80 FLORIDA GEOLOGICAL SUREY-I5TH ANNUAL REPORT

 8. The presence of several per cent of lime and carbon dioxide shows the clay to be of calcareous character, and not only often of buffburning character but with a narrow margin between vitrification and viscosity.
 9. Titanium oxide should be determined, especially in fire clays, as a small quantity may reduce the fusion point of fire clay several cones.
 It might be seen from the above that the ultimate analysis yields us much, and yet it leaves us practically in the dark as to the plasticity, air and fire shrinkage, density and hardness of burning, tensile strength, etc."
 In modern clay investigation emphasis is placed on the physical tests and the ultimate analysis is rarely made because it is of little practical value except in rare cases.
 The rational analysis attempts to resolve the clay into its constituent minerals, and in one sense of the word gives a more accurate conception of the true character of the material. The ordinary analysis divides the clay into: clay substance, quartz, and feldspar. The method as at present used is not by any means satisfactory, nor is it altogether safe to figure the mineral composition from the ultimate analysis.'
 Methods of making both the ultimate and rational analysis are conveniently outlined in a numl:er of publications dealing with clays and clay technology.2

 CHEMICAL EFFECT OF VARIOUS CONSTITUENTS IN CLAYS
 Silica-Silica may occur in a clay in the free state as quartz or it may occur in a combined state in the form of the silicate minerals. The common silicate minerals found in clays are kaolinite, hydromicas, feldspar, mica, hornblende, etc. The total silica is usually given in.the ultimate analysis and includes both the free and the combined forms. Except in the case of kaolinite the silicate minerals occur in a more or less sandy condition and exert an influence upon the plasticity and shrinkage as does quartz. All of these affect the fire shrinkage and fusibility of the


 'Washington, H. S., The Calculation of the "Rational Analysis" of Clays, Jour. Am. Ceramic Society. Vol. I, p. 405, 1918.
 2Washington, H. S., Manual of the Chemical Analysis of Rocks, 1919.
 Hillebrand, W. F., The Analysis of Silicate and Carbonate Rocks, U. S. Geol. Survey Bull. 700, 1919.
 Hillebrand, W. F., Some Principles and Methods of Rock Analysis, U. S.. Geol. Survey Bull. 176, 1900.