142 MISCELLANEOUS NOTICES. 



causes may operate on the lime salts of the vice includ- 

 ed with the breeding pearls, and that so the air may 

 become charged with an infinitesimal quantity of lime dust. 

 During the day the temperature of the air is higher than at 

 night, the range being greatest in tropical latitudes. As before 

 remarked, the quantity of aqueous vapour capable of being 

 held in suspension by the atmosphere varies with the temper- 

 ature. At 32 decrees Fahrenheit it is about 2 grains to the cubic 

 foot ; at 77 degrees Fahrenheit, 10 grains ; at 100 degrees 

 Fahrenheit it is about 20 grains* Should the temperature 

 during the night fall below the point of saturation for the 

 vapour contained in the air, the latter is condensed into 

 liquid globules, and dew begins to fall, carrying with it in its 

 descent the floating dust particles. In such a case, within the 

 box conta : uing the pearl there will fall a fine moisture and 

 lime dust, and the pearl will receive its share, becoming 

 coated with a delicate film. Sometimes no such deposit will 

 take place, and sometimes it will be more appreciable than at 

 others, according to the amount of vapour with which the air 

 is charged and the variation in the temperature. During the 

 night the moisture, together with the carbonic acid of the air, 

 will act on the lime particles, dissolving them. Those of the 

 carbonate of lime will enter into solution as the bicarbonate, 

 in exactly the same way as water passing over a calcareous 

 soil acquires the property known as hardness. The phosphate 

 will he partially decomposed by the carbonic acid, and also 

 become dissolved as the bicarbonate. Hence ultimately the 

 pearl is covered with an exceedingly weak solution of the 

 bicarbonate of lime. Next day, with returning heat, the 

 moisture evaporates, the carbonic acid is given off, and car- 

 bonate of lime is precipitated in a manner exactly 

 analogous to the way- in which stalactites are formed, except, 

 that in the latter the deposit is amorphous, while on the pearl 

 the molecules are induced by the pre-existing crystals to 

 assume a definate polar arrangement which results in crystalli- 

 sation. The effect of all this would be that a uniform or nearly 

 uniform deposit would take place over the whole of the pearl. 

 But suppose that on its surface there should happen to be a 

 slight irregularity, such as might be caused b} r the projecting 

 angle of any crystal, the moisture, according to the laws of the 

 surface-tension of a fluid, will run together, and cling around 

 the prominence. (This is simply illustrated by spilling a little 

 water on a plate, and introducing a pellet into its midst, when 

 the water will be seen to be heaped up round the pellet. 



