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1829.] difference, I found they were anhydrous; consequently incapable 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 temperature 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 careihlly melted; a portion dissolves, and a portion separates, 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 required. Whenever a soda salt is to be distinguished from one of potash, in the manner above described, this effect of temperature must be carefully guarded against.

—On the Manufacture of Glass for Optical Purposes

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

Introduction.

as ls the manufacture of glass for all ordinary purposes, 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 properties of light; but when he desires to apply it, according to the laws he has discovered, in the construction of perfect instruments, and especially of the achromatic telescope, it is