670 GLASS 



with anything like definite proportion. That it should be full of strife, and, therefore, 

 refract the rays of light unequally, as it does, so as to produce the most hideous ap- 

 pearances of distortion, is a mere natural consequence of its mechanical composition, 

 which might, and must one day be corrected ; but that whole nations should have 

 come to view this defect as an unavoidable peculiarity, is precisely one of those sur- 

 prising facts which demonstrate the influence of habit over the powers of the mind, 

 and show how easily human reason can reconcile itself to the most gross inconsisten- 

 cies. If window-glass had one uniform atomic composition, the tendency to form 

 these striae would nowhere exist in excess ; and, therefore, their production would 

 diminish as the skill of the workmen increased ; but, with the present variable com- 

 potind, the glass stretches unequally in different parts, by an equal application of 

 force, and, in spite of human skill, presents a result alternately thick or thin, as 

 accident determines. That these striae have not the same composition as the parts 

 surrounding them is very obvious, from the circumstance that, if striated glass bo cut 

 to an uniform thickness, and polished on both sides, the optical defects remain but little 

 changed, and occasionally they are found to be increased. Again, it is known that 

 the more complex the composition of any glass may be, the greater the liability to this 

 striated structure of which flint-glass offers an apposite illustration ; for here, in 

 addition to the ordinary components of glass, the silicate of lead is superadded. Now 

 the specific gravity of silicate of lead is very high compared with that of silicate of 

 soda, potash, or lime ; hence unless employed in the exact quantity to form a 

 chemical combination with the other silicates, a mere mechanical mixture is produced, 

 of very different densities throughout ; and the product, under the action of light, dis- 

 plays, permanently, that peculiar fugitive appearance seen when syrup and water, or 

 alcohol and water, are mixed together ; that is to say, a series of curved lines are 

 formed by the unequal refraction of the two fluids, which entirely disappear, so soon 

 as perfect admixture has taken place, but which remain in the case of flint-glass, 

 from the utter impossibility of effecting the necessary union between its various parts. 

 Although, however, this cannot be done mechanically, yet, in a chemical way, nature 

 performs such operations with ease and unerring fidelity. The French chemist, 

 Berthier, long ago proved that many neutral salts combine together by fusion in 

 atomic proportions, and form new and definite compounds. Thus, carbonate of potash 

 and carbonate of soda when mixed, atom for atom, unite and produce a compound 

 more easy of fusion than the more fusible of the two ; similarly, either of these car- 

 bonates will act with carbonate of baryta or strontia ; and again, fluor-spar and 

 sulphate of lime, two remarkably infusible substances, when mixed, melt readily, at 

 a low red heat into a fluid as mobile and transparent as water. It is useless to mul- 

 tiply examples of this kind, for thousands exist ; and the alkaline and earthy silicates 

 form no exception to this almost universal rule. A mixture of silicate of potash and 

 silicate of soda will, if in atomic ratios, fuse much more readily than either of them 

 alone. But now, let us imagine an attempt to fuse these two bodies together, in any 

 other proportion than that in which they are naturally disposed to combine ; say 

 that the silicate of soda is in excess ; then the silicate of potash would unite with 

 exactly sufficient of the silicate of soda to produce the extremely fusible compound 

 above spoken of; whilst the less easily fusible silicate of soda, added in excess, would 

 form a kind of network throughout the mass. It may be said, that a higher heat 

 would overcome this difficulty, by thoroughly liquefying the silicate of soda ; and this 

 is really the plan now used with that view ; but, independent of the fact, that the 

 mixed silicate of potash and soda would also undergo a corresponding liquefaction, 

 and, therefore, favour the separation of the silicate of soda, yet, as chemical union is 

 impossible, from the very conditions of the experiment, even the most perfect me- 

 chanical mixture, under the greatest advantages of fluidity, would never generate a 

 homogeneous body." The striae might, indeed, be diminished in size ; but this would 

 imply a correspopding increase in their number; and, if carried very far, complete 

 opacity would result from such an endeavour to subvert the laws of nature. The 

 power of the workmen to remedy this defect is therefore limited to the capability of 

 modifying its more salient features; he can never remove nor destroy it. What 

 we have here illustrated by the simplest of all assumptions, gathers and accumulates 

 into a formidable evil when several silicates are fused together, having considerable 

 differences of specific weight. Thus, in the case of flint-glass before alluded to. then- 

 are generally three, and sometimes five, of these silicates fused together, into, pro- 

 bably, one of the most antagonistic compounds that could be conceived, refracting MTU! 

 dispersing the ray of light in fifty directions, and demonstrating the unfriendly 

 nature of its coerced union, by flying in pieces from the most trivial applications of 

 heat or violence. Yet in flint-glass wo are not surpassed, nor indeed equalled, by 

 any other nation ; and so thoroughly has this beautiful sub&tance become associ.itc'l 

 with our industrial reputation, that the British name, flint-glass, has been adopted into 



