To the same.

Cambridge, Dec. 11, 1866.

Is it not a pity that, after having written at such length, I should be obliged to rightly earn your thanks by a supplement!

In attending too exclusively to the general principles involved in the solution of your problem, I overlooked a circumstance of main importance to the special question about the candy.

I have nothing to retract as to the principle that a fine mechanical subdivision of the mass is the cause, on the optical principles which I explained, of the whiteness of the worked candy. But the admixture of air with it, though one of the means of effecting this subdivision, is by no means the most important. A breaking up of the mass, as it stiffens or becomes of a vitreous consistency, is effected by the working. This is analogous to the effect of pounding ice or cakes of maple-sugar, when the pounding does not pulverize, but simply cracks the mass into minute but coherent fragments. You know that in freezing ice-creams the constant stirring prevents solid congelation, and breaks the freezing mass into minute crystals of ice. Something similar to this takes place in working the candy. For though the candy does not crystallize, it is still vitrified, and the working prevents it from vitrifying into a continuous, translucent body.

This explanation can be tested in practice; for, according to it, the critical time for working the candy to its utmost whiteness is when the whole mass is nearly stiff, and when the parts which become quite stiff or vitreous break and mingle with the still pliant mass without being remelted by it.