382 M. Diitroehet en Ike Irritability of' the Sensitive Plant. 



iXHindiog temperature. Thus, at a temperature of from 20 to 

 ^ degrees of Reauoiur, it requires only four days of darkness 

 to destroy completely the irritability of a sensitive plant ; while 

 fifteen days of darkness are required to produce the same effect 

 when the surrounding temperature is within the limits of 10 and 

 15 degrees ; so that, on only taking the degrees of temperature 

 in which tlie sensitive plant can Hve, it may be established that 

 the extinction of the irritability of that plant in darkness is ope- 

 rated in &. period, the duration of which is in the inverse ratio of 

 the elevation of the temperature. 



M. Dutrochet has observed, that the sensitive plant, deprived 

 of its irritability by means of darkness, recovers it by exposure 

 to light ; and that this restoring of the conditions of irritabihty 

 is more rapidly effected, by exposing the plant to the direct 

 ligfat of the sun, than by exposing it merely to the light of day, 

 such as it exists in the shade. From these observations, M. 

 Dutrochet considers light as the external agent from the in- 

 fluence of which vegetables draw the renewal of the conditions 

 of their irritability, or, more generally, of their motility^ — con- 

 ditions which are subject to deperdition in the natural state, and 

 which thus require to be continuaJly repaired. 



Description of an Improved Air-Pump. By Mr John Dunn, 

 Optician, Edinburgh *. With a Plate. 



JLn the course of my business, having often heard it regretted 

 that the cost of apparatus prevented many gentlemen from en- 

 gaging in philosophical pursuits, I have made it my study to 

 simplify the construction of those which I have been employed 

 to make, wlierever this could be done without impairing the 

 accuracy of their performance. 



One of my first efforts was directed to that most valuable in- 

 strument the air-pump, which I shall endeavour to shew I have 

 improved &o very materially, as to be able to furnish one capa- 

 ble of effecting as complete an exhaustion as the most perfect 

 form of the instrument hitberto devised, and, at the same time, 

 nearly as simple and as cheap as its most imperfect form. 1 



" Read befpre the Society for the Encouragement of the Useful Arts, 

 19th December 1827- 



