ATMOSPHERIC INFLUENCE. 



ATMOSPHERIC INFLUENCE. 



702 



8 Ib. per cubic foot. The difficulty of injecting so large a quantity of 

 oil is overcome by exhausting the sap and moisture from the wood m 

 vacua, and then forcing in the oil under great pressure ; a species of 

 artificial drying is, however, frequently necessary, and indeed the 

 success of this process appears to depend entirely upon the extent to 

 which the original moisture is withdrawn. Both corrosive sublimate 

 and oil of tar are capable of resisting the causes of decay communicated 

 by the atmosphere, and the latter is said to be an effectual preservative 

 against the attacks of boring animals ; but it is to be feared that the 

 ordinary manner of applying them does not insure their penetration to 

 a sufficient depth to attain the objects desired. The use of the sul- 

 phate of copper and of the other metallic salts has hitherto been 

 unsuccessful. 



In the bent timber bridges which have been constructed on some 

 of the modern lines of railway, although every ordinary precaution was 

 taken in selecting the timber, immersing it in solutions of the metallic 

 salts, and in painting it when in place, the wet rot has exhibited itself 

 in so many instances as to render it almost necessary to abandon a 

 system which appeared to have many recommendations. It is, how- 

 ever, to be observed, that these bridges decayed solely because their 

 elasticity caused them to yield upon the passage of every train. The 

 play thus produced caused the joints to open, and moisture, furnished 

 by rain or the condensation of vapour, found its way into the interior 

 of the beams. The failure of the bent timber ribs in such situations 

 does not, therefore, in any manner affect the propriety .of using that 

 construction on more suitable occasions. A valuable lesson is, how- 

 ever, to be learnt from the above fact, namely, that it is difficult, if not 

 impossible, to protect complicated systems of carpentry from the effects 

 of the atmosphere, when exposed to the occasional action of heavy 

 loads able to produce a disturbance of their main parts. 



The action of the atmosphere upon metals is even more complicated 

 than that which takes place upon stones, because the electro-chemical 

 changes are more decided, and the metallic bases are susceptible of 

 combining with a greater number of gases than are the earthy bases or 

 the metalloids. In building operations the metals commonly used are 

 iron, lead, copper, tin, zinc, with the mixed metals, brass or bronze, 

 and to them we will confine our attention. 



Iron, whether cast or wrought, becomes rusted on exposure to the 

 air or water under certain conditions ; that is to say, the outer portions 

 of the metal are converted into a hydrous oxide, and can be detached 

 in scales or flakes. Many systems have been proposed to obviate the 

 danger, and many substances applied to correct the evil, arising from 

 this cause, full information concerning which is contained in Mr. Robert 

 Mallet's papers in the Transactions of the British Association for the 

 Advancement of Science. His experiments appear to show that gas tar 

 applied hot is the most efficacious protection for iron work exposed to 

 c"M water, and that a coating of caoutchouc varnish resists the longest 

 in hot water ; but that neither of them can be considered a durable 

 defence. Cases cited by M. Vicat are within the knowledge of every 

 architect who has examined this class of phenomena, which prove that 

 in some waters, as in some positions in the open air, iron work, totally 

 unprotected, will last an indefinite period. These exceptions are, 

 nevertheless, so rare and the destruction of iron unprotected is gene- 

 rally so rapid both in air and in water, that constant care and attention 

 are required to guard against the destructive tendencies of those agents. 

 If iron work be exposed to the air in positions which render the 

 renewal of the air difficult, and at the same time retain it in a marked 

 degree of dryness, the iron will become covered with a coat of rust, 

 through which the atmosphere cannot penetrate to attack the metal 

 beneath. If the water in which iron is immersed contains a very 

 small portion of some of the earthy salts, the decomposition will take 

 place slowly. But it is necessary to observe that these remarks only 

 apply to iron of considerable dimensions : small wires decay rapidly on 

 exposure to either of these causes of disintegration. 



Should iron, however, be exposed to confined air in damp positions, 

 the decay attains its maximum. Carbonic acid gas contributes much to 

 this destruction ; for under its influence iron passes (to use the words of 

 Vicat) into the state of the carbonate of protoxide, which, absorbing fresh 

 doses of oxygen, transforms itself into a hydrated peroxide. It is 

 indeed generally considered that oxidation cannot take place to a 

 dangerous extent unless carbonic acid be present ; and it is precisely 

 for this reason that iron bedded in fresh masonry or concrete resists 

 the action of the air, because either the mortar absorbs the carbonic 

 gas, which has a greater affinity for lime than it has for iroB, or the 

 masonry is sufficiently dense to protect the iron from contact with the 

 atmosphere. The investigations of the Commission in 1850, respect- 

 ing the fall of the bridge over the Maine, at Augers (recorded in ' Les 

 Annales des Fonts et Chausse'es,' vol. xxix. seconde sene, page 394), 

 appear to prove that the preservative action of the lime depends upon 

 its being in immediate contact with the iron, and that if a space, how- 

 ever small, be left between the two substances moisture will insinuate 

 itae^f, and in course of time produce active oxidation. M. Vicat, in a 

 note inserted in the ' Annaleu des Fonts et Chaussdes ' for May and 

 June 1853, appears to doubt the correctness of this conclusion, but 

 even he admits that the preservative action of the lime depends upon 

 ito absorption of carbonic acid gas, and that directly its hardening 

 ceases, it loses ito power of resisting the action of the atmosphere on 

 the iron. 



