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42.
NOTES ON THE SAFEGUARDING AND CONSERVING CULTURAL MATERIAL IN THE FIELD

them near objects.
2. [[underlined]]Extremes of dryness or dampness[[/underlined]] in the air are both dangerous, particularly for organic substances such as wood, paper and textiles [[footnote:]](1).
3. [[underlined]]Changes in relative humidity[[/underlined]]are more dangerous still. (For definition of relative humidity, see Section IV.1.iii above.)
(i) These changes are in part influenced by changes in temperature.  This makes control of temperature doubly important.
(ii) Various substances, e.g. mixtures of salt hydrates, can be placed about a room, which will absorb moisture from the air, and yield it up again when the relative humidity falls below a certain point.  One of the simplest means to this end is to hang up considerable quantities of any absorbent cloth, e.g., blankets.  But it should be remembered that the efficiency of such devices depends on a fairly rapid oscillation of the relative humidity. A long spell of humidity will saturate
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[[footnote]](1) Experiments in connection with the storage of the paintings belonging to the National Gallery, London, established that in air at c.47 F., and approximately saturated, mold was observed well within a month, and in one case in eighteen days.