The composition of this gem is undoubtedly carbon, seeing that the sole result of its combustion in oxygen is carbonic acid gas: but the origin of the diamond is a subject of much curious speculation. Seeing that its structure is crystalline, the diamond should have been at some early period in a liquid or semi-liquid condition; a state which presupposes fusion by fire, or solution in some menstruum. Opposed to the first hypothesis is the circumstances that within the structure of many diamonds are seen remains of organic beings-appearances scarcely consistent with the assumption that the diamond was once in a state of igneous liquidity. Sir David Brewster inclines to the opinion that the diamond is a drop of fossilised gum.

Although the diamond has been celebrated for its beauty in all countries and all ages of which we have any notice, yet the extreme beauty which this gem is capable of assuming can only be developed by an ingenious and tedious process of cutting unknown even to this day in its full perfection by Eastern nations, and of somewhat modern introduction to Europe, viz., in the year 1456 by Louis Berghen, of Bruges, who accidentally discovered, that by rubbing two diamonds together a new face was produced. In other words the diamond is so hard that it can only be abraded by portions of its own substance: hence, diamond powder is universally employed for that purpose; such stones as, on account of their inferior colour or their flaws, are valueless as gems, being broken down into powder for the purpose of

cutting others. At present, and for a long time past, the head-quarters of the diamond cutting operation are at Amsterdam, where the operation is conducted by Jews exclusively. Their machinery is of the roughest kind, and horses are still employed as the propulsive force. The exact method of procedure is this :—the diamond to be cut is attached by means of a very fusible species of solder to the face of a sort of hammer, the extremity of the handle of which being supplied with a hinge by which it is restrained as regards lateral motion, but allowed free liberty to move vertically. The face of the hammer being now turned towards the flat surface of a horizontal wheel, primed with diamond-dust and oil, and the wheel being put in motion, that portion of the diamond which comes in contact with the wheel is ground away. This gem, however, being subjected to definite crystalline forces of aggregation, cannot be ground into any form at pleasure-but only certain varieties of forms correlative with the exercise of crystalline force. Subordinate modifications of form being omitted, cut diamonds may be described as falling under the category of brilliants and roses. The former, which are the most beautiful and valuable, may be recognised by their flat summits, whereas the summits of rose-diamonds are peaked.

The weight of diamonds is estimated in carats-150 of which are equal to one ounce troy or 480 grains. These carats are subdivided into halves, quarters, or carat grains, eighth, sixteenth, and thirty-second parts.

The rule for the estimation of the value of diamonds is peculiar, and supposing the gems under comparison to be equal in quality, may be expressed as being in the ratio of the squares of their respective weights. Thus, supposing three diamonds to exist, weighing respectively one, two, and three carats, their respective values would be as one, four, and nine. The value of a brilliant diamond is considered equal to that of an uncut diamond of double the weight. This rule, however, can only be considered as applying to gems of moderate dimensions: very large diamonds, if estimated according to this mode of calculation, would become expensive beyond the means of the richest to command.

The history of the large diamonds known to exist in different states possesses great interest, as showing the vicissitudes through which some of these beautiful gems have passed before arriving at their final resting-place. It is a subject, however, which scarcely comes within the scope of this volume. The celebrated Koh-i-noor, which probably may now be regarded as the most beautiful diamond in the world, is believed by Professor Tennant and some other competent authorities to be only one portion out of three of a diamond of extraordinary size*; the

* Having weighed, according to Tavernier, 787 carats; and being worth nearly two and a half millions of pounds sterling. The Persian diamond weighs 130 carats; the Russian diamond, 196. The Koh-i-noor as exhibited in 1851 weighed 186 carats. Since then it has been recut, and now it only weighs 102 carats. For this information I am indebted to Professor Tennant.

second portion he agrees with Dr. Becke in considering a diamond slab, belonging to Persia, described at the meeting of the British Association at Ipswich, in 1851; the third he considers to be the great Russian diamond.

RESPIRATION.

THIS important function is, so far as we know, absolutely necessary to the existence of all living beings, whether animal or vegetable, but it is in the economy of the animal world that the function is most distinctly marked.

Although every animal accomplishes the act of respiration, the organs by which the act is performed differ essentially in the various divisions, and even in the various sub-divisions of the animal world.

The lowest type of animal organisation is usually considered to be that of the polypus, and respiration in these animals is accomplished exclusively by the skin. They are totally devoid of lungs or gills, or any other localised organisation for performing the respiratory function, the skin constituting a respiratory apparatus sufficiently powerful for their wants.

To those who may not have devoted some consideration to the subjects of physiology and comparative anatomy, it may seem extraordinary that respiration should be accomplished by the skin; yet the functions which we recognise under an extreme manifestation

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