176 JOURNAL OF CONCHOLOGY, VOL. 9, NO. 6, APRIL, 1899. 



The fact that the shells exposed to daily public inspection in the 

 top cases are less attacked is explained on the hypothesis that the 

 light acts as a deterrent. 



In one case of three shells mounted on the same tablet, only the 

 middle one is corroded. It surely will be granted that from some 

 yet unexplained cause some shells will be more liable to attack 

 than others. 



The question of the dryness or moisture of the room or gallery 

 does not arise — for, according to the equation expressing the reaction, 

 for every hundred grammes of calcium carbonate acted upon, eighteen 

 grammes of water are set free. Moisture is thus continually supplied. 



I have also come to the conclusion that the gum used in attaching 

 the shells to the tablets has something to do with the corrosion. This 

 is supported by the following arguments : — 



1. — The majority of the shells affected in the National Collection 



are gummed to tablets. 

 2. — As far as can be ascertained, the corrosion has never occurred 

 in private collections where the shells are, and always have 

 been, loose. 

 3. — It has not occurred, so far as I am aware, in the extensive 

 series of mollusca in the Manchester Museum. The speci- 

 mens of the Swainson Collection have been for more than 

 forty years stored there, and those of the Cholmondeley and 

 Walton Collections not so long. The Swainson shells were 

 formerly affixed to glass tablets, cemented with Canada Bal- 

 sam, a cement that does not ferment, although its effects in 

 other ways are prejudicial to the specimen, breaking away the 

 epidermis, and leaving an indelible mark at the point of attach- 

 ment. All shells in the above museum are, however, now 

 loose, and placed on cotton wool in cardboard boxes with 

 glass lids. A far different state of affairs exists, unfortunately, 

 in another museum with which I am acquainted. The disease 

 we are now striving to combat has existed there, unchecked 

 and increasing, for many years, and the results are more than 

 deplorable. 

 The gum ferments, acetic acid being formed. This eats away the 

 calcium carbonate, forming calcium acetate ; this latter substance 

 was found in several of the shells examined, in addition to the cal- 

 cium butyrate. Many recipes for gum contain acetic acid, which is 

 added as a preservative. Probably some was used forty or fifty years 

 ago at the British Museum, although I am assured that none has been 

 used of late. 



