38 BULLETIN 155, U. S. DEPARTMENT OF AGRICULTURE. 



growth of fungi, though possibly certain forms of bacteria may assist 

 in the final destruction of the wood cells. The growth of fungi to 

 an extent detrimental to the life of the wood requires a favorable 

 combination of moisture, air, and heat. The exclusion of any one 

 of these beyond certain limits inhibits their growth. 



From this it follows that with pipes buried in the ground the wood 

 will endure longest where the air is most nearly excluded either by a 

 high internal pressure which completely saturates it or by a deep 

 covering of very fine soil. In accordance with the foregoing state- 

 ment, experience, which might be illustrated by many specific ex- 

 amples, shows that in contact with the soil wood pipe decays more 

 rapidly under a light head than it does under heavy pressure, and 

 other things being equal, it usually decays more rapidly in a porous 

 open soil, such as sand or gravel, than it does in a fine soil of silt or 

 clay, because the finer soil is more effective in excluding the air. 

 Experience appears to indicate also that wood decays more rapidly 

 in a loamy soil, rich in humus or partially decayed organic matter, 

 than it does in one containing little or none. This is probably due 

 to the f aet that the presence of organic matter affords more favorable 

 conditions for the development oi fungus growths and bacteria. 



Pipes fully exposed to the atmosphere and free from contact with 

 the soil will, as a rule, be too dry on the exterior to favor the develop- 

 ment of fungus spores, and so long as the outside of a pipe remains 

 dry no appreciable deca} r will occur, even though the internal pres- 

 sure is very light. Decay of exposed pipes almost invariably starts 

 at the ends of staves, as a result of leaky joints. Where water leaks 

 out and runs down over the outside of the pipe favorable conditions 

 are afforded for the growth of algae, which usually get a start, then 

 mosses may begin to grow in the soil that collects on such spots, and 

 decay spreads to adjoining staves. Bruising the staves in handling 

 or injuring by too tight cinching of bands renders them more suscep- 

 tible to infection by the spores of wood-destroying fungi, thus has- 

 tening decay. The life of exposed pipes may be prolonged by 

 promptly stopping all leaks as they develop and by keeping the ex- 

 terior dry. The decay of buried pipes has also in some instances been 

 arrested by removing the covering and leaving them exposed. 



The asphaltum or tar coating applied to machine-banded pipe, 

 while intended primarily as a protection against corrosion of the 

 bands, doubtless helps also to some extent in preserving the wood. 

 Until recently the practice has been to leave the ends of wooden 

 sleeve couplings untreated. These couplings almost invariably decay 

 long before the main pipe. This may indicate that infection by 

 wood-destroying organisms starts principally where the coating is 

 absent, though less perfect saturation of the wood in the sleeves may 

 be the more largely responsible for the early decay, as it may be noted 