M. Payeu found that the addition of very small quantities of the 

 sub-carbonate of potassa, or of sodium, to pure water, was sufficient to 

 render it innocuous either to cast or wrought iron, and the same 

 property existed in nearly all alkaline solutions ; whilst, on the con- 

 trary, the addition of a small quantity of the chloride of sodium 

 rendered the process of oxidation much more rapid than it usually is 

 in pure water. It appears that gray cast-iron is more susceptible of 

 destruction by oxidation than either wrought iron or white cast-iron ; 

 and that wrought iron resists the action of sea water more effectually 

 than cast. It is a peculiar fact connected with this subject, that iron, 

 exposed to frequent displacements, shocks, or vibrations, is less affected 

 by oxidation than when it remains constantly in one place, without 

 disturbance. Thus, anchors in constant use are less exposed to rust 

 than those preserved in the magazine ; the rails of the main lines of 

 railroads are less corroded than those in the sidings ; iron steamers rust 

 less when in active service than when in dock. But the positions in 

 which iron decomposes with the greatest rapidity are those where it is 

 fixed, and alternately exposed to the air and immersed in sea-water. 

 Ammoniacal and sulphuric acid gas exercise very serious effects upon 

 the durability of cast and wrought iron. It is important, therefore, to 

 prevent their use, cither in urinals, roofs over gasworks, or the engine 

 sheds of railway stations. There appears also to be some danger in 

 using iron hi contact with sulphate of lime, or in fact under any 

 circumstances where it is likely to take up sulphuric acid gas, for which 

 it has great affinity. 



The galvanisation of iron, which, as generally practised, consists in 

 forming a superficial coating upon the metal by immersing it in melted 

 zinc, appears to constitute an efficient protection, so long as the iron is 

 covered. The contact with the zinc brings the iron into an electro- 

 negative state, and it is known that so long as the latter prevails there 

 is little tendency on the part of the iron to combine with oxygen. 

 From a series of experiments made at Brest between the years 1842 

 and 1851 (see note by M. Dehargne, 'Annales des Fonts et Chausse'es,' 

 1851), it appears that the zincing process does not, in any sensible 

 degree, affect the tenacity or the ductility of the iron ; but it is important 

 to secure the protected metal from any shocks or friction likely to 

 remove the surface. 



Zinc, when exposed to the atmosphere in its ordinary state, becomes 

 rapidly covered by a whitish efflorescence, which adheres to the metal, 

 and forms, as it were, a species of varnish, capable of arresting any 

 further decay. This efflorescence is considered to be a carbonate of 

 zinc ; but if the atmosphere should contain any sulphuric or hydro- 

 chloric acid (as in London and in the immediate neighbourhood of the 

 sea), compounds are formed of a nature to compromise the solidity of 

 the metal. In the purer atmosphere of Paris, and other Continental 

 towns, where wood is the ordinary fuel, and to which the sea air does 

 not reach, zinc is employed successfully for roofing purposes ; in London, 

 and on the sea shore, its durability is very limited ; aud at all times its 

 fusibility, and even its combustibility must be a serious objection to 

 its use externally, especially for roofs. 



Copper resists the action of the atmosphere very successfully, and 

 the presence of some of the gases mentioned above does not seem to 

 affect it in any material degree. A film of oxide, or carbonate, of copper 

 is rapidly formed over the surface, and secures the metal from further 

 decay. It is found, however, that a mixed metal or bronze, made of 

 copper and zinc, resists the influence of the atmosphere and sea water 

 more successfully than pure copper alone. 



Lead undergoes little change upon exposure either to air or water, 

 especially when the latter contains small proportions of the salts of 

 lime. According to Brande, when lead is kept in distilled water to 

 which air has access, small crystalline scales of oxide of lead are formed, 

 a portion of which dissolves in the water, and is again slowly precipi- 

 tated in the form of a carbonate. Soft water also, or that without the 

 salts of lime, appears to be more likely to attack lead than that con- 

 taining lime. The use of lead for cisterns must, therefore, be regulated 

 by the nature of the water to be preserved in them ; for all roofing or 

 analogous purposes there do not appear to exist any philosophical 

 reasons to object to the use of this metal, or to limit its application to 

 any particular districts. 



A very important remark with respect to the use of metals must be 

 made, namely, that when two of them are used in contact, in positions 

 where moisture in any form has access to them, a species of galvanic 

 action is established, which causes them to decay with great rapidity. 

 Illustrations of this may be observed in iron railings, when the bars 

 are secured to the stone curb with lead, and the decay is most evident 

 when the iron is of the best and most malleable description. A similar 

 phenomenon may also be observed when copper or bronze is in contact 

 with iron in sea water ; though the iron decays rapidly, it appears to 

 exercise a protective influence upon the copper. 



The laws of electricity developed by the contact of two metals with 

 a liquid containing a solution of an alkaline salt, are treated at some 

 length in vol. 1, Gmelin's ' Handbook of Chemistry,' p. 364. From 

 this authority it appears that zinc, tin, and iron, protect copper in sea 

 water. Zinc protects iron aud tin plate, but it is not so effective for 

 the defence of iron in sea water if air be present, and it is itself rapidly 

 corroded when used with iron in the sea. In that element tinned iron 

 decays unequally, the iron oxidating whilst the tin remains intact ; and 



appears that the decay, superinduced by the contact with tin, is 



