OFFICE FULFILLED BY DEW. 279 



helps to fertilize his neighbours' fields as well 

 as his own, by rising into the air and diffusing 

 therein. That is, where science has not come 

 to his aid, and put a stop to the appropria- 

 tion by a shower of dilute acid, or a sprink- 

 ling of powdered gypsum, by means of which 

 the evaporation of ammonia is prevented, in 

 consequence of its being compelled to assume 

 a new and less volatile form. A certain amount 

 of moisture is almost essential to the escape of 

 odour from many bodies. The cause of this 

 appears to be, that the vapour forms a sort of 

 vehicle for the escape of volatile organic matter ; 

 and also, that moisture favours the decomposition 

 of bodies, so that as they decompose the vapour 

 is given out. Much of that pleasure which we 

 ourselves derive from the perfume of plants, 

 depends on the assistance to its vaporization 

 rendered by dew. " Who," writes the late Pro- 

 fessor Fownes, " does not inhale with rapture 

 the perfumes of a flower-garden, when the dews 

 of night, or the refreshing summer shower, 

 have awakened the thousand sweet odours of 

 its fair inhabitants? The breath of the haw- 

 thorn and of the rose have been always one of 

 the most favourite themes of the poet's song; 

 and yet this endless succession of pure and 

 simple pleasures is but a mere consequence of 

 the law which bids a vapour, arising by its own 



