NATURAL PHILOSOPHY. 17-"> 



conductive power of the block to be infinite, that is to say, that the heat 

 imparted by the rocker is instantly diffused equally throughout the block ; 

 then, though the general expansion might be very great, the local expan- 

 sion at the point of contact would be wanting, and 110 -vibrations would be 

 possible. The inevitable consequence of good conduction is, to cause a 

 sudden abstraction of the heat from the point of contact of the rocker with 

 the substance underneath ; and this the lecturer conceived to be the precise 

 reason why Professor Forbes had failed to obtain vibrations when the cold 

 metal was a good conductor. He made use of blocks, and the abstraction 

 of heat from the place of contact by the circumjacent mass of metal was so 

 sudden as to extinguish the local elevation on which the vibrations depend. 

 In the experiments described by the lecturer, this abstraction was to a 

 great extent avoided by reducing the metallic masses to thin laminae ; and 

 thus the very experiments adduced by Professor Forbes against the theory 

 supported by Professor Faraday appear, when duly considered, to be 

 converted into strong corroborative proofs of the correctness of the views 

 of the philosopher last mentioned. 



ON THE EFFECT OF PRESSURE ON THE TEMPERATURE OF FUSION 



OF DIFFERENT SUBSTANCES. 



Mr. Hopkins, before the British. Association, in a paper on the above 

 subject, described the apparatus he had used, and the successive steps by 

 which fusion in some contrivances had led him to that which was ulti- 

 mately found to answer. In particular, how, from the enormous pressures to 

 which the substances were subjected, they found it impossible to use glass 

 to see what was going on within the cylinders in which the substance to be 

 experimented upon was enclosed ; which difficulty had been got over by 

 causirg an iron ball to rest on the top of the substance within the cylinder, 

 while its presence deflected a small magnetic needle outside ; but the instant 

 the melting of the substance inside permits the ball to fall, the magnetic 

 needle returning to its position of rest indicated the fact. The use of this 

 needle made it necessary to make the cylinder of brass ; and Mr. Hopkins 

 stated that, with the first cylinder they used, they were surprised to find 

 when enormous pressures were laid on that the liquid within wasted ; the 

 cause of this they long sought to discover in vain, until at length they 

 found that it was escaping through the very pores of the metal in thou- 

 sands upon thousands of jets so minute as to be almost imperceptible. 

 This they remedied by greater care in the casting of the cylinder, and 

 hammering it well on the outside. The method of laying on the pressure 

 was by a piston well packed aud forced down by a lever. This they 

 adopted as the simplest means of getting a numerical estimate of the actual 

 compressing force. Mr. Hopkins, then described the method by which the 

 friction had been determined which opposed the motion of the piston, and 

 so diminished the pressure by so much. This was done ,by noting the 

 weight required to drive the piston in a certain small distance : this, less by 



