128 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



that takes place between molecules when they are only for 

 an instant in close contact, which action, in fact, depends upon 

 the particular constitution of the encountering molecules. 

 The first contribution to a dynamic theory of liquids is made 

 by Maxwell in some comments on the labors of Vander- 

 waals, where he takes occasion to show that we have evi- 

 dence that the molecules of gases, besides encountering each 

 other in their motions, also attract each other at a certain 

 small distance, but when they are brought still nearer they 

 repel each other, a conclusion in accordance with Boscovich's 

 theory of atoms. On the other hand, the molecules of 

 liquids, or even these same gaseous molecules, when reduced 

 to the liquid condition, apparently repel each other at a cer- 

 tain small distance, which repulsive forces between contig- 

 uous molecules are overcome by the general attractions of 

 the mass of the body. 12 A, X., 479. 



ON THE REFLECTION OF SOUND FROM A LAYER OF FLAME 



OR HEATED GAS. 



Mr. Cottrell has observed the reflection of sound from a 

 coal-gas flame in the following manner: Sonorous pulses sent 

 through an open tube agitate a sensitive flame placed at its 

 other end ; but when a coal-gas flame is placed between 

 the end of the tube and the flame, the latter is no longer 

 affected by the sound sent through the former. He then 

 placed two tubes so that they were equally inclined to one 

 face of the gas flame, and, sending the sound into one of 

 these tubes, it was reflected from the flame, passed up the 

 other tube, and agitated a sensitive flame placed at its mouth. 

 In a similar manner he has shown that part of the sound is 

 reflected from the flame, and part is transmitted by the 

 flame, thus giving a complete analogy between the reflec- 

 tion of sound from a flame and the reflection of light from 

 a transparent plate. He obtained the same effects, as given 

 above, when he replaced the flame by the sheet of heated 

 gases rising from it. 



THE EVAPORATION OF METALS BY ELECTRICITY. 



Mr. Hopkins describes an interesting experiment, which 

 consists in passing a charge of electricity through a very 

 fine thread of platinum, or other metallic foil, the thread 



