64 



make tlu- .yla.S!^ tit air tiglit to the metal. The gas is now pa8se<l into tlie chaniher 

 from a generator or reservoir through one tube, escaping through the i>ther. The 

 apparatus has been in use some time in European lalxiratories. 



4. Thk Olass Si-idk with I)1NDIX(; Posts, lo he used when it is desired 

 fo pass an electrie eurrent llirough a microscopit' ohjeet. eonsistsof the usual form 



ol mierosco{)ie slide, witli a small luass 

 binding post at either end. conneeled 

 with a pair of clips. Po jiut into use, 

 Hf-MiTc 4. {^^,,, small wedge-shaped pieces of tin 



foil are placed under the clips, so that the points nearly touch. The object is 

 then mounted between them and covered with a cover glass in the usual manner. 

 ■'>. Thk Awn HY(;Roi«KrEK is used to indicate the loss of moisture from a 

 leaf surface. It consists of a thin glass chamber, across the mouth of wliich 

 extends an adjustable metal rod. An awn of stipa is supported from the middle 

 of the rod by ;i set screw, and from the other end of the awn an index projects 

 3= 



Figure o. iJarwin Awn Hygrometer, 

 at right angles. .\s the air of tlie chamber becomes (^ 

 moister the awn untwists, and the index is carried 

 around. I'he most satisfactory way of using this 

 instrument is to fasten a pair of hygrometers of ecjiial 

 sensitiveness to a leaf, one on either side, by means 

 of a mixture of wax and oil. The leaf is eitlier 

 left attached to the plant or dipped into water to 

 prevent wilting, as shown in figure oa. The c(mi- 

 parative rate of transpiration from the upper and lower surfaces of a le:if is iliu> 

 obtained. 



This instrument in the crude form was seen by the writer when vi-itiiig the 

 laboratory of Mr. Francis Darwin at Cambridge in 188S, and has bee ii in n>e in 

 the physiological laboratory of Purdue University since that time. I be present 

 form is the result of this experience. 



Kiffure oa. Hygronirter 

 in Use. 



