Hygrometric Properties of Insoluble Compounds, 93 



siderable importance, and likely to offer many new and useful 

 results, a more extended series of experiments were undertaken, 

 with the view of determining the actual increase, sustained by 

 given weights of various insoluble bodies, on exposure to a very 

 damp atmosphere. 



Portions of the various oxides, chlorides, and salts of earths and 

 metals, were reduced to fine powder, and dried on a sand bath, 

 till they ceased to deposit moisture upon a cold glass plate, held 

 over their surfaces; 100 grains of each were then accurately 

 weighed, and placed in small glazed earthenware pans, about three 

 inches in diameter, and a quarter of an inch deep; these were 

 all respectively munbered, and a list of their contents provided. 

 The substances being thus far arranged, were placed in parallel 

 rows at the bottom of a shallow box, which was now taken into a 

 small out-building, about six feet square, the atmosphere of 

 which was saturated with moisture. This was completely eflfected 

 by covering one of its sides with flannel, kept constantly wetted 

 from a vessel of water placed near the roof of the building. 



The cover of the box was slightly raised, so as to allow free 

 access of air, and at the same time exclude any particles of dirt 

 or dust that might otherwise have fallen on its contents. 



Upon making an experiment with an hygrometer in this atmo- 

 sphere, the temperature (45°) closely coincided with the dew 

 point ; thus proving its saturation with aqueous vapour. Under 

 these circumstances the various substances remained for a month, 

 during the whole of which time the atmosphere was kept satu- 

 rated ; at the expiration of this period the temperature of the air 

 and dew point being 37°, they were again all accurately balanced, 

 and their increase of weight carefully noted. These results, 

 together with the names of the substances employed, are given 

 in the first of the annexed Tables 



Spongiform silver, powdered bismuth and antimony, of each 

 50 grains, dried, and exposed with the above substances, acquired 

 no increase in weight. A similar quantity of spongy platinum 

 gained however . 1 of a grain. 



It is well known that newly-made charcoal has the property of 



