PHYSICS. 331 



the supposition tliat there are no vacant spaces between them. (Anz. 

 Ak. Wien, ISSd-'Sl ; J. IMiys., May, 1880, 11, v, 240.) 



Mach and Wenzel have studied the raechanism of explosions with a 

 view to explain their action in rupturing the solid bodies on whicli they 

 are j)laced, even when the explosive is unconfined. The experiments 

 were made with silver fulminate. The authors show, in the first place, 

 that a visiting- card is perforated <piite as readily by the explosion of a 

 small quantity of the fulminate placed upon it when the experiment is 

 performed in vacuo as Avhen it is made in the air, thus proving that the 

 presence of air is not necessary to- the result. In the next place they 

 established the fact that the velocity of propagation of the explosive 

 wave is ver}^ great by placing a few grains of the fulminate in the mid- 

 dle of a train of powder upon a sheet of paper. After ignition the 

 direction in which the combustion was propagated through the powder 

 could be clearly recognized, but the position of each mass of fulminate 

 was marked by a circle with divergent stria?, equally distinct and well 

 marked in all directions. Hence the combustion of the fulminate had 

 not occupied an appreciable time. To measure this velocity of the com- 

 bustion in the case of the fulminate, the authors laid two paraHel trains 

 of fulminate upon a strip of paper and fired them at opposite ends 

 simultaneously by the spark of a Leydeu jar. Above these trains was 

 a plate of smoked glass, upon which after the explosion an oblique in- 

 terference band could be seen, making with the trains themselves an 

 angle a. If c be the velocity of sound (supposed with these wreat ampli- 



tudes to be not less than 400 meters),* then sin a = from which v. 



V 



the velocity desired, can be calculated. This the authors found to be 

 between 1,700 and 2,000 meters per second. In further proof of the 

 extremely brief duration of the explosion, a ballistic pendulum was at- 

 tached at its top to a horizontal rod, upon the end of each arm of which 

 was a card. On firing a small discharge of fulminate upon one of these 

 the card itself was perforated, but no impulse whatever was given to 

 the i)endulum, the velocity of propagation of the explosive wave being 

 too great to communicate any motion to the mass in the time the com- 

 bustion occupied. If however the cards be replaced by brass cups the 

 metal is not ijerforated, but the pendulum receives a strong impulse. 

 If it be assumed that in this ease the amount of motion corresponding 

 to the lower half of the wave has been communicated to the pendulum, 

 this amount can be calculated from the impulse given to the pendulum, 

 and so an approximate value of the velocity of this wave obtained 5 but 

 the value thus calculated is too great. Hence it seems probable that 

 the wave is reflected by the metal without diminution of strength. In 

 this case the amount of motion communicated to the pendulum is twice 

 that of the half wave, and the calculated velocity on this supposition is 

 reduced to one half or to 1,750 meters per second. ( Wied. Ann., 1885, 

 XXVI, 028 ; J. Phys., November, 1886, II, v, 477.) 



