-1859] WRITINGS OP JOSEPH HENRY. 245 



of oily matter by washing it in ether, and the beard of the 

 wild oat ; the change in length of the first two and the twist- 

 ing of the latter furnish the indications required. 



Different materials absorb moisture in different degrees ; a 

 fact which is evident in passing along the sidewalk of a street 

 at the beginning of a rain. While some of the bricks of 

 which the pavement is composed are entirely wet at the sur- 

 face others appear dry, because the water which has fallen 

 upon them has been absorbed. It is scarcely necessary to 

 add that after perfect saturation has taken place, and the 

 surface is exposed to the heat of the sun, the appearance of 

 wetness is exhibited in a reverse order. The relative absorp- 

 tive power of different materials is frequently a matter of 

 considerable practical importance, which can be readily ascer- 

 tained by weighing equal bulks of the material previously 

 dried in an oven, and again after having been thoroughly 

 soaked under the pressure of several feet of water. The 

 absorption of water and its subsequent expansion by freezing 

 is the most efficient agency in the gradual destruction of the 

 architectural monuments by which the ancients sought to 

 impress upon the future a material evidence of their power 

 and wealth. 



Constitution of clouds. — Water in the state of vapor (as has 

 been stated,) is perfectly transparent, and this may be con- 

 clusively proved, even of steam at a high temperature, by 

 boiling water in a glass vessel with a long neck or by fasten- 

 ing a glass tube to the spout of a tea kettle. The vapor 

 within the glass will be entirely invisible, and that peculiar 

 condition called cloud will not be assumed till the transparent 

 steam mingles with the cooler atmosphere and is partially 

 condensed. The appearance of a cloud is also produced if 

 a portion of transparent air is suddenly cooled, either by ex- 

 pansion or mingling with a portion of air of a lower tem- 

 perature. Much speculation has arisen in regard to the 

 nature or condition of water when in the intermediate state 

 of cloud, and though the subject has occupied the attention 

 of scientists for more than a centur}'' it is still not fully settled. 



Saussure, the celebrated Swiss meteorologist, states that in 



