8 



BURTON EDWARD LIVINGSTON 



lies o\'er the vulcanite and is provided with a paper screen on its 

 upper surface. This screen is perforated with two circular win- 

 dows corresponding to the openings in the vulcanite below. On 

 the lower side of the glass plate, under one of the windows, is 

 gummed by its margin a piece of the cobalt chloride paper large 

 enough to cover completely one of the windows. This gives the 

 standard color of the hj^drated paper. The test paper is quickly 

 inserted over the remaining opening (under the second window) 

 the glass is returned to place, and observation is made of the time 

 required for the test paper to become of the same color as the 

 standard. Tests are more readily carried out with this apparatus 

 than with the vials. 



For applying the paper slips to the plant leaves, small spring 

 clips are used, formed of hard brass wire, to either arm of which 

 is cemented — by means of De Khotinsky cement — a small glass 

 plate. The glass plates used in the present work were cut cross- 

 wise from ordinary inicroscopic slides and are about 12 mm. 

 wide and 21.5 mm. long. About a third of its outer surface is 

 occupied, at one end of each plate, by the cement attachment of 

 the corresponding arm of the wire part. The two inner faces of 

 the plates lie in contact when the clip is closed, its position of 

 rest, and pressure upon the looped portion of the wire separates 

 the plates, keeping their surfaces nearly parallel. The general 

 form of the wire is not unlike that of an ordinar}^ pinch-cock for 

 rubber tubing, the wire loop being the handle in the present case. 

 The cobalt paper slips should be small enough so as not to reach 

 to the edges of the glass plates when held between them. 



It is expeditious, after drying a paper, to insert it between the 

 plates of a clip, and so bring it to the leaf to be tested. If the 

 leaf be held horizontally, the clip may now be opened so that the 

 paper rests on the lower plate, and the clip may then be moved 

 forward till the leaf lies over the paper and between the plates. 

 On closing the clip the paper is brought into the desired position 

 on the under side of the leaf. A very little practice leads to facil- 

 ity in these operations. Only a single paper should usuall}^ be 

 applied with one clip. To place a paper on the normally upper 

 side of a leaf, the latter may be turned till this surface comes to 



