But why is froth _white_? Porter, ale, tea, coffee, champagne, water, differ very considerably in colour; yet in all of these, when froth is produced it is white. This appears to be due to the reflection of the light from the outer surface of the several bubbles. When the surface is thus broken up, we have a cluster of little spheres, each of which presents a reflection to the eye from some part of its surface; and as there may be tens of thousands of these in a very small space, the effects become united, and are recognised as a whiteness. It is simply an aggregation of small effects to produce one more conspicuous. If the bubbles are large, then fewer of them can be contained in a given area; consequently the number of convex reflecting surfaces is smaller, and the united effect less brilliant--in other words, less white. But it may still be asked by some of us, how is it that the froth of a _reddish_ liquid, such as beer, is white? The phenomena of reflected light must again be appealed to for the means of solving this problem. The colour of a liquid (not its froth) is determined by the transmitted light, not the reflected. If liquor be contained in a transparent glass vessel held between the eye and the light, and we look _through_ it, the eye receives the light transmitted by the liquid, and deems the colour of that light to be the colour of the liquid itself; but if we pour the liquor into an open vessel, and look obliquely at the surface, we shall find that the colour does not deviate much from whiteness, whatever the transmitted colour may be. The liquid, whatever be its body colour, is when agitated broken up into detached portions at its surface by the formation of bubbles, and each bubble reflects to the eye a portion of the light which falls upon it. Consequently, if this reflected light is nearly white in all cases, the resulting assemblage of bubbles, generally known as froth, must always appear white or nearly white. We can easily understand the greater yellowness of the froth on strong Dublin stout than that on pale Burton ale; the more tenacious liquor forms a thicker and consequently less clearly reflecting bubble than that on the more limpid. A CITY WEED. I passed a graveyard in a London street, Where 'stead of songs of birds, the hoarse sad cries Of wretched men echoed from morn to night. Locked were its gates, and rows of iron bars Fenced in God's Acre from tired wanderers' feet. All broken lay the slabs which love had raised; But on a mound where fell a patch of light, A Bindweed grew; and on its flowers, with eyes O'erflowing with a wintry rain of tears, A pale-faced, miserable woman gazed, Heart-sick with longings for the nevermore, And faint with memories of bygone years: A breezy common with a heaven of stars, And lovers parting at a cottage door. * * * * *