162 BRUSH— SPONTANEOUS GENERATION OF HEAT [Ap"l 22, 



the shrinkage was again measured. The rod shrank very consid- 

 erably in each operation, as indicated quantitatively in Table 2, in 

 which the annealed length is taken as unity or 100 per cent. 



Table 2. 



Length of rod after hardening 100.383 



After spontaneous shrinking 100.332 



After tempering to light straw 100.182 



After tempering to light blue 100.131 



After annealing 100.000 



Of course the shrinkage in volume must have been very nearly 

 three times the linear shrinkage, or considerably more than one per 

 cent, from the hardened to the annealed condition, which is more 

 than double that observed in the bar steel used in the first experi- 

 ment. Doubtless this was due to the higher carbon content of the 

 small rod, and more uniform hardening owing to its small size. It 

 is highly probable also that more heat was generated per unit of 

 mass. 



Linear Shrinkage x 100 



Length of Rod After Hardening 

 After Spontaneous Shrinking 

 After Tempering to Light Straw 

 After Tempering to Light Blue 

 After Annealing 



Fig. 6. 



In Fig. 6 I have visualized the stages of shrinking of the small 

 rod by magnifying a hundred-fold the observed quantities in 

 Table 2. 



I have already pointed out the close similarity in character of 

 the spontaneous-shrinkage curve (Fig. 5) and the computed curve 

 of total heat generation ; and there seems little room for doubt that 

 the two phenomena are quantitatively related. But it is equally 

 clear that spontaneous shrinking is only incident to, and is not the 

 prime cause of the generation of heat, because the internal work 

 represented by the heat generated is hundreds of times more than 

 necessary to bring about the accompanying change in volume. This 



