502 



THE PRESERVATION OF NATURAL HISTORY SPECIMENS 



IN AIRTIGHT CASES. 



BY 



John Wallace, C. E. 



The Preservation of Natural History Specimens, especially in the Tropics, 

 involves very often far more trouble than does their capture, for air, sunhght, 

 dust, fungoid gL'owths and various insects are grrayed constantly against 

 the collection. Moist air softens them and assists fungoid growths, dry air 

 shrivels and distorts them, sunlight causes their colours to fade, dust by 

 requiring cleansing involves wear that is ultimately destructive, and insects 

 effect the most rapid destruction of all by eating them. A cool, dry, atmos- 

 phere, uniform in temperature and humidity, and suitably protected against the 

 invasion of fungi and insects, offers ideal conditions for the dry preservation 

 of specimens, while immersion in some preservative liquid within an airtight 

 glass vessel has been the favourite means employed for creatures whose natural 

 appearance it is desired to preserve without stuffing. My remarks will be 

 confined to the first category which demands the protection of airtight boxes 

 or cases. In Bombay the temperature of the air ranges from 53*ii to 100*2 

 degrees Fahrenheit and varies from that cause alone by about 8i per cent, of 

 its original volume. The moisture ranges from 34 per cent, of relative 

 humidity to 98 per cent, and to actual saturation on the rare occasions of a fog, 

 that is to say, from 1*61 grains of water vapour per cubic foot at 34 per cent, 

 to 1 1*79 grains at 98 per cent. 



The normal pressure of the air on the objects about us is 14*7 lbs. per 

 square inch or 2,1 1 6 lbs. per square foot, a force that would crush many hollow 

 objects were it not balanced by a similar interior pressure. The pressure of 

 the atmosphere varies very slightly in Bombay, the range being 5*89 tenths of 

 an inch of mercury with corresponding changes of volume. 



When the barometer falls 0*589 inch at, say, 80 degrees Fahrenheit, the 

 pressure in pounds per square inch of the air will fall from 14*7 to 14"41 and 

 the pressure on the exterior surface of a completely airtight case would be 

 reduced by 41*76 pounds per square foot. A perfectly tight and empty 

 kerosene tin would, in such a circumstance swell visibly and, were it perfectly 

 elastic, it would increase in volume by 2-04 per cent. On the return of normal 

 pressure it would resume its original size and shape. 



A change in temperature has a similar effect on the volume of the au\ 

 Arise from 50 degrees to lOu degrees Fahrenheit would, in a rigid closed 

 vessel, cause an increase of pressure equal to 1*24 lbs. per square inch or 

 17S lbs. per square foot. The increase in volume, if expansion were allowed, 

 would be 9*1 per cent. 



Such a tin is on the table along with a delicate anemometer gauge which 

 will measure pressures from 12 inches of water down to ^^5^0 ^^ ^^ inch. 

 When the tin and the gauge are coupled by means of a rubber tube the 



