The Rev. P. Keith on the Internal Structure of Plants. 181 



changes in temperature and pressure, and especially for mois- 

 ture*. For the latter object the volta-electrometer (fig. 11.) 

 is most accurate, as its gas can be measured over water, whilst 

 the others retain it over acid or saline solutions. 



738. I have not hesitated to apply the term degree, in ana- 

 logy with the use made of it with respect to another most im- 

 portant imponderable agent, namely, heat; and as the de- 

 finite expansion of air, water, mercury, &c, is there made 

 use of to measure heat, so the equally definite evolution of 

 gases is here turned to a similar use for electricity. 



739. The instrument offers the only actual measurer of 

 voltaic electricity which we at present possess. For without 

 being at all affected by variations in time or intensity, or alter- 

 ations in the current itself, of any kind, or from any cause, or 

 even of intermissions of action, it takes note with accuracy 

 of the quantity of electricity which has passed through it, and 

 reveals that quantity by inspection ; I have therefore named 

 it a Volta-electrometer. 



740. Another mode of measuring volta-electricity may be 

 adopted with advantage in many cases, dependent on the 

 quantities of metals or other substances evolved either as pri- 

 mary or as secondary results ; but I refrain from enlarging 

 on this use of the products, until the principles on which their 

 constancy depends have been fully established (79 1. 843.). 



741. By the aid of this instrument I have been able to 

 establish the definite character of electro-chemical action in 

 its most general sense ; and I am persuaded it will become of 

 the utmost use in the extensions of the science which these 

 views afford. I do not pretend to have made its detail per- 

 fect, but to have demonstrated the truth of the principle, and 

 the utility of the application. 



[To be continued.] 



XXVI. On the Internal Structure of Plants. By the Rev. 

 Patrick Keith, F.L.S. 



[Continued from p. 121.] 



Composite Organs. 



The Epidermis.— HPHE epidermis of the vegetable, a term 

 ■*■ borrowed from the anatomy of animals, 

 is the external envelope or integument of the plant, extending 

 over its whole surface, and covering the root, stem, branches, 

 leaves, flower, and fruit, with their appendages, the summit 



* For a simple table of correction for moisture, I Dray take the liberty 

 of referring to my Chemical Manipulation, edition of 1830, p. 376. 



