion. Owing to the form of 

 the drop, it is apparent that all these layers meet together at 

 the origin of the neck, so that, on destroying this, the com- 

 mon point of resistance vanishes. 4 Z>, 18*73, 232. 



THE ATOMIC THEORY OF PROFESSOR CHALLIS. 



The various forms in which the atomic theory is developed 

 by all the different authors who have made this the subject 

 of mathematical or experimental investigation, have appeared 

 unsatisfactory to Professor Challis, w^ho has, in a recent work 



H 



