of the Specula of the Reflecting Telescopes. 259 



should not be satisfied as to this, from having produced by 

 those different treatments, a perfectly good and an extremely 

 bad metal out of the same pots at the same time. 



There ought now to be no difficulty in seeing what the 

 efficient cause of this evil is. It is the crystallization of the 

 alloy, permitted or encouraged by the application of too high 

 a heat in the annealing, and by continuing that heat too long. 

 Nor is fluidity necessary to this process ; it occurs after the 

 metal is consolidated, as I have amply proved ; since it is even 

 easy to bring a metal to this state by annealing, after having 

 been thoroughly well cast and annealed, and after having been 

 tried by fracture. Nor is it a fact to excite any surprise, since 

 it belongs to a wide train of analogies on which my experi*- 

 ments, long since ready to be made public, amount perhaps to 

 thousands. 



We can now, therefore, trace the cause of all those vari-' 

 ations of defect, which occur in this case, between the best and 

 the worst specimens ; as it will be found that the degree of 

 crystallization on which depends the faults, are always pro- 

 portioned to the extent to which the annealing process has 

 been carried; or, on the duration and the intensity of the heat 

 applied to the metal after consolidation. The reason of this, 

 also, is apparent : but I need not, in this merely practical 

 notice, enter into a subject which I have treated fully in 

 a paper of another nature. 



But to convince observers that this is the real cause of the 

 defect of such metals, it is always easy to dissect the crystal- 

 lization, if I may use such a term, by means of an acid, and 

 thus to see how the defect goes hand in hand with the crys- 

 tallization, while the variety of texture in speculum metals thus 

 discovered, will also be found very considerable, and often, I 

 may add, very unexpected ; since, among other singular 

 varieties not easily described, I have found even examples of 

 what mineralogists call the spheroidal concretionary structure. 



But if I dare not here dwell on the properly theoretical 

 part of this subject, I must suggest one circumstance or 

 change, which, under high or extreme degrees of anneahng, 

 seems to occur, and which is probably the real cause of the 

 defect as to colour, since it is not so easy to suppose that mere 



