This so-called bone is really composed of the purest chalk, for which reason it is in great request as a dentifrice, being easily scraped to almost impalpable powder when wanted, and not liable to be spilled, as is the case with any ordinary tooth-powder. It is exceedingly light--so light, indeed, that it floats like a cork, even in fresh water. Now, as chalk is very much heavier than water, we may naturally ask ourselves how this lightness is obtained. If the upper surface be examined, it will be seen to be traversed by a vast number of wavy lines, something like the markings of “watered” silk. These show the lines of demarcation between the multitudinous rows of pillars of which the whole structure is formed. If the “bone” be sharply snapped in the middle, and the particles of white dust blown away, a wonderful structure presents itself, which can be partially discerned by the naked eye, though a microscope is required to bring out its full beauties. Even with an ordinary pocket lens we can make out some of its wonders. The object looks like a vast collection of basaltic columns, except that the pillars are white instead of black, and they are arranged in rows with the most perfect accuracy, just as if the place of each had been laid down with rule and compass. They are scarcely thicker than ordinary hairs, but they are beautifully perfect, and rise in tier after tier as if they were parts of a many-storied building. As a definite space exists between the pillars, the reader will understand why the whole structure should be so much lighter than water. In order, however, to see these wonderful pillars in perfection, a very thin section should be taken, and viewed with polarised light. * * * * * ANOTHER excellent example of Pillars and Flooring is to be found in the nests of various Wasps, including that of the Hornet. In these nests the combs are arranged horizontally, and not vertically, like those of the bees, and in consequence they have to be supported in some way. This object is achieved by means of multitudinous pillars made of the same papier-mâché of which the combs are formed, and attached to the successive rows of combs. There is, however, one curious point of difference between the Wasp-comb and human architecture, namely, that the pillars do not support floors, or rest upon them, but sustain the weight of those which hang from them. The mouths of the cells are all downwards, and the combs are therefore suspended from the pillars, instead of being supported by them. TUNNEL ENTRANCE TO THE DWELLING. We have already found occasion to treat of the snow-house, or igloo, of the Esquimaux, and have now to speak of a subsidiary, though necessary, part of Esquimaux architecture.