ATMOSPHERIC INFLUENCE. 



ATMOSPHERIC INFLUENCE. 



by the development of some chemical affinity between the ultimate 

 particles, similar to that which is noticed with respect to the various 

 forms of silica, and which enables them afterwards to resist more ener- 

 getically external causes of decay. In some marbles the joints assume 

 different directions from either the lines of stratigraphical deposition, 

 or from the planes of cleavage ; but in the practical operations of 

 building the consideration of this peculiarity may be neglected, because 

 they are usually so distinctly marked as to place in reality the natural 

 limits to the sizes of the blocks raised. 



M. Durocher communicated to the Acade'mie des Sciences a memoir 

 "sur Vabsorption de 1'eau atmospherique par les substances mine'rales," 

 which throws some light on the subject of the decomposition of silicated 

 materials, or those containing metallic oxides. From his experiments 

 it appears that all substances of this description actually absorb water 

 from the atmosphere, and undergo a commencement of hydration, 

 which must necessarily facilitate the decomposition of mixed minerals, 

 such as the silicates, and is apparently the commencement of that 

 action. Messrs. Jamin and Bertrand have also shown that in porous 

 bodies gases are condensed with remarkable facility, so that it is 

 possible that the conditions of molecular aggregation influence the 

 durability of mineral substances in the direct proportion of the 

 facilities which are offered to the passage of air to the interior. From 

 this we may infer that the existence of a crystalline structure is in 

 itself a protection to the materials in which it may occur, because 

 under such circumstances the porosity is usually less ; or at any rate, 

 the intervals between the ultimate molecules are proportionately 

 smaller than in the case of materials held together simply by aggre- 

 gation without crystallisation. Sir H. de la Beche notices the uni- 

 versality of this law (' Geol. Observer,' ed. 1851, p. 8), but he does not 

 attempt to account for it. 



The magnesian, the oolitic, and the ordinary secondary and tertiary 

 limestones are liable to decay under the influence of the atmosphere w it li 

 very various degrees of rapidity ; and it is moreover to be noted, that 

 the same formations yield materials varying singularly in their powers 

 of resistance according to the position they occupied in the quarries, 

 and the exposure of the building in which they are employed. The 

 results obtained by the use of the Anston magnesian limestone, and 

 the marked differences to be observed in the Bath, Caen, and Portland 

 oolites, as well as in the several members of the tertiary series of the 

 Paris basin, appear to show that no a priori laws can be laid down with 

 respect to their durability when exposed to the air. All these classes 

 of materials are supposed to have been deposited by waters containing 

 their elements in solution ; and if they were once in that state (that is, 

 of solution), it must be evident that they are always susceptible of 

 passing into it again if the necessary conditions are presented, unless 

 the deposition should have been effected by the introduction of some 

 additional element, or such element should have been supplied at a 

 later period. There is another chemical agent at work in many cases 

 to hasten the decomposition of the sedimentary rocks unaffected by 

 Plutonic action, namely, the animal matter which they so often con- 

 tain ; and as the distribution of this matter is not subject to any 

 definite law, it necessarily produces irregular effects. Occasionally, 

 also, the body of the stones is traversed by numerous fissures, which 

 have been subsequently filled in by more crystalline materials of greater 

 powers of resistance ; and again, the chemical nature of these stones 

 often varies, owing to the presence of more or less silica in combination 

 with the ordinary bases. Every possible variety in the mode of 

 disintegration may therefore be observed in these stones. 



Experience has shown that the magnesian limestones are not more 

 capable of resisting the effects of our London atmosphere than the 

 carbonates of lime formerly used, when proper care has been exercised 

 in the selection of the latter. Mr. Kogers indeed remarks (' Brit. 

 Asaoc.' 1849), that rain water slightly carbonated takes up the carbonate 

 of magnesia more rapidly than it does the carbonate of lime, and that 

 the magnesian silicates are easily soluble even in pure water. Forch- 

 hammer also observes, that water which contains carbonate of soda 

 acts easily upon magnesian rocks. Now, it is very probable that the rain 

 water falling through the atmosphere of London may become charged 

 at any rate with carbonic acid gas, and thus be rendered extremely pre- 

 judicial to this class of materials ; and it is possible that the rain may 

 give rise to a catalytic action between the lime and magnesia contained 

 in this class of stones, which would facilitate their decomposition. 

 Every stone, as may be gathered from what has been said above, is 

 exposed to decay in the precise ratio of its porosity, but in addition to 

 this cause of disintegration, which acts principally by allowing water 

 to exercise its natural mechanical powers, all the ordinary building 

 stones are exposed to the peculiar process known by the name of nitri 

 fication, which, in consequence of the formation of the crystals of nitre 

 combined with a base, tends to disintegration of the surface. The 

 magnesian limestones allow the development of this process as freely 

 as the carbonates of lime, even if they do not offer extraordinary 

 facilities for its action. The generally received opinion is, that the 

 azote, furnished by the decomposition of the animal matters diffuse< 

 through the rocks, combines with the oxygen of the atmosphere to form 

 nitrogen, which latter gas in its turn combines with the soda, existing 

 in small quantities in all sedimentary deposits, to form the nitrate o 

 oda. Dumas says that azote and oxygen combine most readily under 

 th influence of electricity, but that the energetic bases, lime aw 



nagnesia, may suffice, especially when moisture is present, to replace 

 hat intermediate agent. At any rate, this chemical operation takes 

 >lace in nearly all building stones of a porous nature, and it may 

 :onfirm the common opinion that of stones of the same description 

 jeologically, the densest are the most likely to resist the action of the 

 atmosphere. 



