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

89. 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 case with which they can be reduced to a powder.

90. 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.

91. 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 Some of the balls are withdrawn as soon as their color indicates that they are underburnt; others when they have the appearance of well-burnt brick; and others when their color shows that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.

92. 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.

93. 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.

94. 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.

95. 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 hydraulic lime can be obtained.

MORTAR.

96. 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.

97. 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.

98. 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.

99. 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 proportion;-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 lime-stones, 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.

100. 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 deposites 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 th andth of an inch in diameter; the term fine sand, when the grain is betweenth and th of an inch in diameter; and the term mixed sand is used for any mixture of the two preceding kinds.

101. The silicious sands, arising from the quartzose rocks, are the most abundant, and are usually preferred by builders. The calcareous sands, from hard calcareous rocks, are more rare, but form a good ingredient for mortar. Some of the argillaceous sands possess the properties of the less energetic puzzolanas, and are therefore very valuable, as forming, with common lime, an artificial hydraulic lime.

102. The property which some argillaceous sands possess, of forming with common, or slightly hydraulic lime a compound which

will harden under water, has been long known in France, where these sands are termed arènes. The sands of this nature are usually found in hillocks along river valleys. These hillocks sometimes rest on calcareous rocks, or argillaceous tufas, and are frequently formed of alternate beds of the sand and pebbles. The sand is of various colors, such as yellow, red, and green, and seems to have been formed from the disintegration of clay in a more or less indurated state. The arènes are not as energetic as either natural or artificial puzzolanas; still they form, with common lime, an excellent mortar for masonry exposed either to the open air, or to humid localities, as the foundations of edifices.

103. Pit-sand has a rougher and more angular grain than river or sea sand; and, on this account, is generally preferred by builders for mortar used for brick, or stone-work. Whether it forms a stronger mortar than the other two is not positively settled, although some experiments would lead to the conclusion that it does.

104. River and sea sand are by some preferred for plastering, because they are whiter, and have a finer and more uniform grain than pit sand; but as the sands from the shores of tidal waters contain salts, they should not be used, owing to their hygrometric properties, before the salts are dissolved out in fresh water by careful washing.

105. Pit-sand is seldom obtained free from a mixture of dirt, or clay; and these, when found in any notable quantity in it, give a weak and bad mortar. Earthy sands should, therefore, be cleansed from dirt before using them for mortar; this may be effected by washing the sand in shallow vats, and allowing the turbid water, in which the clay, dust, and other like impurities are held in suspension, to run off.

106. Sand, when pure or well cleansed, may be known by not soiling the fingers when rubbed between them.

107. Hydraulic mortar. This material may be made from the natural hydraulic limes; from those which are prepared by M. Vicat's process; or from a mixture of common, or feebly hydraulic lime, with a natural or artificial puzzolana. All writers, however, agree that it is better to use a natural than an artificial hydraulic lime, when the former can be readily procured.

108. When the lime used is strongly hydraulic, M. Vicat is of opinion that sand alone should be used with it, to form a good hydraulic mortar. General Treussart has drawn the conclusion, from his experiments, that the mortar of all hydraulic limes is improved by an addition of a natural or artificial puzzolana. The quantity of sand used may vary from 1 to 2 parts of the lime, in bulk, when reduced to a thick pulp.

109. For hydraulic mortars, made of common, feeble, or or

dinary hydraulic limes, and artificial puzzolana, M. Vicat states that the puzzolana should be the weaker as the lime is more strongly hydraulic; using, for example, a very energetic puzzolana with a fat, or a feebly hydraulic lime. The proportion of sand which can be incorporated with these ingredients, to form an hydraulic mortar, is stated by General Treussart to be one volume to one of puzzolana, and one of lime in paste.

110. In proportioning the ingredients, the object to which the mortar is to be applied should be regarded. When it is to serve to unite stone, or brick work, it is better that the hydraulic lime should be rather in excess; when it is used as a matrix for beton, no more lime should be used than is strictly required. No harm will arise from an excess of good hydraulic lime, in any case; but an excess of common lime is injurious to the quality of the mortar. 111. Common and ordinary hydraulic limes, when made into mortar with arènes, give a good material for hydraulic purposes. The proportions in which these have been found to succeed well, are one of lime to three of arènes.

112. Hydraulic cement, from the promptitude with which it hardens, both in the air and under water, is an invaluable material where this property is essential. Any dose of sand injures its properties as a cement. But hydraulic cement may be added with decided advantage to a mortar of common, or of feebly hydraulic lime and sand. It is in this way that it is generally used in our public works. The French engineers give the preference to a good hydraulic mortar over hydraulic cement, both for uniting stone, or brick work, and for plastering. They find, from their practice, that when used as a stucco, it does not withstand well the effects of weather; that it swells and cracks in time; and, when laid on in successive coats, that they become detached from each other.

General Pasley, who has paid great attention to the properties. of natural and artificial hydraulic cements, does not agree with the French engineers in his conclusions. He states that, when skilfully applied, hydraulic cement is superior to any hydraulic mortar for masonry, but that it must be used only in thin joints; and, when applied as a stucco, that it should be laid on in but one coat; or, if it be laid on in two, the second must be added long before the first has set, so that, in fact, the two make but one coat. By attending to these precautions, General Pasley states that a stucco of hydraulic cement and sand will withstand perfectly the effects of frost.

113. Mortars exposed to weather. The French engineers, who have paid great attention to the subject of mortars, coincide in the opinion, that a mortar cannot be made of fat lime and any inert sands, like those of the silicious, or calcareous kinds, which