440 P. G. Leavitt — Psychrometer applicable 



Art. JAY. — A Psychrometer applicable to the Study of 

 Transpiration ; by Robert G. Leavitt. 



The rate at which the vapor of water is exhaled from plants 

 has been quantitatively examined by three principal methods, 

 viz.: (1) by noting the weight of water taken up by the plant, 

 (2) by weighing the growing plant, with pot or jar, before and 

 after the loss of the vapor in question, and (3) by collecting 

 and weighing the escaped vapor itself. The first mode was 

 employed by Woodward in 1699. The second method has 

 been in general use since Hales's early experiments (1724). 

 The collection of the transpired water has been accomplished 

 either by means of an hygroscopic substance (Garreau, 1849), 

 or by condensing it on the walls of a receptacle (Deherain, 

 1873). 



There remains the possibility of testing the air brought 

 from a transpiring part for the amount of vapor present, with- 

 out actually collecting the vapor. If the dew-point could 

 always be speedily and accurately ascertained, it was thought 

 that this would be the foundation of a method having some 

 advantages; as, for example, in case it were desired to detect 

 changes in short intervals of time. 



The dry hygrometers in use, such as Dines's and Alluard's, 

 require a steady fall of temperature until the dew is perceived 

 upon the collecting surface. This changing of temperature 

 creates a considerable uncertainty as to the actual point at 

 which dew begins to form. The difficulty may be overcome 

 by employing several hygrometric surfaces, at adjustable fixed 

 temperatures, side by side. In the instrument to be described 

 there are four nickel-plated tubes passing vertically through a 

 glass chamber. Each conducts a stream of water and has a 

 thermometer within it. The water is derived from two 

 sources, warm and cold, in order that the temperature may be 

 regulated at will. The water-tanks are of such capacity and 

 the flow is so even, that the mixed streams maintain sensibly 

 constant temperatures for many minutes together. Above the 

 chamber the hygrometric tubes are of glass, allowing the ther- 

 mometers to be read. 



Through the chamber a steady current of air is drawn, 

 bringing the transpired vapor from the receptacle holding the 

 plant. The draft having been so suited to the activity of 

 the plant as to give a dew-point within the range of tempera- 

 tures at command, the tubes are set, let us say, at 6°, 10°, 14°, 

 and 18°. If No. 1 alone shows dew the fact is noted, the 

 warm water turned on to throw off the gathered moisture, and 



