ATMOSPHERIC INKH KNCE. 



ATMOSPHERIC INFLUENCE. 



1 ; 



nitrous 



thoa* found near Paris, are impregnated with an extraordinary ] 

 tiuo uf organic matter; thai decompose* on exposure to the 

 warm and damp position*, and give* rise to the formation of i 

 alta to net) an extent, that the saltpetre used by the French powder 

 manufacturers, during the last war, was almost entirely obtained from 

 cellar* constructed of rubble masonry, set in plaster in the style usually 

 adopted in France. It i* possible that the gaae* which are present in 

 rain water, or in the atmosphere, may give rise to a specie* of decom- 

 n by relative affinity, in combination with the sulphuric acid of 



Atmospheric influence upon brick*, tiles, and other building materials 

 * A *-'ir J by the bunting of plastic clay*, depends very much on the 

 dumiral nuiipnajtlnn nf thn rlajri and mi thn drgrnr nf Inirninr; Thua 

 any dwtinct turtion* uf limestone present hi them would be converted 

 int.. quirk lime in the kiln, and when the briclu were thoroughly 

 wetted, would expand in such a manner as to disintegrate the mass. 

 If the clay u*ed be too poor, that is to say, if it contain an excess of 

 sand, the bricks will not become aufficiently fused, and upon exposure 

 to the weather their constituent parts will neparate. It U to be 

 observed that in bricks, as in stones, decomposition does not take place 

 with the greatest rapidity where constant moisture exist*, but rather 

 where, front the influence of capillarity, variable according to the 

 moisture furnished by the atmosphere either directly or indirectly, a 

 series of alternation* of dryness and humidity prevail. The foundation 

 walk of building* do not in fact suffer so much in the parts imme- 

 diately upon the ground, aa they do in those at a height of from on.- 

 to three Met, according to the permeability of the materials employed. 

 Whru bricks mad* of clay containing free silica are laid in mortar, 

 and moisture can pas* freely from either one or the other, it mar be 

 observed that the edge* in contact become harder than the body of the 

 brick*. No doubt thia arises from the formation of a silicate of lime 

 and alumina, the lime being furnished by the passage of the water 

 through the bed of mortar. 



I'l-.u lime* and cements the effects of the atmosphere are very 

 marked, although at present they are considered to be of far leas 

 importance than formerly. All the materials of thia class, whether the 

 hydraulic or the rich limes, the cements or the plasters properly so 

 called, have a remarkable aridity for water, and abstracting it from 

 the surrounding atmosphere assume the form of hydrates. If these 

 occur in the condition* requisite to enable them to pass into the car- 

 bonate* or sulphates, a species of confused crystallisation, or aggrega- 

 tion, takes place. But it must be evident, from what was said in the 

 commencement of thia notice, that the quantity of carbonic acid gas 

 the atmosphere is capable of furnishing within a moderate period is 

 exceedingly small, and the theory that limes or cements harden 

 originally by the absorption of that gas, and the consequent conversion 

 of the lime into carbonate of lime, must be abandoned. There can be 

 no doubt but that the conversion of the lime into the carbonate must 

 increase its cohesion, but the rapid setting, to use a workman's phrase, 

 cannot be accounted for in this manner. It appears, so far as we are 

 able to judge in the actual state of applied chemistry, that mortars 

 harden hi consequence of the formation of an insoluble silicate of lime 

 and alumina, the silica being either furnished by the limestone itself, 

 or by the sand, poxzolano, or other ingredient mixed with the slaked 

 lime. When the setting ha* been once effected, the absorption of 

 carbonic acid gas from the atmosphere may tend to harden a thin 

 external film ; but the very perceptible character of thia film militates 

 against the auppoaition that the cause producing it can act upon the 

 interior of the mas*. 



Neverthele**, the numerous classes of limes, cement*, and plasters 

 exhibit very marked difference* in their manner of resisting the action 

 of the atmosphere. The purer carbonates of lime, when used for the 

 preparation of mortar, are excessively soluble in water ; and if the 

 mortar obtained from them be much exposed to the weather or to the 

 action of running water, it will be rapidly removed. The argillaceous 

 limestones, on the contrary, furnish the elements necessary to form an 

 inxoluble double salt of lime with the silica and alumina ; and if the 

 mortar* made from them be protected from running water during the 

 period required for their setting, the action of either weather or 

 running water subsequently, will rather tend to increase their powera 

 of resistance. The practical lesson to be drawn from these facts would 

 appear to be, that none but the lime* obtained from argillaceous lime- 

 atone* ahinild be employed in damp situations. With respect to the 

 o*e of planter, the observations before made upon the gypseous stones 

 will apply here perhaps with even greater force. Indeed, the marked 

 difference between the external aspect* of the applied plaster and the 

 natural (tone, would show that in the process of calcination some 

 lament we are unable to ascertain must be driven off. It may be 

 that the burnt plaster yields more readily on account of the absence of 

 thi* very element; but certain it U that plaster of Paris very rapidly 

 decay* when exposed to the influence of the atmosphere. 



