Woody fiber alone will not decay, but when associated with the sap, fermentation takes place in the latter (with such energy as may depend upon its constituent elements), which acts upon the woody fiber, and produces decay.  In order that this may take place, it is believed that there must be a concurrence of four separate conditions: 

1st.  The wood must contain the elements or germs of fermentation when exposed to air and water.

2d.  There must be water or moisture to promote the fermentation.

3d.  There must be air present to oxidize the resulting products.

4th.  The temperature must be approximately between 50 deg. and 100 deg.  F. Below 32 deg.  F. and above 150 deg.  F., no decay occurs.

When, therefore, wood is exposed to the weather (air, moisture, and ordinary temperatures), fermentation and decay will take place, unless the germs can be removed or rendered inoperative.

Experience has proved that the coagulation of the sap retards, but does not prevent, the decay of wood permanently.[1] It is therefore necessary to poison the germs of decay which may exist, or may subsequently enter the wood, or to prevent their intrusion, and this is the office performed by the various antiseptics.

[Footnote 1:  Angus Smith, 1869, “Disinfectants.”  S.B.  Boulton, 1884, Institution Civil Engineers, “On the Antiseptic Treatment of Timber.”]

We need not here discuss the mooted question between chemists, whether fermentation and decay result from slow combustion (eremacausis) or from the presence of living organisms (bacteria, etc.); but having in the preceding pages detailed the results of the application of various antiseptics, we may now indicate under what circumstances they can economically be applied.

(To be continued).

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THE SPAN OF CABIN JOHN BRIDGE.

To the Editor of the Scientific American Supplement:

Your issue of 17th October contains the fifth or sixth imprint of Mr. B. Baker’s, C.E., recent address at the British Association of Aberdeen which has come into my hands.

In speaking of stone bridges, he alludes to the bridge over the Adda as 500 years old.  It was never more than 39 years old as stated in the same address, and he belittles the American Cabin John Bridge by making its span "after all only 215 ft." As the builder of this greatest American stone arch, I regret that on so important and public an occasion the writer was not accurate.

The clear span of Cabin John Bridge is 220 ft.  The difference is not great, but in the length of a bridge span it is the last foot that counts, as in an international yacht race to be beaten by one minute is to fail to capture the cup.

M.C.  MEIGS.

Washington, D.C., Oct. 16, 1885.

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