1886. J on Properties common to Fluids and Solid Metals. 



399 



taining treacle is provided with a cylindrical hole in its base, and on 

 the removal of the plug which now closes it, the treacle will flow 

 out, the end of the stream being rounded. If a similar vessel be 

 filled with lead it will, at the ordinary pressure, remain there, but if 

 pressure be applied the lead will prove by its behaviour that it is 

 really a viscous solid, as it flows readily through the orifice ; the end 

 of the jet is rounded, and, as has been shown by the beautiful 

 researches of the late M. Tresca, of Paris,* all the molecules which 

 compose the original block place themselves in the jet absolutely 

 as the molecules of a flowing jet of a viscous fluid would. If the 

 metal has a constant " head," as it would be termed in the case of 

 water, that is, if the vessel be kept filled with solid lead up to a 

 certain level, then you have a continuous stream, the length de- 

 pending on the constancy with which the " head " and the pressure 

 are maintained. If, on the other hand, the head is diminished so 

 that nearly all the solid lead has been allowed to flow away, you 

 have a folding of the jet, and vertical corrugations, exactly such as 

 would characterise the end of the flow of cei'tain other viscous fluids, 

 and finally the jet forms a distinct funnel-shaj)ed tube, concentric 

 with the jet. It is also seen that when the formation of these 

 cavities takes place, the jet is no longer equal to the full diameter 

 of the orifice, as is shown in Fig. 3, the formation of the contracted 



Fig. 3. 



vein is manifest, and a new analogy is thus obtained between the flow 

 of solids and liquids. The application of this fact, that solid metals 

 flow like viscous fluids, is of great importance in industry, and the 



* These researches extend through a long series of memoirs ; those relating 

 to the flow of metals are well summarised in tlie 'Proc. Inst. Mech. Engineers,' 

 1867, p. 114, and in the report of tlie Science Conferences held in connection with 

 the Loan Collection of Scientific Apparatus (Phvsics and Mechanics), London, 

 1876, p. 252. ff ~ 



Vol. XL (No. 80.) 2 d 