Temperature appear* to act in a distinct manner upon lime* and 

 cemenu ; for if they are used in summer, without the adoption of any 

 precaution* to defend them from the *un, they invariably crack ; and 

 of course, if their water of crystallisation become* frozen in winter, the 

 whole maa> will disintegrate. The more rapidly limes or cement* 

 olidify, the more they appear to bo exposed to the danger and incon- 

 venience of cracking in warm weather, and it would appear that the 



most favourable eonditi.ni for their resistance is when a certain degree 

 of uniform moderate dmpnesa prevails. It must also bo o!> 

 that aea air has a marked influence upon the durability of aome lim. -, 

 because the minute particles of sea-water it contains hold in solution 

 many aalU with which free lime has more affinity than with silica. 



The decay of wood superinduced by atmospheric action, is affected 

 by a different class of phenomena from those which ton. I to destroy 

 stones and metals, namely, those connected with organic < li. < 

 although at the same time the change* produced by inorgiuii.- .-I. m. nt 

 are as powerful in the one case a* in the other. From the day wl>< n 



felled to the day it is used, it requires care and attenti. < 

 when it is in [place, precautions should still be taken to inmii, it* 

 dmal.ility. Current* of air which are either renewed with too great 

 rapidity or are too dry, a temperature too elevated, constant moisture 

 at high temperatures, alternations of drrneas and humidity, ak-. 

 ventilation producing wet rot, the accidental transport by the . 

 phere of the seeds of certain cryptogamous plants producing a - 

 of dry rot, and the attacks of insects, together with the fermcir 

 of the sap of the trees, may be cited amongst the numerous sources of 

 danger from which it is necessary to secure wood, either when in store 

 or when employed. 



AVlien wood is exposed to frequent currents of air, especially at high 

 temperatures, the moisture it contains evaporates too rapidl 

 gives rue to cracks and fissures which either .1. -troy the rosistn 

 the material, or open a paasage for the water contained at <>th< T 

 in the atmosphere, to penetrate to the interior of the mass. It tin- 

 temperature to which wood ia exposed, whilst any sap remains in it, i 

 too elevated, the vegetable fluids ferment, the tenacity is dimi 

 and when the action U carried to its full extent, the w.l ijui.-kly 

 becomes affected by the dry rot. Kxposurv to tl 

 positions where rain can lodge in quantity , contact wit h tin- ground. 

 and application in damp situations deprived of air, will render \vo<l 

 liable to the wet rot ; and however well seasoned it may have been 

 previously to being brought within the influence of any of these 

 causes, it will infallibly suffer. It U therefore of the highest import- 

 ance, that whether in the merchants' stores, or subsequently w lu-n 

 placed in a building, wood should be preserved from contact with the 

 ground, and that air should have free access to it in every direction. 

 The germs of destruction ore often communicated whiNt the wood is 

 in store, from neglect of these simple precautions ; if they be once 

 implanted, the progress of decay can never be subsequently arrested. 

 It has been supposed that keeping wood in water tends to prevent the 

 commencement of dry rot, because in that position the sap is washed 

 out of the pores. If this theory is correct, it must be evident that the 

 oftencr the water is changed, the greater will be the probability of its 

 producing the desired effect ; because if it be allowed to stagn 

 must become saturated with the sap in course of time, and unable to 

 take up any additional quantity which may be present. Duhamel 

 observed, that if wood were immersed immediately after it was tvil.-.l. 

 it would be less liable to decay than if put in water at a subs, 

 period; he also found that immersion tended to preserve the wood 

 from the attacks of insects, and even to arrest the progress of conn- 

 kinds of decay, but that a notable portion of the strength was thus 

 lost. The drying and seasoning take place with greater rapidity after 

 immersion, probably because the water displaces the sap, which does 

 not evaporate so rapidly as the thinner fluid. Duhamel asserts that 

 the process of charring the ends of posts, 4c., built into the ground, is 

 very inefficient, and that it is only of use to the extent of interposing 

 an extraneous substance between the wood and the earth ; in his 

 opinion it would be better to inclose the lower ends in sand, stone, 

 cinders, or other materials which would easily carry off the water 

 supplied by the surrounding media. 



When wood is converted and placed in a building, its durability 

 may be greatly increased by covering it with a coating of paint or 

 other substance which will prevent the moisture of the atmosphere 

 from obtaining access to it. But it is essential that the wood so 

 covered should be free from sap or internal moisture, or the very 

 perfection of the coating will be found to accelerate its decay. Care 

 must be token to prevent water from finding its way into the joint*, 

 and if the wood be exposed directly to the action of the sun, it xh.mld 

 be painted of a colour able to reflect, rather than to alorb, boat. It is 

 desirable that it should be planed before being painted, in order that 

 the paint may be applied in an equal manner over the aurfoce. It is 

 important also to observe, that the moisture in the atmosphere not 

 only affects the volume of the wood, but frequently alters the p. 

 of the fibres, by producing a torsion analogous to that which may be 

 observed in hygronietric cords. 



Of late years the processes of Kyanising, creosoting, and immersing 

 timber in solutions of mineral salts, have been applied with various 

 success for preserving it from rot and the attacks of worms or ants. 

 Of these, Kyanising, which employe a solution of deutochlori.l. 1 of 

 mercury, appears to be most satisfactory; and among some striking 

 illustrations of it* results may bo cited the fence of the Hr < ' I arie, 

 the post* of which were inserted in the ground, without being painted, 

 at least twenty-throe years since, and rc-inain at the present day in very 

 tolerable condition. For railways and harbour works, Englicl. 

 neers appear to prefer the system of creoxoting, or immersing tli 

 timbers in the rough oil of tar, until it has absorbed at least 7 or 



