102 ILORIDA GEOLOGICAL SURVEY-I5TH ANNUAL REPORT

of water absorbed expressed in percentage terms of the dry weight of the fired test piece and is obtained by the following formula:

 Absorbed weight-dry weight
 X100=% absorption.
 dry weight
 A knowledge of the amount of water possible for a clay to absorb is very desirable as this property may seriously affect its usefulness. A small percentage of absorption indicates that the ware will absorb a correspondingly small amount of moisture. This in turn indicates a better resistance to frost action, less likelihood of the appearance of soluble salts and also indicates the general hardness of the ware as well as the general progress of vitrification.
 The porosity of a clay is an expression of the pore space it contains. The porosity is determined after each firing by boiling the test piece in water for one hour after it it has absorbed as much water as it will. This causes the piece to become saturated. Its weight suspended in water is also obtained. The porosity is expressed in percentage terms of the original weight of the test piece and obtained by the following formula:
 Saturated weight-dry weight
 X100=% porosity.
 Saturated weight-suspended weight

 By use of the porosity determination the progress of vitrification is Known. The point of minimum porosity, or maximum density, is the point of complete vitrification. If plotted on cross-section paper where percentage of porosity and temperature are the factors, the resulting curve drops more or less uniformly:until it approaches the temperature of vitrification, risin,.again as the temperature f xiscosity is reached. When the porosity. incTcases the clay is said to be overfti;ed. The porosity of a clay prod'ict'is.cegarded as an importaLmfactor in its durability and non-conductivity to heat. . :
 Porosity and absorption stand in direct relation to each other. The curves, when plotted, are essentially parallel. The determination of absorption is essential in structural materials and porosity in fireproofing ware.
 The transverse strength or tensile strength indicates the ability of a clay to. withstand the shocks and strains of. handling. The transverse