Petot will give, by calcination, like results with the same proportions when found naturally combined.

104. Puzzolana, etc. The practice of using brick or tiledust, or a volcanic substance known by the name of puzzolana, mixed with common lime, to form an hydraulic lime, was known to the Romans, by whom mortars composed of these materials were extensively used in their hydraulic constructions. This practice has been more or less followed by modern engineers, who, until within a few years, either used the puzzolana of Italy, where it is obtained near Mount Vesuvius, in a pulverulent state, or a material termed Trass, manufactured in Holland, by grinding to a fine powder a volcanic stone obtained near Andernach, on the Rhine.

Experiments by several eminent chemists have extended the list of natural substances which, when properly burnt and reduced to powder, have the same properties as puzzolana. They mostly belong to the feldspathic and schistose rocks, and are either fine sand, or clays more or less indurated.

The following Table gives the results of analyses of Puzzolana, Trass, a Basalt, and a Schistus, which, when burnt and powdered, were found to possess the properties of puzzolana.

105. Whether natural puzzolanas occur in the United States, is not known. The great abundance of natural hydraulic cements would probably cause no demand for them, nor for artificial puzzolanas for building purposes.

106. All of these substances, when prepared artificially, are now generally known by the name of artificial puzzolanas, in contradistinction to those which occur naturally.

107. General Treussart, of the French Corps of Military Engineers, first attempted a systematic investigation of the

properties of artificial puzzolanas made from ordinary clay, and of the best manner of preparing them on a large scale. It appears from the results of his experiments, that the plastic clays used for tiles, or pottery, which are unctuous to the touch, the alumina in them being in the proportion of one fifth to one third of the silica, furnish the best artificial puzzolanas when suitably burned. The clays which are more meager, and harsher to the touch, yield an inferior article, but are in some cases preferable, from the greater ease with which they can be reduced to a powder.

108. As the clays mostly contain lime, magnesia, some of the metallic oxides, and alkaline salts, General Treussart endeavored to ascertain the influence of these substances upon the qualities of the artificial puzzolanas from clays in which they are found. He states, that the carbonate of potash and the muriate of soda seem to act beneficially; that magnesia seems to be passive, as well as the oxide of iron, except when the latter is found in a large proportion, when it acts hurtfully; and that the lime has a material influence on the degree of heat required to convert the clay into a good artificial puzzolana.

109. The management of the heat, in the preparation of this material, seems of the first consequence; and General Treussart recommends that direct experiment be resorted to, as the most certain means of ascertaining the proper point. For this purpose, specimens of the clay to be tried may be kneaded into balls as large as an egg, and the balls when dry, be submitted to different degrees of heat in a kiln, or furnace, through which a current of air must pass over the balls, as this last circumstance is essential to secure a material possessing the best hydraulic qualities. Some of the balls are withdrawn as soon as their color indicates that they are underburnt; others when they have the appearance of well-burnt brick; and others when their color shows that they are overburnt, but before they become vitrified. The burnt balls are reduced to an impalpable powder, and this is mixed with a hydrate of fat lime, in the proportion of two parts of the powder to one of lime in paste. Water is added, if necessary, to bring the different mixtures to the consistence of a thick pulp; and they are separately placed in glass vessels, covered with water, and allowed to remain until they harden. The compound which hardens most promptly will indicate the most suitable degree of heat to be applied.

110. As the carbonates of lime, of potash, and of soda, act as fluxes on silica, the presence of either one of them will

modify the degree of heat necessary to convert the clay into a good natural puzzolana. Clay, containing about one tenth of lime, should be brought to about the state of slightly-burnt brick. The ochreous clays require a higher degree of heat to convert them into a good material, and should be burnt until they assume the appearance of well-burnt brick. The more refractory clays will bear a still higher degree of heat; but the calcination should in no case be carried to the point of incipient vitrification.

11. The quantity of lime contained in the clay can be readily ascertained beforehand, by treating a small portion of the clay, diffused in water, with enough muriatic acid to dissolve out the lime; and this last might serve as a guide in the preliminary stages of the experiments.

112. General Treussart states, as the results of his experiments, that the mixture of artificial puzzolana and fat lime forms an hydraulic paste superior in quality to that obtained by M. Vicat's process for making artificial hydraulic lime. M. Curtois, a French civil engineer, in a memoir on these artificial compounds, published in the Annales des Ponts et Chaussées, 1834, and General Pasley, more recently, adopt the conclusion of General Treussart. M. Vicat's process appears best adapted when chalk, or any very soft lime-stone, which can be readily converted to a soft pulp, is used, as offering more economy, and affording an hydraulic lime which is sufficiently strong for most building purposes. By it General Pasley has succeeded in obtaining an artificial hydraulic cement which is but little, if at all, inferior to the best natural varieties; a result which has not been obtained from any combination of fat lime with puzzolana, whether natural or artificial.

