STEEL AT WELDING TEMPERATURES. 601 
No doubt a series of experiments to determine the exact limits of the critical state 
would be full of interest. 
Between the temperature of 1400° and that of melting wrought iron (stated to be 
1600°) there are doubtless increasing degrees of mobility in the material. When 
pressure is put on the bar, say at 1400°, it not only lowers the temperature of the 
melting point, but increases the mobility at lower temperatures, so that if before 
pressure the temperature be 1400°, after pressure there may be a condition of mobility 
between the molecules which corresponds to a temperature without pressure of 1500°, 
although the temperature has been reduced by pressure to 1848°. 
If two pieces of iron, or almost any metal, be raised to the melting point, union can 
no doubt be effected, but this is by melting together and not by welding. The pro- 
cess of welding appears to be that by which complete union can be effected- by 
hammering or pressure, at a temperature considerably below that required to melt 
the material. The heat of the fire having raised the bars within the critical range of 
temperature above described, the smith in striking with his hammer is assisted by the 
special properties of this welding material in producing an increased mobility of the 
molecules, which approach though never arrive at liquidity. This condition is favour- 
able to the interpenetration of the molecules, and consequent adhesion of the surfaces 
on hammering. 
Since I made this experiment in Professor RoBERTSs-A usTEN’s laboratory, I have had 
the opportunity of experimenting in steel works upon the behaviour of solid masses 
of soft weldable rolled steel when lowered into molten steel of the same quality. The 
mass at first sinks, and then quickly floats to the surface, where it remains until 
melted, showing that weldable low carbon steel follows the same law as cast iron, and 
therefore possesses that property which it is contended is essential to bodies which 
can be united by the process of regelation or welding. The increase of carbon in 
steel appears to prevent welding just as it does in the case of the more highly car- 
bonized form of iron known as cast iron. 
In conclusion J would observe that if I have been able to demonstrate this remark- 
able property in wrought iron of being cooled by pressure when at the welding 
temperature, it would not have been possible to attain this result without the aid of 
Professor Roperts-AUSTEN’S admirable pyrometer, and without the help which he 
and his able assistants, Mr. STANSFIELD and Mr. RreGinaLp Roperts, have contributed. 
EXPLANATION OF PLATE. 
Effect of Pressure on an Iron Bar. 
Fig. 1.—Current cut off before pressure. _ 
Fig. 2.— ,, 2 at a lower temperature. 
In each experiment pressure applied at point marked P. 
MDCCOXOV.—A, 4 4 
