Re-coat over the iodine full one fourth as long as first coating." This exceedingly volatile compound is difficult to control from its instability; it is but little used.

The impressions successfully produced by this mixture are very brilliant, and possess a pleasing peculiarity.
DRY SENSITIVES.
Hydrate of Lime .-- The operation by which water is combined with lime is called slaking.

Take a piece of quick lime, common lime used in mortar, and immerse it in warm water for about fifteen seconds; then place it in an iron or tin vessel.

It will soon begin to swell, evolving a great deal of heat and emitting steam, and soon falls into a fine powder, hydrate of lime.

This should be well stirred and allowed to cool, and then bottled in order to prevent it from giving off the hydrate and recovering the carbonic acid from the atmosphere.