60 Mr John Adie on Dew-poini Instruments. 



engaged, finds itself in an opposing electric condition. This 

 reciprocity shows in what the nascent state differs from the de- 

 finitive state of a body. 



2. It follows that the action of vegetables upon the oxygen of 

 the air is one of the most permanent and powerful causes of at- 

 mospheric electricity ; and if we consider, on the one hand, that 

 a gramme of pure charcoal passing into a state of carbonic 

 acid disengages electricity sufficient to charge a Leyden phial ; 

 and, on the other hand, that the charcoal which is engaged in 

 the constitution of vegetables does not give less electricity than 

 the charcoal which burns freely, we may conclude, as my ex- 

 periments all tend to establish, that upon a surface of vegeta- 

 tions of 100 metres square, there is produced in one day more 

 vitreous electricity than is wanted to charge the strongest 

 electrical battery. 



This origin of the electricity of the atmosphere being once 

 demonstrated by rigorous experiments, it remains to be seen 

 what becomes of it, — by what laws and what properties it is 

 propagated in the air, — disperses, ascends, and accumulates 

 in the highest regions of the atmosphere. I have already col- 

 lected some fundamental data upon this subject, and I hope that 

 my other occupations will allow me to prosecute the inquiry. 



Art. VIII. — Comparative experiments on different Deiv-point 

 Instruments ; with a description of one on an improved con- 

 struction.* By Mr John Adie. Communicated by the 

 Author. 



Having had occasion to make use of dew-point instruments 

 in some late experiments with the barometer for measuring 

 heights, and having observed in the public journals various 

 objections to different constructions of that instrument now in 

 use, my attention was called to these defects ; and the results 

 which I obtained during this examination will form the sub- 

 stance of the following paper. 



Mr Daniel objects to the simple instrument constructed by 

 Mr Jones of London, and at the same time by Dr Coldstream 

 of Leith, because a part only of the oblong bulb is exposed to 

 a depression of temperature produced by the evaporation of 



• Read before the Royal Society of Edinburgh, Feb. 2, 1829. 



