ATMOSPHERIC INFLUENCE. 



ATMOSPHERIC INFLUENCE. 



previously existing. Under tame oooditioM, however, the rerjr matt 

 ag*oU which are nocasimully so injurioiu, may produce advantageous 

 rwulu; utJ as they thus act to consolidate u well u to destroy, the 

 tudy of their mod* of action becomes one of great inters* tad import- 

 ance to UM architect and builder especially. Strange to lay, nothing 

 ( any scientific nine fa known u ( K>n the subject ; and there is upon 

 record nothing beyond a few empirical observations, to guide the 

 student in hi* inquiries The following brief remarks will, it U 

 bettered, be found to contain all that ia known with respect to the 

 changes produced by the atmosphere upon the various descriptions of 

 materials enumerated. The choice of those materials has been made 

 distinctly on account of their frequent use in the arU of construction. 



Granites, which are usually considered to be amongst the most durable 

 of building materials, preeent nevertheless many varietiee differing 

 greatly in their composition, and the mechanical arrangement of their 

 clement* ; and consequently also in their durability. Thus the granites 

 from Devonshire and Cornwall often contain a large proportion of 

 achori, and they are frequently perraded by mniann of felspar, of such 

 dimension* and so distinctly crystallised as to cause the mass to 

 assume a porjihyritic character. According to Sir H. de la Beche, the 

 granites of Ireland are of the same character, but the schorl occurs 

 in smaller proportions ; whilst the granites of Aberdeen are more 

 decidedly micaceous, and schorl is rarely found in them. The Aberdeen 

 granites also iliffer from those of the west of England and Ireland in 

 their mechanical structure, as they have their component parts more 

 equal in volume and more evenly distributed. In some cases horn- 

 blende takes the place of mica ; and in others, the quartz and felspar 

 are so much aflected in colour, by the presence doubtlessly of the 

 hydrous oxide of iron, as to assume a general rosy hue, o in the case 

 of the Peterhead granites, and the red Egyptian porphyries. The 

 granites of Guernsey, Jersey, and of the French coast, as well as those 

 of the north-western ports of Spain are very similar in their compo- 

 sition to those of Cornwall ; and the decomposition of the excess of 

 felspar they contain, gives rise to the large deposits of kaolin clay to be 

 found in some of those respective localities. 



Sir H. Davy (' Agr. Chem.,' p. 189) explains the mode of decom- 

 position of granites as follows : " The quartz in them is almost pure 

 aUicious earth in a crystalline form. The felspar and mica are very 

 compounded substances ; both contain silica, alumina, and oxide of 

 iron. In the felspar there is usually lime and potassa ; in the mica, 

 lime and magnesia. When a granitic rock of this kind has been long 

 exposed to the influence of air and water, the lime and the potassa 

 contained in ita constituent parts, are acted upon by water and carbonic 

 acid ; and the oxide of iron, which is almost always in its least oxidised 

 state, or in that of the protoxide, tends to combine with more oxygen : 

 the consequence is, that the felspar decomposes, and likewise the mica, 

 but the first the more rapidly." There does not appear to be any reason 

 to question the correctness of this explanation, and indeed it is still 

 received as being the true mode of accounting for the decay of granite ; 

 but there appear to be different conditions in the combinations of the 

 bases of felspar, which give rise to some apparent anomalies. Thus, 

 the Egyptian porphyries, which contain a notable excess of rose-coloured 

 felspar, resist the influence of the atmosphere in an extraordinary man- 

 ner. Possibly this may be accounted for by the closeness of the grain 

 (so to speak), which would to a certain extent prevent atmospheric 

 moisture from communicating with anything beyond the immediate 

 surface ; or the more simple character of some of these porphyritic 

 rocks may be favourable to their preservation ; whereas the different 

 rates of expansion of the ingredients must have a material mechanical 

 influence in disintegrating rocks when they exist in considerable num- 

 bers. The difference between the rapid rate of decomposition of the 

 porphyritic granites of Spain, Brittany, and Cornwall, and that of the 

 Egyptian specimens, appears to justify the inference that the cause 

 last suggested has great influence upon their durability. The former 

 are, in fact, composed of a greater number of ingredients than the 

 fetter, in which both mica and hornblende are wanting. Practically it 

 is found that the uniform character of the Aberdeen granites resists 

 atmospheric Influences in our latitude better than the more easily 

 worked stones of Devonshire and Cornwall 



Whinrtones.br basalts, become disintegrated unequally tinder the 

 fleet* of exposure to the atmosphere, in consequence of the variable 

 proportions of felspar they contain, and perhaps also of the particular 

 combinations of alumina, lime, and magnesia, which, in connection 

 with silica, constitute their base. If any potassa be present in combi- 

 nation with the silica in the shape of felspar, the action is more rapid 

 than under ordinary circumstances, the humidity of the atmo.-ph. -r. 

 decomposing the felspar into two substances, one of which is soluble 

 and easily removed by succeeding rains. Serpentines, and the diallage 

 rocks, which are silicates of magnesia combined with hydrates of that 

 base, yield easily to the attacks of acids when the silicates are in certain 

 proportions and alumina is present. The most beautiful varieties ol 

 UMM materials employed in the arts are obtained from the environs of 

 Oenaa, and from ZojbliU in Saxony ; but it is much to be feared that 

 the atmosphere of large towns, especially when containing sulphuric 

 add gas from the combustion of coal, renders the employment of even 

 the best of these decorative materials very hazardous. It appears also 

 that in parting with their water of crystallisation upon exposure to the 

 Air, an unequal construction takes place in tho constituent part* of the 



serpentine obtained in certain localities, which in consequence breaks 

 n an irregular manner. 