113. All the puzzolanas possess the important property of not deteriorating by exposure to the air, which is not the case with any of the hydraulic limes or cements. This property may render them very serviceable in many localities, where only common or feebly bydraulic lime can be obtained.

114. The well-known artificial Portland cement, manufactured in England, is composed of an intimate mixture of chalk and clay, in the state of paste, which is then dried and burned. in kilns or ovens; the product of the calcination being flinty, or like vitrified brick. This degree of calcination is essential to the excellence of the material, of which its weight, or specific gravity, is one of the best tests.

Another more recent method of giving a certain degree of hydraulicity to common limes, and of improving that of hy

draulic limes, is to place the calcined stone, after it has been drawn from the kiln, in arched ovens which can be made airtight, in which it can be subjected to the action of a fire, from a grate beneath; so that the heat can be equally diffused throughout the mass, which is brought only to a slight glow, as seen by the eye. When in this condition, iron pots containing sulphur are placed underneath, and the sulphur, converted into vapour, allowed to permeate the mass of lime; the escape of the vapour from the oven having been previously provided against. After the sulphur has been consumed the mass is allowed to cool, and is then ground fine like other cements. This product is known in commerce as Scott's cement, from the name of the inventor, an officer of the Royal Engineers. See Professional Papers of the Corps of Royal Engineers. Vol. X. New Series.

IV.

MORTAR.

115. Mortar is any mixture of lime in paste with sand. It may be divided into two principal classes; Hydraulic mortar, which is made of hydraulic lime, and Common mortar, made of common lime.

116. The term Grout is applied to any mortar in a thin or fluid state; and the terms Concrete and Beton, to mortars incorporated with gravel and small fragments of stone or brick.

117. Mortar is used for various purposes in building. It serves as a cement to unite blocks of stone, or brick. In concrete and beton, which may be regarded as artificial conglomerate stones, it forms the matrix by which the gravel and broken stone are held together; and it is the principal material with which the exterior surfaces of walls and the interior of edifices are coated.

118. The quality of mortars, whether used for structures exposed to the weather, or for those immersed in water, will depend upon the nature of the materials used; their proportions; the manner in which the lime has been converted into a paste to receive the sand; and the mode employed to mix the ingredients. Upon all of these points experiment

is the only unerring guide for the engineer; for the great diversity in the constituent elements of limestones, as well as in the other ingredients of mortars, must necessarily alone give rise to diversities in results; and when, to these causes of variation, are superadded those resulting from different processes pursued in the manipulations of slaking the lime and mixing the ingredients, no surprise should be felt at the seemingly opposite conclusions at which writers, who have pursued the subject experimentally, have arrived. From the great mass of facts, however, presented on this subject within. a few years, some general rules may be laid down, which the engineer may safely follow, in the absence of the means of making direct experiments.

119. As to the action of salt water on artificial hydraulic limes made by mixing common lime with a natural or artificial puzzolana, opinion among European engineers seems divided. Some state that they withstand well the action of salt water; others that they only resist this action after the exposed surface becomes coated with barnacles, oysters, etc.

120. The view now generally taken of mortar is, that being an artificial sandstone, the nearer its constituents approach those of the natural sandstones, the better will be the result obtained; and that therefore the best proportions for its ingredients are those in which each grain of sand is enveloped with just sufficient lime, in a barely moist state, to cause the whole mass to cohere and set quickly. Too much lime causes shrinkage and cracks; and when too much water is added the mass in drying is found to be porous.

121. Sand. This material, which forms one of the ingredients of mortar, is the granular product arising from the disintegration of rocks. It may, therefore, like the rocks from which it is derived, be divided into three principal varieties -the silicious, the calcareous, and the argillaceous.

Sand is also named from the locality where it is obtained, as pit sand, which is procured from excavations in alluvial, or other deposits of disintegrated rock; river sand, and sea sand, which are taken from the shores of the sea, or rivers.

Builders again classify sand according to the size of the grain. The term coarse sand is applied when the grain varies between 4th andth of an inch in diameter; the term fine sand, when the grain is betweenth andth of an inch in diameter; and the term mixed sand is used for any mixture of the two preceding kinds.

122. The silicious sands, arising from the quartzose rocks, are the most abundant, and are usually preferred by builders.