lime has been exposed to any cause of deterioration, it is prudent to mix with it, a certain dose of puzzolana, without any regard to its primitive degree of energy. Artificial hydraulic limes should be treated in the same way.

§ 29. On the preservation of Puzzolanas.

As to factitious puzzolanas, it may readily be conceived that their preservation requires but little care, for we know that from their situation in nature, the natural puzzolanas are exposed to all the vicissitudes of the seasons without any apparent loss of energy after a great lapse of time. We will say, however, that a newly made and perfectly dry puzzolana, from its possessing a higher degree of absorbent power, must have a favourable influence on the setting of the mortars. The mortars are often, in fact, tempered too soft; and, to lime already slaked to cream, the workmen add more water to lessen their labour; this excess of water being absorbed in part by the puzzolana, the mortar preserves a strong consistence, favourable to the reaction of its elements. In this respect it is therefore advantageous to keep the puzzolanas in dry situations, or at least if they have been moistened, to dry them in the air, or in the sun, before using them.

$30. On the Solidification of Fat Lime.

The preparation and the preservation of lime, puzzolana, and plastic cements, are connected so intimately with a knowledge of their properties, that we think it will be useful to add to the indications already given, some developments relative to the theory of mortars in general.*

Proceeding from the simple to the compound, let us explain what relates to common, or fat, lime. When it is plunged into water, it absorbs rapidly, and solidifies, a quantity of water equal nearly to 0.22 of its weight. Withdrawn, then, and left in contact with the air, it slakes with disengagement of heat, and is reduced to dry and impalpable powder. In this state it is capable of absorbing much water still, but without sensible disengagement of heat, and there results a paste more or less stiff. The first portions of the fluid, form, with the lime, a true chemical combination, known by the name of hydrate of lime: the other portions of water are, simply, interposed. Thus, lime in stiff paste will throw out, on working it, so much water that it is unnecessary to add more on making the mortar. The hydrate of lime, on the other hand, can only be decomposed at a high temperature.

This hydrate being a dry powder, its molecules are too far apart to be able mutually to approach each other, and to pass into the state of a compact mass; it is only after being brought to the state of paste that the hydrate is in a condition to be used. That being premised, it is well known that fat lime, if kept from contact with the air, may be preserved an indefinite time in paste; that this same lime at the ordinary temperature, or at a higher temperature, in paste, or dissolved in water, is without any chemical action on quartzose sand, whether the sand be in fine powder *In a manuscript note communicated to M. Vicat early in 1826, and mentioned in his last work on mortars, we have already stated some of the propositions which follow.

AUTHOR.

or not; that the mortars which result from the mixture of these two substances, remain soft, like lime alone, as has been ascertained by Dr. John, on examining thick masonry two hundred years old. But if the lime in paste, or the mortar, be left in contact with the air, it will solidify; and if the air be replaced by pure carbonic acid gas, the solidification will take place with great rapidity. In both these last cases the carbonic acid is absorbed by the lime, and this absorption will go on till the acid is to the base, in the ratio, approximately, of 43 to 57, as in the natural subcarbonate of lime. But it is worthy of remark that the proportion of water appropriated to the conversion of lime into a hydrate, is not rejected: the carbonate is not, therefore, regenerated, as before the calcination; and it is, in fact, a double salt, which might be called the hydro-carbonate of lime.

We see then that for the solidification of fat lime, 1st, the proportion of water must be greater than in the dry hydrate: 2d, there should be contact of the air, or, better still, of pure carbonic acid gas. 3d, the mixture of quartzose sand, without the contact of the air, would not have the least influence. Hence comes the superiority that lime slaked spontaneously, and consequently already somewhat carbonated, imparts to mortars. Hence also the impossibility that these mortars should harden under water, since water, in general, contains only inappreciable quantities of carbonic acid in solution.

The solidification, as Mr. Vicat remarks, spreads from the surface towards the centre of the specimen: but the quantity of gas that can pass through the voids of mortar beyond a short distance, is too small for its direct influence to be sensible: if then, there is not a total cessation of absorption, it must be admitted that the transmission must go on by the play of affinities-by the tendency that the several concentric layers of lime have to an equilibrium of saturation, like the transmission of heat in solid bodies. It must also be admitted that the equilibrium is the more difficult to attain, as the parts requiring saturation are more remote from the surface, and the dose of acid already received, is the greater, so that the thickness of mortars is injurious to their solidification: that in equal times, the increase in hardness will be far from being equal, and will progressively become less and less, and, lastly, though it may be exact to say that mortars made of fat lime improve as they grow old, still the improvement may not be at all sensible at the expiration of periods of only a few years each.

