238 Dr. Mac Culloch on the Annealing 



occurs in the larger ones, and increases rapidly with their in- 

 crease of size. And the failure consists in the reverse of what 

 I have just stated. The fracture becomes granular or rough, 

 and is at the same time dull, as might be anticipated ; while 

 the degree of this fault varies with the extent of the mis- 

 management ; which is a mismanagement in the annealing 

 alone, as 1 shall presently show, while the nature of the metals 

 used, or something in the flux, or in the casting, is often 

 accused for faults depending on a much simpler cause. 



Nor is it only that the metal is dull from the nature of the 

 fractured surface, or that this is a mere deception which the 

 polishing will remove. So far from that, the absolute colour 

 of the alloy itself is imperfect or bad ; it has no longer that 

 clear silver whiteness which will take what is technically called 

 a black polish, but is grey, and sometimes of different tones — 

 often so grey and so dark, as to be palpably unfit for its 

 purpose, though often, in the least imperfect instances, re- 

 tained and finished for use, with the production assuredly of 

 some disappointment, even at first, and of somewhat more, I have 

 fully ascertained, after long use, as I shall presently explain. 



In such cases of extreme failure, as the blame is generally 

 laid on the metals of the alloy, the fault remains uncorrected ; 

 and if others have found the same difficulties as I at first en- 

 countered myself, and as Mr. Ramage had always done, 

 when I inspected his processes, they will be well pleased to 

 know where the error lies ; though it must still depend on a 

 nicety of attention which it is impossible to describe, to find 

 the remedy. 



Could the speculum be suddenly or quickly cooled after 

 casting, it would be always perfect ; the evil is the result of 

 slow cooling — of that very process which is necessary, or 

 thought necessary, to secure its integrity : and the slower 

 the cooling, the greater the evil or the failure. Or, by rapid 

 cooling, the texture and the colour are the most perfect 

 which can be obtained from this alloy : while it is possible, by 

 a proceeding as highly reverse as possible, to render it abso- 

 lutely useless, and utterly unlike to what is desired ; and even 

 while the most rigid analysis shows, that the bad metal and 

 the good are precisely the same after having been cast, if we 



