This ſpecies reſembles aluminous ſchiſtus, but upon examination is found to contain more magneſia than clay.
All the ſpecies, except the firſt, are more or leſs contaminated by iron, but they do not owe all their colour to this ſubſtance. The green colours altogether vaniſh during ignition, and leave only a white opake maſs.
ARGILLACEOUS EARTH,
OR
ARGILLA.
By earth of alum (argilla) I do not mean common clay, which is never free from ſiliceous matter, but a pure clay, unmixed, at leaſt, with any other earth. It may be readily obtained by diſſolving Roman or roach alum in diſtilled water, filtering, and precipitating by mild volatile alkaly.
The ſpecific gravity of this pure clay, or earth of alum, is 1,305. It diſſolves in acids, with a little efferveſcence. With the vitriolic acid it forms alum; with the nitrous, muriatic and vegetable acids, deliqueſcent ſalts.
When dry, it abſorbs water greedily, becomes ſoft, and, with a due quantity of water, gains ſuch a tenacity, that it may be moulded at pleaſure. This maſs contracts greatly in the fire, from whence ariſe numerous cracks; and with a due degree of heat, it becomes hard enough to ſtrike fire with ſteel. By this burning it loſes its glutinous tenacity, and the water is excluded by the approach of the particles; nor does it again aſſume its former properties, but by ſolution and precipitation.
It may be diſſolved in the dry way, by means of fixed alkaline ſalt, as well as in the liquid way, by acids. The vitriolic acid is better than the others for this purpoſe, becauſe more eaſily concentrated.