A PRELIMINARY REPORT ON CLAYS OF FLORIDA 95

bons, etc.), derived from the original minerals should have been converted into stable forms, . and nothing should be left except what may enter into a silicate solution. The normal beginning of the reaction in the vitrification process is from little spots or foci scattered throughout the body, each focus being represented by some easily fusible mineral grain, or the juxtaposition of two or more mineral grains which combine to form a. eutectic or the most fusible ratio in which these minerals can combine. The spread of the glassy cement from focus to focus in a clay of good vitrifying character is slow and steady, and the proportion of grains which will not readily dissolve is such that they readily form a sort of skeleton or frame work, holding the mass in its shape, while the glassy cement slowly decomposes them and fills up the voids, causing the well-known phenomenon called shrinkage. Practically all silicates when passing from the solid state to a state of complete fusion, give off some gaseous matter. It may be the gas which they have held in solution and which is then occluded, or it may be from remnants of volatile matter not hitherto expelled, or it may be due to the swelling of gases caught in the interstitial voids of the mass during the shrinkage and unable to escape. Probably all three causes are responsible in most cases. . This swelling agency is at work as soon as the formation of glassy cement begins. . If the process of fusion be carried along steadily until a fluid bath is obtained, the liquid will pass into a frothy stage in which the gas bubbles work their way to the top and escape, . but with continued heat and liquidity the bubbles finally cease to form. . It can thus be seen that the clay product, in reaching its point of greatest density, does not reach the point where the gases are fully expelled but only the highest point attainable without causing their evolvement to seriously begin. This maximum density is found at a point where the reduction in volume due to shrinkage is equalized by the expansion due to gases evolved. One force balances the other and for a time the volume of the clay remains constant. This time may be long or short. In some clays of most excellent vitrifying habit, a heat treatment represented by five or six cones may occur with scarcely any change in size. In others the volume diminishes rapidly and at the minimum point begins at once to swell again, with no appreciable interval. Such clays cannot be burned profitably into hard products. There is no margin in which the burner can regulate his kiln, and a part of every kiln would