The next description of building materials to be considered, in 

 ollowing the. ascending order of the geological series, are the slate 

 rocks. Those worked for the supply of the London market are 

 winci pally situated in North Wales, Cumberland and Westmoreland, 

 he former being a tolerably homogeneous clay slate, the latter being 

 nuch more chloritic. Exposed freely to the action of the atm..- 

 rath varieties appear to be tolerably durable, but Westmoreland slates 

 decay very rapidly in damp positions, when the air around them is not 

 requently renewed. They decay, in fact, in the covered parts, proba- 

 jly from the decomposition of tho silicate of iron to which they owe 

 .heir peculiar colour. The combination of silica and alumina in tho 

 Welsh slates appears to be more stable than that which prevails in any 

 other material of the some nature ; for their powers of resistance to 

 atmospheric influence are greater than those of any slates employed in 

 Western Europe or North America. As a general rule, the denser t he 

 slate the more durable it will be ; and the most important condition to 

 MI observed in its use is, that water should not be allowed to percolate 

 between the edges of the strata. Practically, it is found that tin- 

 smoother the surfaces of the slates, and the closer they lie, the greater 

 is the pitch requisite to bo given to the roof ; because, under such cir- 

 cumstances, capillary action takes effect to a greater extent than when 

 the slates present irregular and perceptibly large intervals. 



The various sandstones, millstone grits, and conglomerates are 

 affected by the atmosphere, either through the decomposition of the 

 material cementing their particles together, or by the mechanical 

 effects of moisture. These last may consist either in the removal of 

 the cementing material, or in tho destruction of the cohesion of the 

 particles by the expansion of the water in freezing. Many of the sand- 

 stones occur in regular layers, separated by thin films of a species of 

 clay, as in the common Yorkshire flagstones ; and when these films are 

 sufficiently thick to offer an efficient resistance to the passage of water, 

 to retain it in fact tinder the upper shale, the expansion during frost 

 will almost inevitably detroy the stone. If the water should change 

 iU form from other causes, such as heat, the some effect will be pro- 

 duced ; and it is therefore found that the Yorkshire stones of a very 

 porous, and at the some time of a very fissile character, are unable to 

 resist the extremes either of cold or heat. 



The best materials of this description are those of an homogeneous 

 nature, such as the Park Spring, the Idle, and the Parley Dale stones, 

 in all of which the minute subdivisions appear, and the mass may be 

 described as consisting of an assemblage of distinct molecules of similar 

 nature, united by mechanical juxtaposition. The sandstones, in which 

 the silicious molecules ore united by a calcareous cement, are however 

 far more susceptible of decomposition than those united by a si lit -i< HIM 

 cement; for it appears that in the former cases the chemical combi- 

 nation is rarely effected between the lime and the silica, as the la' 

 not in the state known as free silica, nor such as can easily form com- 

 binations with other materials. In the case of some of the millstone 

 grits, however, the cementing substance, which is silicious, forms so 

 energetic a combination with the molecules it envelopes, that both 

 resist the action of extemal causes in an equal manner, and the dura- 

 bility of the Bramley Fall stone may fairly be compared even to that 

 of granite itself. These remarks may be extended to the conglome- 

 rates, in all essential respects at least ; because in fact, generally speak- 

 ing, they differ from the millstone grits only in the size of their 

 elements. 



The crystalline marbles vary in such a very marked manner, both in 

 the nature and the proportion of some of their constituents, and they 

 have been so modified by the accidental circumstances of their geological 

 position, that it is more than usually difficult to trace the laws of their 

 decomposition under the action of atmospheric causes. In many 

 instances they are traversed by veins filled in with more perfectly 

 defined crystals of the base of the rock, or the fissures caused by these 

 veins are filled in with extraneous matters, such as clay, and sometimes 

 even with minerals. Again, however regular the structure of the mass 

 may seem, there U always a tendency in the more crystalline rocks to 

 assume a distinct cleavage, which is often different from the <lire<-ti..ii 

 of the planes of stratification. Under such circumstances, the inoi.-t nre 

 furnished by the atmosphere either furnishes the elements necessary 

 for the decomposition of the materials introduced into the veins, or by 

 the mechanical effects it produces in consequence of the changes of 

 temperature, disintegrates the mass. The former class of phenomena 

 may be moat distinctly observed in the Breccias, or such marbles as 

 the Brocatello and the Oriental Jasper ; the white marbles very 

 frequently exhibit the cleavage above alluded to, and arc the most 

 exposed to the injuries produced by the infiltration of water between 

 the planes of division. It is to be observed, that the species of false 

 stratification indicated by the cleavage is most apparent in the rocks 

 which, like the slate rocks, liave been affected by proximity to igneous 

 formations, and that the more crystalline the marble the more exposed 

 it is to this inconvenience. At the same time, it must be observed, 

 that the more perfectly crystallised carbonates of lime, under vhieh 

 ill-nomination in fact nearly all the finer marbles may be included, 

 resist the chemical influences of the atmosphere more successfully than 

 similar substances where the forms ore not so distinctly marked. It 

 would appear a* though the process of crystallisation wcru accompanied 



