366 Mr. John Aitken on Dew. 



Near the end of the second part of his ' Essay ' he refers to 

 the dew on plants, but it is simply for the purpose of refuting 

 the opinion that " the dew found on growing vegetables is the 

 condensed vapour of the very plants on which it appears." 

 He says : — " This seems to be erroneous, for several reasons. 

 (1) Dew forms as copiously upon dead as upon living vege- 

 table substances. (2) The transpired humour of plants will 

 be carried away by the air which passes over them when they 

 are not sufficiently cold to condense the watery vapour con- 

 tained in it." I draw attention to these passages to 



show that the distinction between true dew and the dew-drop 

 was not recognized at the time Wells wrote his ' Essay/ All 

 his arguments are directed against the opinion that the dew 

 formed over the whole surface of the blades of plants was 

 produced by the condensation of the vapour transpired by the 

 plants themselves, which he pointed out would be a small 

 quantity at night on account of the absence of light. He 

 makes no reference, so far as I am aware, to the dew-drop, 

 which my investigations tend to show is the result of exuded 

 liquid, and not of the transpired vapour to which Wells di- 

 rected his criticisms; so that my observations are in a different 

 field from those of Dr. Wells. 



But even supposing my conclusions regarding the source 

 of the dew-drop to be correct and to be in opposition to recog- 

 nized authority, still, if we wish to be accurate, we shall not 

 be entitled to call it a new theory of dew, as it is a theory of 

 the dew-drop as distinct from dew. 



If Mr. Tomlinson will change his style of criticism, and 

 will explain to us whence came the drops which formed on 

 the plants experimented on when they were isolated in dry 

 air, and all supply of moisture cut off except that which came 

 up through the tissues of the plants ; and if he will show us 

 that we have misinterpreted the teaching of the experiments 

 made by weighing turfs and others, which we have adduced 

 to show that vapour is given off from the ground while dew 

 is forming at night, he will be entitled to be listened to ; but 

 purely literary criticism is a mere jangling of words, and 

 seldom leads to satisfactory conclusions. 



I fear Mr. Tomlinson has misunderstood the bearing of my 

 remarks with regard to what takes place in Persia and the 

 African Desert. The impression I wished to convey was, that 

 we cannot conclude from experiments made in this climate as 

 to what takes place in arid regions. I am happy to say I can 

 assure my critic that " the great forces of Nature rule as im- 

 partially in Persia and in Africa as in Scotland;" is it 

 not for this very reason that we cannot conclude from what 



