256 Dr. Mac Culloch on the Annealing 



often mismanaged : the result being the production, by acci- 

 dent, of good metali, perhaps one good metal only, among 

 many failures, when, if I am right, those may be produced 

 with comparative certainty, and possibly, in such hands as 

 those of Mr. Ramage himself, with absolute precision. It 

 may perhaps, indeed, be considered that it is wrong here to 

 suppose that because this excellent constructor has overlooked 

 a necessary portion of his process, others should be supposed 

 to have done the same : but as I have no means of ascertain- 

 ing what is the knowledge and practice of the different makers, 

 I must even hazard the chance of here attempting to teach 

 any one what he knew before ; while I must still trust that 

 some may hereafter derive, from my experience, precautions 

 by which they will be saved considerable disappointment and 

 some expense. Such is the apology for this paper, should it 

 prove superfluous to any one : while I cannot help considering 

 the subject as of some importance, as, not merely the colour 

 of the metal, but, as I think, its durability, and its power of 

 giving a clear image, depend much on the nature of the ma- 

 nagement in that part of the process to which I allude. 

 , I am not aware that it is necessary to say anything respect- 

 ing the proportions of the two metals in the alloy ; though I 

 would caution makers of specula respecting the purity of the 

 tin and copper, being convinced by some trials that the pre- 

 sence of arsenic is apt to lead, in time, to the tarnishing of the 

 surface. I must equally presume that every one is acquainted 

 with the temperature required for the separate metals before 

 mixing, with the necessary flux, the method of pouring in the 

 alloy, the position of the mould, and so forth ; though with 

 respect to the best form of the metal itself, that is, as to 

 relative thickness in its several parts, I must confess that I 

 have never yet been able to satisfy myself: such are the com- 

 plicated difficulties, entangled between the cooling and anneal- 

 ing first, and the optical consequences afterwards depending 

 on changes of temperature, and on weight or flexure, that 

 beset any calculations on this subject; while it is probable 

 that they are really irreconcileable, or that there is no one 

 form which will be the best, as concerns the original nature of 

 the metal and its action when completed. 



