1829.] On the Manufacture of Optical Glass. 231 



difference, I found they were anhydrous ; consequently in- 

 capable of efflorescing, and indeed exactly of the same nature 

 as the crystals obtained by Dr. Thompson from certain hot 

 saturated leys*. 



Hence it would appear that a mere difference in the tempe- 

 rature at which a solution of sulphate of soda is evaporated, 

 will cause the formation of hydrated or anhydrous crystals at 

 pleasure, and that whether the quantity of the solution be large 

 or small. This, indeed, might have been expected from that 

 which takes place when hydrated crystals of sulphate of soda 

 are carefully melted ; a portion dissolves, and a portion sepa- 

 rates, the latter in an anhydrous state*. I find that, if it were 

 desirable, crystallized anhydrous sulphate of soda might easily 

 be prepared for the market; though, as the pure salt is now 

 but little used, it is not likely this condensed form will be re- 

 quired. Whenever a soda salt is to be distinguished from one 

 of potash, in the manner above described, this effect of tempe- 

 rature must be carefully guarded against. 



THE BAKERIAN LECTURE. On the Manufacture of Glass for 



Optical Purposes-^-. 

 [Read November 19, December 3 and 10, 1829.] 



Introduction. 



PERFECT as is the manufacture of glass for all ordinary pur- 

 poses, and extensive the scale upon which its production is 

 carried on, yet there is scarcely any artificial substance in 

 which it is so difficult to unite what is required to satisfy 

 the wants of science. Its general transparency, hardness, 

 unchangeable nature and varied refractive and dispersive 

 powers, render glass a most important agent in the hands of 

 the philosopher engaged in investigating the nature and pro- 

 perties of light ; but when he desires to apply it, according to 

 the laws he has discovered, in the construction of perfect in- 

 struments, and especially of the achromatic telescope, it is 



* Quarterly Journal of Science, xxii. 399, or Ann. Phil. N. S. xx. 401. 



t Philosophical Transactions, 1830, p. 1. The use of the glass manufac- 

 tured as described has since become so important in diamagnetic and magneto- 

 optical researches, that I deem the paper worthy of insertion at full length in 

 cthis collection. M. F. 1858. 