§ 31. On the distinctive characters of Meagre lime and Hydraulic lime.

Natural limestones often contain earthy or metallic oxides, which by calcination combine with the lime. Whence result modifications in its properties. Thus it is known that lime will remain a fat lime so long as the foreign substances do not form a tenth of its weight; but beyond that it becomes meagre, that is to say, it swells much less on slaking; and, if amongst these foreign bodies, silex should predominate, the paste, with, or without, sand, will acquire the property of hardening in water. It was for a long time thought that other foreign bodies acted like silex; but the method of investigation followed by Mr. Berthier, leaves no doubt in this respect. We will give a summary of his results.

Comparing, first, the quality of various limes with their chemical composition, Mr. Berthier found:

Fat lime from Chateau-Landon to contain 96.4 pure lime-1.80 of magnesia-1.80 of clay (silex and alumine.)

Meagre lime from Coulommiers 78.00 pure lime-20.00 of magnesia— 2.00 of clay (silex and alumine.)

Lime moderately hydraulic from Saint-Germain-89.00 of pure lime1.00 of magnesia-10.00 of clay (silex and alumine.)

Lime very hydraulic from Senonches-70.00 of pure lime-1.00 of magnesia-29.00 of silex. To these analyses we will add:

Meagre lime of Brest-82.30 of lime-10.00 of oxide of iron-7.70 of clay.

We see from these analyses that silex, whether pure or mixed with alumine, renders lime hydraulic; and that magnesia, or the oxide of iron, renders it meagre and not hydraulic. Mr. Berthier found the same consequences when proceeding synthetically: he ascertained that silex in jelly, calcined with pure lime, gave an hydraulic product; that alumine, magnesia, oxide of iron, and oxide of manganese, calcined, one by one, with pure lime, gave a meagre lime only: that alumine or magnesia mixed with silex increased the hydraulic property, and, lastly, that the proportions the most favourable for the mixture were equal parts of silex on the one hand and alumine or magnesia on the other.

A consequence results from these considerations which it is important to mention: it is this, that the process of Mr. Vicat for preparing artificial hydraulic lime, does not answer equally well on taking any limestone or clay that may present itself: that it is, with difficulty, applicable to meagre limes mixed with ochreous clays, and that this is the case at Brest, where, the matters being charged with oxide of iron, nothing passable was obtained, and we were obliged to resort to puzzolanas.

§ 32. On the solidification of hydraulic lime.

Hydraulic lime slaked in the ordinary manner solidifies a certain quantity of water as fat lime does; and forms, with an excess of water, a paste, more or less stiff. If left exposed to the air, it absorbs less carbonic acid than fat lime; and, like fat lime, it retains the water it had solidified. According to Mr. Vicat there are in 100 parts of fat lime = 100.00 Absorbed carbonic acid

76.00

And in 100 parts of hydraulic lime which contains a fifth of its

weight of clay:

Absorbed carbonic acid

54.00

Retained water

15.00

But this last result may be put under the following form:

Pure lime

100.00

It differs therefore, in this respect, very little from fat lime, so that it is equally a hydro-carbonate of lime, the clay appearing not to enter into the combination.

On the other hand, when the paste remains immersed in water, the aid of the carbonic acid is no longer possible, and that of the silex becomes indis

pensible to solidification. It remains to seek the cause of this phenom

enon.

1st. Pure lime is soluble in five or six hundred times its weight of water, and the product is called lime water.

2nd. Pure lime combined by calcination with gelatinous silex is only partially soluble in water, and leaves a residue, composed of sixty-five parts of silex and thirty-five parts of lime, which is known under the name of neutral silicate of lime.

3rd. Pure lime combined in the same manner with alumine, magnesia, oxide of iron, or oxide of manganese, although it has lost the property of swelling much, or slaking, is still soluble in water, and the residue contains nothing but pure alumine, or magnesia, or the oxide of iron or manganese.

4th. In order that a lime may be hydraulic, it will suffice that it possesses six or seven per cent. of silex, a quantity that can render only a very small dose of lime insoluble.

5th. Plastic cements, at the point of complete calcination, may be assim ilated to ordinary lime-they are but slightly hydraulic, although they contain a considerable portion of silex.

