To Miss Catherine L. Howard.

Cambridge, Dec. 6, 1866.

. . . But I wander into moral philosophy: your question is one in natural philosophy: “Why does molasses-candy grow whiter from our working it?” This admits, it seems to me, both of a poetical and a scientific answer. After the manner of our countrymen, it might be answered by asking other questions. Why does the sea grow white when the winds work it into foam? Why are beaten eggs white? Why is snow white? or, to descend to bathos, Why is soapsuds white?

In the three cases where fluids are concerned, the sea, the eggs, and the soapsuds, the admixture of air with them is the property common to them all. Let us believe, then, that a pure aerial spirit enters into, and purges these gross fluids of their dark and evil properties. This might satisfy the poetical imagination, and every good Christian conscience.

But the sceptical, scientific understanding demands to know more about it. That which is common to all the five cases (the sugar and the snow, as well as the three fluids) is the fine mechanical subdivision of these substances, — which in the case of the fluids is effected by the admixture of air, but in the case of the solids is independent of this circumstance.

Solid ice is not white, but pulverized ice and snow are; and a large list of substances might be added which grow white, and many others which become lighter in color, on being pulverized.

But what has this all to do with molasses-candy? Just this: the process of working it is one by which it becomes finely subdivided by the admixture of air with it. Melt the worked candy, and it becomes a thick, creamy foam, from which the heat would soon expel the air, and it would then relapse into its pristine dark and evil state. Fine mechanical subdivision in the worked candy, or breaks in the continuity of its solidity by the interposition of minute particles of air, is, then, the condition in which molasses-candy agrees with all the cases analogous with it in respect to the property in question. (I am following the Baconian method, — perhaps I ought to tell you.)

But the sceptical spirit is not even satisfied with this. Why should minute mechanical subdivisions, interruptions of the continuity of a solid or fluid substance, produce whiteness? Here optics comes to our aid. In a finely divided substance there is a correspondingly greater number of reflecting surfaces. Every minute bubble of air makes such a surface. The amount of white light, or sunlight, which is reflected near the surface of such a substance, bears a much larger proportion to that which is reflected from greater depths, and after transmission through the substance, than when the substance is continuous in its texture. A continuous substance, like undisturbed water, reflects principally, almost exclusively, from one surface. But foam reflects daylight, or white light, from innumerable surfaces, and reflects light which has penetrated only a little way into it. The color of any substance is composed of light from two sources, — of the light which it reflects and the light which it transmits, either directly or by internal reflection. Directly transmitted light, as in the case

of colored glass, is that which the substance does not absorb. In all other cases, we have a union of the two kinds of color, — the color which is reflected unaltered from the outmost surface of the substance, and that which is reflected from the interior of the substance. The proportion of these two sources of color depends chiefly on the mechanical constitution of the substance. In homogeneous, continuous substances, internal reflection predominates. In heterogeneous, discontinuous substances, like the worked candy, external reflection predominates. Snow and refined sugar are white because their light is chiefly that which is reflected from innumerable crystalline surfaces. In unrefined or brown sugar, the crystals are covered by a film of treacle or molasses, which is washed off in the process of refining. The internally reflected light from the treacle gives the brown color, the other constituents of white light being absorbed by this substance.

Molasses-candy, therefore, grows whiter from our working it, because the working introduces air into it, and thereby increases the number of reflecting surfaces near its exterior, from which white light is reflected without the loss, by absorption, of any of its constituents.

In your note, you come very near to this explanation: you say, “I have bothered you so many times before this”(so far you are wrong, — but you add) “to get a few rays of light thrown into the depths of my ignorance, &c.” I am surprised that you did not see in this the answer to your question! It is because light is thrown into the sweet depths of the unworked candy, and is not reflected, that it looks so dark. It is only by reflected and superficial light that it grows in whiteness.

... I shall be pleased to answer, if I can, any other questions — not metaphysical—which you may still have in reserve.