CALCAREOUS EARTH

CALCAREOUS EARTH, (Encycl.) Though this kind of earth cannot be vitrified by itself in the heat of our common furnaces, it is remarkable, that by mixing it with argillaceous earth, which is as unvitrifiable as itself, both will melt together into glass. To illustrate this, Dr. Watson relates the following experiment. "Mix well together equal weights of powdered chalk or other calcareous body, and powdered pipe-clay; put the mixture into a crucible. Any where towards the middle of the crucible put two solid pieces; one of pure chalk, the other of pure clay, so that they may not be contiguous to each

other, but be every where severally surrounded with the mixture of chalk and clay. Then by exposing the whole to a strong degree of heat for a sufficient length of time, it will be found that the mixture has been converted into glass; and upon breaking the vitrified mass, we shall discover the two solid pieces of chalk and clay unvitrified and unaltered in their figure, tho' every where surrounded with glass.

It is remarkable, that some kinds of calcareous earth, after being deprived of all their fixed air by calcination, will regain, upon being exposed to the atmosphere, the very same quantity that they lost, while others will not. Dr. Watson informs us, that having calcined into lime 204 grains of dove marble, the lime weighed whilst hot 116 grains. This was done on the 6th of March; and on the 5th of November following it weighed 203$\backslash frac\{1\}\{2\}$ grains. As it had been frequently weighed during that interval, it is very probable that the half grain had been lost by repeating this operation. A second experiment turned out, as accurately as could be expected, the same. Some dry chalk, weighing 120 grains, was reduced by calcination to 68. This quantity of lime increased only to 104 grains; so that 16 parts in 120, or between one seventh and one eighth of its weight, were never regained. The Doctor gives a reason for this defect of increase in the following manner. "The chalk or clunch (says he) of this country, is not a pure calcareous earth, but contains a portion of sand; and hence, when a definite weight of this chalk-lime is exposed to the air, it cannot acquire as much increase of weight as the same quantity of purer lime would do; for the sand which is mixed with the lime is of a very different nature from it, and does not attract any thing from the air." It is difficult, however, on this supposition, to see why the sand should lose any thing by calcination; for as it can regain no air, so it can lose none in the fire. The following is a table of the weight regained by lime made from different materials, according to Dr. Watson's experiments. He observes, however, that there is great reason to believe, that the quantity of matter which the

Calcination the same sort of lime can attract from the air, depends very much on the degree of heat with which the lime has been burned. There is a certain definite degree of heat requisite to make the lime attract the greatest possible quantity from the air. When the heat is not sufficient to convert the whole of the substance into lime, then that part which is not converted into lime will not contribute any thing to the increase of the lime when exposed to the air, inasmuch as it possesses no quality of attracting any thing from the air. Our author observes also, that the same effect takes place when the heat has been more than sufficient to convert the whole of the substance into lime; for then a partial vitrification takes place. The following substances were changed into lime by being exposed to a very fierce fire for 24 hours. The experiment was made in November 1779; and in three months they had all acquired the greatest increase of weight. The first column of the table expresses the increase of weight which a ton of the same substances gained; and the second expresses that which they would have gained, had they been exposed to the fire only for four hours.