We see then that the combination of silex with lime has, alone, the advantage of resisting the attacks of water: that if alumine and some other oxides raise the hydraulic energies, it arises probably from this, that the obstacle they interpose to the swelling of the lime tends to the concentration of the molecules, thereby helping their predisposition to submit to the influence of the silicate of lime: and that free lime, notwithstanding its solubility, ought always to predominate in the immersed paste.

The question being thus stated, it is necessary to explain two effects; the insolubility, and the hardness, acquired by mortars under water.

In the first place, it cannot be admitted that the silicate of lime solidifies separately, and that it envelopes the hydrate as a gangue; for the last would not be less soluble, and the hardness acquired would be proportionate to the quantity of silicate, which, in general, is not true. Every thing leads to the opinion rather, that the molecules of silicate are so many centres of attraction, with respect to the soluble molecules, and that, within the sphere of activity of each of these centres, there is an arrangement which may be assimilated to a true crystallization.

On this hypothesis, we conceive, the most proper proportion of silicate, is that which leaves around each molecule thereof, a layer of hydrate equal in thickness to the radius of the sphere of attraction: that below this term, that is to say, in measure as the silicate becomes more abundant, the layers of interposed hydrate are submitted to attractions which interfere with and disturb, instead of mutually aiding, each other, that above this term, that is to say when the silicate is in too small a quantity, the layers of hydrate may still solidify, not so soon, nor wholly by the direct action of the silicate, but by the influence of the particles nearest the silicate, which, in measure as they solidify, react in their turn upon others: as in saline solutions, a crystal already formed may be the proximate cause of crystallization.

It may in the same manner, be conceived that hydraulic mortars have need of moisture rather than dryness, because the water preserving to the molecules a certain mobility, permits their arranging themselves in juxtaposition by the proper facets. It must not however be concluded that soft mortars would be preferable to mortars in stiff paste: because the water

augmenting too much the distance of the molecules, would throw them beyond the sphere of mutual attraction: hence one of the causes why hydraulic mortars are generally less hard on the surface than in their central parts, while it is the reverse with all mortars made of fat lime and hardened in the air.

§ 33. On the influence of the dissolving action of water.

This fact has relation to another cause on which it is important to insist.

When hydraulic mortar is immersed in the bottom of any vessel, two opposite molecular forces are set in action: on the one hand, the action of the silicate of lime on the hydrate, and on the other, the solvent force of the water with respect to this same hydrate. The water being supposed tranquil, the lower portions of it dissolve the lime: but as this water becomes more dense it remains at the bottom, without power of removal; and the lime thus dissolved cannot be transferred to the upper portions of fluid except by the play of affinities-by a transmission analogous to that of carbonic acid in the interior of mortars made of fat lime. However that may be, the portion of water in immediate contact with the mortar losing its solvent power in proportion as it approaches the term of saturation, an equilibrium will soon be established between the two opposite molecular forces, and then all solution will cease. But if the superficial layer of mortar has lost a small portion only of its hydrate, this equilibrium, will, as regards the progress of solidification of the mortar, be only instanta neous: this layer might in fact continue to take cohesion from the influence of the under layers of mortar not attacked by the water.

If, on the other hand, the liquid be agitated, in order to render the saturation uniform throughout the mass of fluid- -or rather, if the mortar be immersed in running water, it may happen that the exterior layers of mortar will, little by little, lose all, or the greater part, of their hydrate; but this effect not being brought about instantly, the interior layers would be protected for a time sufficient for them to harden beyond its solvent power before becoming, in their turn, exposed to the action of the water. can conceive, therefore, that mortars may wash away for a certain depth: and that thickness may be an indispensable condition to the success of the solidifying process.

§ 34. Influence of Quartzose Sands.

We

As the practice is to use hydraulic limes, not pure, but mixed with sands, it is important to study the influence of these sands on the induration of mortars. We shall speak here only of quartzose sands, which are the

most common.

It is demonstrated by experiments that caustic lime, cold or hot, is without any action on quartz in discernible particles: if, then, it adds to the quality of hydraulic mortars, it can only be because, 1st. it augments the density of the mass, and thus prevents its being brought to the state of soft mortar by being too easily permeable to water: or, 2nd. because its adherence to the lime, however feeble it may be supposed to be in the first instance, is an accelerating cause of crystallization. That this should be