in bodies, a property directly opposite to fluidity; by which they resist the impulsion of any other substance, sometimes in an extreme degree. As fluidity has been found to consist in the motion of the particles of a body upon one another in consequence of a certain action of the universal fluid or elementary fire among them; we must conclude that hardness consists in the absence of this action, or a deficiency of what is called latent heat. This is confirmed by observing, that there is an intermediate state betwixt hardness and fluidity, in which bodies will yield to a certain force, though they still make a considerable resistance. This is principally observed in the metals, and is the foundation of their ductility. It appears indeed that this last property, as well as fluidity, is entirely dependent on a certain quantity of latent heat absorbed, or otherwise acting within the substance itself; for all the metals are rendered hard by hammering, and soft by being put again into the fire and kept there for some time. The former operation renders them hot as well as hard; probably, as Dr Black observes, because the particles of metal are thus forced near one another, and those of fire squeezed out from among them. By keeping them for some time in the fire, that element infuses itself again among the particles, and arranges them in the same manner as before, so that the ductility returns. By a second hammering this property is again destroyed, returning on a repetition of the heating or annealing as it is called; and so on, as often as we please.

Hardness appears to diminish the cohesion of bodies in some degree, though their fragility does not by any means keep pace with their hardness. Thus, glass is very hard and very brittle; but flint, though still harder than glass, is much less brittle. Among the metals, however, these two properties seem to be more connected, though even here the connexion is by no means complete. Steel, the hardest of all the metals, is indeed the most brittle; but lead, the softest, is not the most ductile. Neither is hardness connected with the specific gravity of bodies; for a diamond, the hardest substance in nature, is little more than half the weight of the lightest metal. As little is it connected with the coldness, electrical properties, or any other quality with which we are acquainted; so that though the principle above laid down may be accepted as a general foundation for our inquiries, a great number of particulars remain yet to be discovered before we can offer any satisfactory explanation.

All bodies become harder by cold; but this is not the only means of their doing so, for some become hard by heat as well as cold. Thus, water becomes hard by cold when it is frozen, but it becomes much harder when its steam is passed over red-hot iron, and it enters the substance of the metal, by an union with which it becomes almost as hard as glass.

Dr Quist and others have constructed tables of the hardness of different substances. The method pursued in constructing these tables was by observing the order in which they were able to cut or make any impression upon one another. The following table, extracted from M. Magellan's edition of Cronstedt's Mineralogy, was taken from Dr Quist, Bergman, and Mr Kirwan. The first column shows the hardness, and the second the specific gravity.

<table> <tr> <th>Diamond from Ormus</th> <th>20</th> <th>3·7</th> </tr> <tr> <th>Pink diamond</th> <th>19</th> <th>3·4</th> </tr> <tr> <th>Bluish diamond</th> <th>19</th> <th>3·3</th> </tr> <tr> <th>Yellowish diamond</th> <th>19</th> <th>3·3</th> </tr> <tr> <th>Cubic diamond</th> <th>18</th> <th>3·2</th> </tr> <tr> <th>Ruby</th> <th>17</th> <th>4·2</th> </tr> <tr> <th>Pale ruby from Brazil</th> <th>16</th> <th>3·5</th> </tr> <tr> <th>Ruby spinell</th> <th>13</th> <th>3·4</th> </tr> <tr> <th>Deep blue sapphire</th> <th>16</th> <th>3·8</th> </tr> <tr> <th>Ditto paler</th> <th>17</th> <th>3·8</th> </tr> <tr> <th>Topaz</th> <th>15</th> <th>4·2</th> </tr> <tr> <th>Whitish ditto</th> <th>14</th> <th>3·5</th> </tr> <tr> <th>Bohemian ditto</th> <th>11</th> <th>2·8</th> </tr> <tr> <th>Emerald</th> <th>12</th> <th>2·8</th> </tr> <tr> <th>Garnet</th> <th>12</th> <th>4·4</th> </tr> <tr> <th>Agate</th> <th>12</th> <th>2·6</th> </tr> <tr> <th>Onyx</th> <th>12</th> <th>2·6</th> </tr> <tr> <th>Sardonyx</th> <th>12</th> <th>2·6</th> </tr> <tr> <th>Occid. amethyst</th> <th>11</th> <th>2·7</th> </tr> <tr> <th>Crystal</th> <th>11</th> <th>2·6</th> </tr> <tr> <th>Cornelian</th> <th>11</th> <th>2·7</th> </tr> <tr> <th>Green jasper</th> <th>11</th> <th>2·7</th> </tr> <tr> <th>Reddish yellow ditto</th> <th>9</th> <th>2·6</th> </tr> <tr> <th>Schoerl</th> <th>10</th> <th>3·6</th> </tr> <tr> <th>Tourmaline</th> <th>10</th> <th>3·0</th> </tr> <tr> <th>Quartz</th> <th>10</th> <th>2·7</th> </tr> <tr> <th>Opal</th> <th>10</th> <th>2·6</th> </tr> <tr> <th>Chrysolite</th> <th>10</th> <th>3·7</th> </tr> <tr> <th>Zeolite</th> <th>8</th> <th>2·1</th> </tr> <tr> <th>Fluor</th> <th>7</th> <th>3·5</th> </tr> <tr> <th>Calcareous spar</th> <th>6</th> <th>2·7</th> </tr> <tr> <th>Gypsum</th> <th>5</th> <th>2·3</th> </tr> <tr> <th>Chalk</th> <th>3</th> <th>2·7</th> </tr> </table>