3bo PLEASANT WAYS IN SCIENCE. 



their hopes of success in the search for the philosopher's 

 stone, the elixir vita, and the other objects of their pursuit, 

 on occult influences supposed to be exercised by the celestial 

 bodies. It was unlikely, therefore, that they would willingly 

 reject the ancient theory which ascribed dew to lunar and 

 stellar radiations. 



But at length Baptista Porta adduced evidence which 

 justified him in denying positively that the moon or stars 

 exercise any influence on the formation of dew. He dis- 

 covered that dew is sometimes deposited on the inside of 

 glass panes ; and again, that a bell-glass placed over a plant 

 in cold weather is more copiously covered with dew within 

 than without ; nay, he observed that even some opaque 

 substances show dew on their under surface when none 

 appears on the upper. Yet, singularly enough, Baptista 

 Porta rejected that part of Aristotle's theory which was 

 alone correct He thought his observations justified him in 

 looking on dew as condensed not from vapour, as Aristotle 

 thought but from the air itself. 



But now a new theory of dew began to be supported. 

 We have seen that not only the believers in stellar influence, 

 but Aristotle also, looked on dew as falling from above. 

 Porta's experiments were opposed to this view. It seemed 

 rather as if dew rose from the earth. Observation also 

 showed that the amount of dew obtained at different heights 

 from the ground diminishes with the height. Hence, the 

 new theorists looked upon dew as an exhalation from the 

 ground and from plants a fine steam, as it were, rising 

 upwards, and settling principally on the under surfaces of 

 objects. 



But this view, like the others, was destined to be over- 

 thrown. Muschenbroek, when engaged in a series of ob- 

 servations intended to establish the new view, made a 

 discovery which has a very important bearing on the theory of 

 dew : he found that, instead of being deposited with toler- 

 able uniformity upon different substances, as falling rain is, 

 for instance, and as the rising rain imagined by the new 



