STEEL AT WELDING TEMPERATURES. 597 
might probably prove to be identical. I afterwards found that the same suggestion 
had been made by Love.* As, however, this hypothesis was not based on a know- 
ledge of the properties of wrought iron, which can be welded, but upon those of cast 
iron, which cannot be welded, it appeared that the suggestion could only be regarded 
as speculative, until some method of examining the physical properties of wrought 
iron within the range of temperature known as the welding heat could be devised. 
This brief statement of my earlier researchest will serve to lead to the work 
embodied in the present paper, which shows that wrought iron at the welding 
temperature possesses the property of expanding when cooled and contracting when 
heated, and that the welding property is intimately connected with the critical 
condition in which this abnormal behaviour is exhibited. 
Professor JAMES THOMSON was the first to show that although in the case of 
normal bodies which expand by heat and contract by cold, the effect of impact or of 
pressure is to heat them; theoretical considerations rendered it probable that in 
a material which possessed the physical property of expanding during cooling and 
contracting during heating, a contrary effect would be produced. ‘The effect of 
pressure or impact would cool and not heat it. This was subsequently experimentally 
demonstrated, in the case of freezing water, by Lord Krtvin.{ 
These theoretical deductions of Professor JAMES THOMSON, and the well-known 
confirmatory experiments by Lord Ketvin, led to a theory of regelation now 
generally accepted, which depends upon the lowering of the solidifying (or freezing) 
point by pressure. 
The condition known as “ the welding state” of iron or steel is one which exists 
only within a very limited range of temperature. If the smith takes his iron bars 
out of the fire at too low a temperature welding cannot be effected. If, on the other 
hand, the iron is too hot, a failure is also certain. 
The range of temperature during which impact or pressure causes the union known 
as the welding of two masses of iron or steel, is therefore comprised within narrow 
limits, and the familiar operation is really a critical one. 
In order that the phenomenon of the welding of iron may be identified with 
the regelation of ice, it must be experimentally proved that the surfaces of the iron, 
at the moment of welding, are in that peculiar and critical condition in which an 
increase of heat will cause contraction and a diminution of heat will be followed by 
the expansion of the mass. On the other hand the identification of regelation and 
welding will be equally satisfactory if the collateral property of the cooling of the 
hot iron by pressure or impact can be demonstrated. 
The first method of demonstration is impracticable, as the welding state is transient 
* ‘Proceedings of Civil and Mechanical Engineering Society,’ February 19th, 1880. 
+ © Proc. Iron and Steel Institute,’ 1879-80. 
tf ‘ Proc. Roy. Soc. Edin.,’ January, 1850. 
