Additional Notes. 523 



«3ay electricity of days equally dry is always proportional to 

 the heat; that winds lessen the electricity of a clear day, 

 especially if damp ; and that, for the most part, when there 

 is a clear sky, and little wind, a considerable quantity of 

 electricity arises after sun-set, at dew falling. Considerable 

 light has been thrown on the sources of atmospherical elec- 

 tricity, by the experiments of M. Saussure, and other mine- 

 ralogists. Air is not only electrified by friction, like other 

 electric bodies, but the state of its electricity is changed by 

 various chemical operations which often go on in the at- 

 mosphere. Evaporation seems, in all cases, to convey elec- 

 tric matter into the atmosphere; and Saussure has ascer- 

 tained that the quantity of electricity is much increased when 

 water is decomposed, as wdien water is dropped on red-hot 

 iron. On the other hand, when steam is condensed into ve- 

 sicular vapour, or into water, the air becomes negatively 

 electric. Mr. Canton has ascertained that dry air, when 

 heated, becomes negatively electric, and posiuve when cooled, 

 even when it is not permitted to expand or contract; a'Hd 

 the contraction and expansion of air also occasion changes ia 

 its electric state. It is discovered, therefore, by these expe- 

 riments, that there are four sources of atmospheric electricity 

 known; viz. 1. Friction; 2. Evaporation; 3. Heat and Cold; 

 4. Expansion and Contraction ; not to mention the Electricity 

 evolved by the melting, freezing, solution, &c. of various 

 bodies in the contact of air. — -Thomson'^ C/iemistn/. 



Hydrology. 

 Common Waters, p. 199. 



The comparative qualities of common waters, whether 

 falling in rain, or found in springs, wells, or lakes, have 

 been observed and ascertained, during the eighteenth cen- 

 tury, with a degree of intelligence and accuracy never before 

 known. For the experiments and inquiries which have led 

 to our knowledge on this subject, we are chiefly indebted to 

 Bergman, Scheele, Carradori, Hassenfratz, and 



GUYTON-MORVEAU. 



Sea Water, p. 199. 



The taste, specific gravity, and other properties of sea wa- 

 ter, have also been examined with new accuracy, and with 

 new results, during this periods For many enlighteaed ex- 



