144 TENTH ANNUAL REPORT OF 



As the water at tlie earth's surface is everywhere in contact with, 

 the air, it is always more or less perfectly saturated with it. Although 

 the oxygen is continually heing withdrawn by the respiration of 

 aquatic animals, and also by decaying animal and vegetable matters, 

 fresh portions of this element are as constantly taken in from the at- 

 mosphere. This process of absorption is materially facilitated by 

 agitation at the surface of contact between the two fluids. Hence the 

 winds play an imjiortant part in keeping the superficial portions of 

 the oceans, seas, and lakes charged with oxygen. 



Water is thus the great transferer of oxygen from the atmosphere 

 to the various organic and mineral substances entering into union 

 with it. The thinner the stratum of water interposed between the air 

 and the oxydizing body, the more rapidly is the transfer effected. 

 Hence metals with a mere film of water upon their surface, such as 

 they gatlier from a damp atmosphere, corrode much faster than when 

 deeply buried in that fluid. Metals with rough surfaces, also, when 

 exposed to a damp atmosphere, rust sooner than metals whose sur- 

 faces have been polished. These latter, on account of their feebler 

 attraction for moisture, do not so readily gather the requisite film 

 of water ; or if it be precipitated upon them, it quickly passes off 

 by evaporation, as is seen in the case of the highly polished knife 

 or razor. 



If a sheet of iron be placed in a damp atmosphere, or in water con- 

 taining air, the phenomena observed Avill be as follows : For a time the 

 metal will retain its brightness and apparently suffer no change. At 

 length, however, minute spots of rust make their appearance here and 

 there upon its surface. These, when they have once begun to form_, 

 rapidly enlarge and multiply until ere long tlie entire sheet is over- 

 spread by them. This more rapid oxydation is probably caused by 

 a change in the electric state of the metal. Little galvanic circles are 

 formed by the spots of rust on the iron, in consequence of which the 

 latter acquires an increased tendency to unite with oxygen. Whether 

 the incipient oxydation is due to a similar influence of the water upon 

 the iron, or whether it is owing to the oxygen being presented by the 

 water in a state more favorable to combination, or whether both of 

 these causes concur in determining it, may admit of question. 



Copper, lead, tin, and zinc, exposed to a moist atmosphere, or 

 placed in water holding air in solution, exliibit like phenomena. 

 Hence the corrosion of the copper sheathing of vessels by sea-water. 

 Hence, too, the frequent contamination of well and spring water by 

 the leaden pipes employed in conveying it. In both cases, the first 

 step in the series of transformations which occur, is the union of the 

 metal with the oxygen dissolved in the water. Silver and gold, in 

 similar circumstances, experience no change. Sulj^hur, and not oxy- 

 gen, is the agent by which they are tarnished. 



This rapid wasting of the metals, after oxydation has once com- 

 menced, finds an analogy in the moral world. The first spot of rust 

 is the first lapse from virtue, the first stain of vice. And as that spot 

 of rust, if not promptly removed, enlarges and spreads until it soon 

 covers the whole surface of the metal ; so that first act of vice, if not 

 speedily repented of, becomes a habit by repetition, which continues 