Of late years attempts have been made to cast doubts upon the 

 :orrectness of the generally received opinion, that it is essential to 

 Jace stones on the same bed as they occupied in the quarry. With 

 lorne few stones when in place, it may be true that the position of the 

 ayers is a matter of indifference ; for the roche de St. Cloud, and the 

 Villebois stone of the neighbourhood of Lyon, amongst the secondary 

 and tertiary limestones, have been employed without reference to the 

 Janes of bed for many centuries without inconvenience. But these 

 cases are decidedly exceptional, and even in them the powers of the 

 stone to resist a crushing weight are less when it is applied in a direc- 

 tion parallel to the beds, than when it is applied transversely. In 

 almost every other case it will be found that when stones are used the 

 wrong way of the bed, to employ a workman's phrase, they disintegrate 

 in parallel plates. Great care requires to be exercised to ascertain the 

 srecise direction of the natural beds, because many stones present the 

 appearance of inclined planes of deposition, which cut the beds and 

 joints under every modification of angle. When from the stones 

 laving been worked in the quarry it is difficult to ascertain the precise 

 oed, it is possible that the mason may be misled by the greater facility 

 with which they work in one direction, and may mistake this cleavage 

 for the real bed. The only remedy appears to be to cause the stones 

 exposed to this danger to be marked in the quarry, but fortunately 

 the examples of its occurrence are rare. 



The principal danger of exfoliation arises from the expansion of the 

 moisture contained in the stone under the influence of frost, and a very 

 elegant process was invented by M. Brard, for the purpose of ascer- 

 taining the probable extent due to this cause. M. Brard in his experi- 

 ments upon the resistance of stones, caused them to be boiled for half 

 an hour in a saturated solution of the sulphate of soda. They were 

 then withdrawn and .allowed to stand in a flat vessel, at the bottom of 

 which was a small quantity of the same solution, the first efflorescences 

 were washed off, and the degradation of the stones during the next five 

 or six days, under the effects of the continued efflorescence, was taken 

 as an indication of the probable extent to which they would be affected 

 by frost. In the first volume of Rondelet's ' Art de Batir,' p. 307 (ed. 

 1842, Paris), M. Brard's process is described in detail ; but some very 

 curious experiments recorded in vol. vii. premiere seYie des ' Annales des 

 Fonts et Chaussees,' by M. Minard, together with an article by M. Vicat, 

 inserted in the same volume, throw very considerable doubts upon the 

 exact amount of dependence to be placed on its indications. M. Vicat, 

 indeed, very properly observes, that it still remains to be proved that 

 the expansive action of water in freezing is identical with that of 

 crystallisation, which can only produce energetic effect at temperatures 

 between 68 and 86" Fahr. According to this very accurate observer, 

 stones which are exposed to a southerly aspect on the north of the 

 equator, are more affected by frost than those exposed to the north ; 

 and the most efficient protection to materials of this description of a 

 porous nature is a coating of oil paint, or any other fatty pigment 

 which prevents moisture from being driven or absorbed into the stone. 

 M. Minard recommends that stone should be quarried in the spring, 

 and not employed in a building until it has been exposed to the effects 

 of one winter at least. 



In many varieties of the oolites, the fossil shells are to be observed 

 left in high relief upon the surface by the decomposition of the ma- 

 terials in which they were embedded, and in Bath stone the veins of 

 calcareous spar frequently become detached. It appears that the cause 

 is the same in both cases, and that the shells and the veins protrude 

 in consequence of the resisting powers of the more perfectly crystallised 

 carbonate of lime, of which they are composed, exceeding those of the 

 amorphous mass in which they are embedded. When nodules of flint 

 or chert occur in the numerous varieties of carbonate of lime, the 

 stone around them is usually more durable than the general mass of 

 the rock, doubtless because it contains a portion of the silica in com- 

 bination with the lime. It is known, indeed, that the application of a 

 soluble solution of silica will at all times add much to the durability 

 of the purer carbonates of lime ; but it is very desirable that the con- 

 ditions under which such a solution should be employed, should be 

 made the subject of close and careful experimeuta. The application 

 of Kuhlman and Ilansome's processes for the preservation of stone, 

 may hereafter throw light upon this obscure branch of practical 

 chemistry. 



With respect to the gypseous formations, it appears that where they 

 occur in sufficient abundance to be used as ordinary building materials, 

 they absorb moisture from the atmosphere with extraordinary facility ; 

 while they decompose with such rapidity that the municipal authorities 

 of Paris have forbidden their being used for the walls of houses. 

 Occasionally, however, rubble plaster stone is used for inclosure walls, 

 but it rarely lasts more than from twenty to thirty years, when exposed 

 to the weather near Paris. The cause of this rapid decay, according 

 to Gmelin, is to be found in the fact that the sulphates with an earthy 

 base (gypsum being a sulphate of lime), are soluble in water. It is 

 also to be observed that many of the gypseous deposits, especially 



