166 WRITINGS OF JOSEPH HENRY. [1855- 



produce a marked effect upon the character and condition of 

 vegetation. Dr. Dalton found, at Manchester, one part of 

 salt in one thousand parts of rain water. Brandes found in 

 rain water, in Germany, besides common salt, chlorate of 

 magnesia, sulphate of magnesia, carbonate of magnesia, chlo- 

 rate of potassium, sulphate of lime, oxide of iron, oxide of 

 magnesia, and salts of ammonia, the greatest part of these 

 being ingredients of sea-water. This explains the fact that 

 certain plants do not grow luxuriantly near the ocean unless 

 screened by a fringe of trees or houses, or protected in some 

 other way. Near the ocean, a number of garden plants can- 

 not be made to grow unless placed near a fence which inter- 

 cepts the wind from the ocean. We might infer from this 

 that the saline matter is carried mechanically by the air, 

 and not diffused through it, as in the case of vapor. We are 

 informed by Mr. Browne that a gentleman at Nahant has 

 succeeded in raising pears to perfection by protecting the 

 trees on the ocean side by a high brick wall, perforated at 

 intervals with comparatively small openings, sufficient how- 

 ever to keep up the ventilation. 



Mineral matter in the atmosphere. — There is also constantly 

 diffused through the air a considerable quantity of mineral 

 substances, in a state of impalpable powder. This is carried 

 up by the ascending columns of air which are constantly 

 rising under the varying heat of the different portions of 

 the ground due to the influence of clouds and the various 

 conditions of the surface, and is brought down in the rain 

 which falls in the beginning of a shower. The presence of 

 this material at all times, is rendered evident when a ray of 

 liffht enters a small hole in the window shutter of a dark- 

 ened room. By some, it has even been conceived to 'be an 

 essential ingredient of the atmosphere. The amount of this 

 is much greater than we might be led by casual observation 

 to suppose. It falls upon the decks of vessels in mid-ocean, 

 and forms dry clouds, which were observed by Prof. Piazzi 

 Smyth, at the height of several thousand feet, upon the side 

 of the Peak of TenerifFe. 



