THE TECHNOLOGIST. [June 1, 1864. 



504 ON GRANITE AND ITS USES. 



all its threads, but only with that of the overstrained fibres, which 

 quickly give way. If all the threads were equally tight, and the strain 

 borne by them all, the web which before could be rent by the hands 

 might, as the storm-sail of a man-of-war, withstand the fiercest 

 hurricane. 



Now, in the case of glass the unequally strained threads or fibres 

 which make it up are afforded the opportunity of lengthening or 

 shortening themselves till they are of the same length, by heating the 

 completed vessels or other articles of glass up to the temperature at which 

 it begins to soften, but no higher (otherwise they would lose their shape 

 and symmetry), and then allowing it very slowly to cool down to the 

 temperature of the air. Glass for choice optical purposes is thus allowed 

 to fall gradually from a high to a low temperature through the space of 

 many days, and in all' cases hours are allowed to elapse during the 

 cooling. This process is called annealing. Shakspere introduces a most 

 expressive figurative use of it, strikingly bringing out its meaning, when 

 he makes Hamlet's father denounce his murderer for hurrying him into 

 the world of spirits — 



"TJn-annealed ; 



No reckoning made, but sent to my account 



With all my imperfections on my head." 



In other words, unprepared to endure unscathed the powers of so dread 

 a place. 



It is exceedingly probable that in the glass-maker's annealing process 

 there is not merely an equable mechanical arrangement of the glass 

 particles in the way mentioned, but also a combination of heat with 

 them, which is another element of stability. We know, for example,' 

 that when cold iron and other metals are long hammered, they give out 

 so much heat as even to become red-hot ; but at the same time, they 

 grow brittle and lose malleability, and the only way in which this 

 brittleness can be removed, and malleability restored to them, is by 

 heating them red-hot and allowing them slowly to cool. During this 

 process these metals apparently recover and render latent within their 

 substance the heat which is essential to their solidity ; and in the same 

 way glass appears to require and to obtain, during the process of anneal- 

 ing, an amount of heat essential to its stability. At all events, annealing 

 renders glass, which otherwise would be uselessly brittle, wonderfully 

 strong and enduring, as the immense window panes and mirrors which 

 can now be cast in plate-glass strikingly exhibit ; and not less the com- 

 paratively thin tubes employed by the chemist, which he is not afraid to 

 expose to a pressure of several hundred pounds upon each square inch. 



An industrial museum is intended to be a repositoiy for all the 

 objects of useful art, including the raw materials with which each art 

 deals, the finished products into which it converts them, drawings and 

 diagrams explanatory of the processes through which it puts those mate- 

 rials, models or examples of the machinery with which it prepares and 



