PRESERVATION OF NAT. HIST. SPECIMENS. 503 



smallest pressure on ihc tin causes a responsive movement on the gaugt;. 

 A pressure of U-58'J inch of mercury is equal to about 8 inches of water. 

 This pressure applied to the tin may be beard as well as seen. The effect of 

 change of temperature of the air within is made visible by laying a piece of 

 cloth dipped in hot water upon the case. 



An immediate movement is visible on the gauge. These changes usually 

 take place slowly enough to modify their effect on closed cases, owing to 

 leakage, and thus, with every change of temperature or of pressure there is a 

 movement of air in or out of boxes and show cases. 



There is one wave of heat per twenty-four hours having its maximum at 

 •2-30 P. M. and its minimum at average sunrise. In the same period there art 

 two waves of barometric pressure both tending to affect the volume of air 

 surrounding a closed vessel, and these influences sometimes work together and 

 sometimes in opposite directions. If there is any communication at all 

 between the air in the case and that outside it, a breathing action takes place, 

 tending to mix and assimilate the two atmospheres. 



If the outer air be damp and the contents of the vessel are dry, the air 

 with each inflow will leave a little of its moisture on or in the contents, until 

 they will absorb no more, and if the air be drier than the contents, moisture 

 will be carried away as far as the absorbing capacity of the air will allow. A 

 varying humidity would have a corresponding effect. With dust, the effect is 

 not quite the same. The speed of current entering a small hole in a box is 

 quickly dissipated, and the dust it carries is dropped, but as the air only 

 regains its velocity again at the outlet it cannot gather up the dust, which 

 thus accumulates. We are now in a position to understand how articles that 

 have been placed clean and dry m a box or drawer may become damp, dusty 

 and mouldy, and our business is to find out how to prevent these unpleasant 

 manifestations. 



It is by no means easy to make a box or cupboard (unless it is very smallj 

 that is free from the risk of air leakage. Although wood does not alter 

 appreciably in length, it is constantly changing in width with the seasons, 

 influenced mainly by the amount of moisture in the air. 



Moisture will traverse a box of one inch teak during the monsoon, even if 

 sheltered in a house from the rain. This may be prevented to a considerable 

 degiee by varnishing, but comparative security against damp can only be 

 gained by means of a metal lining and a very well fitted door or lid. The box 

 then becomes a frame strong enough to give adequate support to the lining, 

 and thick enough at the edges to form a good joint with the lid. A well made 

 deal packing case may be made reasonably airtight at little cost. The lid is 

 nailed on, and the box put in a good repair ; the lid is then sawn off with about 

 three iaches of the box attached. Next, the mouth of the box is carefully 

 levelled with plane and straight edge all round, after which the lid is fitted to 

 it, the edge of the box being rubbed with chalk to indicate points of contact. 

 The two surfaces must touch all round. Linings of tinplate or zinc are put 



28 



