124 THK NA I' TILTS. 



4. Loose shells are also destroyed, but there are comparatively few 

 of these compared with the number of those mounted on cards. 



.X The shells affected are from twenty to fifty years old, but the 

 corrosion does not appear until after the lapse ot about ten years. 

 The process is thus an extremely slow one. 



('). It occurs principally amongst the shells kept in drawers in the 

 dark, where the air is confined and seldom changed. 



7. If the tongue be placed against one of the shells, an astringent 

 alum-like taste will be observed. 



We now come to the consideration of possible causes of corrosion. 

 They appear to me to be four in number. 



I. Damp It the shells were placed in a room or gallery that was 

 not properly warmed, a very probable reason would be that a fungus 

 had been formed, eating away the surface of the shell. The shell 

 gallery of the Natural History Museum is, I am assured, excessively 

 dry. This cause is, therefore, excluded. 



II. The action of sulphuric acid It is well known that the atmos- 

 phere of cities contains free sulphuric acid (in addition to other 

 sulphur compounds) derived from household fires and the burning of 

 coal-gas. This acting over a peri xl of years would slowly eat away 

 the calcium carbonate of the shell, forming calcium sulphate. If this 

 were the explanation, then the whole of the white powdery substance 

 on the surface of the shell would consist of calcium sulphate. Analy- 

 sis, however, showed that none was present. 



III. Presence of salt. Shells that had not been soaked in fresh 

 water before being placed in the cabinets would contain salt, not 

 only in the epidermis, but also held mechanically amongst the parti- 

 cles of calcium carbonate. It is highly probable that its presence 

 would exert a deleterious influence. 



Chemical tests showed that it was practically absent, in the shells 

 examined, a fact that occasioned some surprise. 



IV. Act ion of an acid snhs/nt/re. After carefully considering all 

 the facts in my possession. 1 have come to the conclusion that the 

 corrosion is due to the action of butyric acid. 



Upon opening the box of shells sent me by Mr. E. A. Smith, I at 

 once noticed a pungent vinegar-like odor, which pervaded the lin- 

 gers and everything that came in contact with them. This pointed to 

 the pre.-ence of acids of the acetic series. Analysis showed that every 

 shell contained butyric acid, as calcium butyrate. A few contained 



