6 



BURTON EDWARD LIVINGSTON 



It is also desirable that it be opaque, so as not to allow trans- 

 mitted light from the leaf to confuse the color judgment of the 

 observer. No doubt the quality of the paper most suitable will 

 vary with the properties of the leaves to be studied ; as will shortly 

 appear, it is not essential always to make use of the same paper. 

 A thin rough-surfaced writing-paper proved satisfactory, but the 

 best success was obtained with the "tissue towel" paper, now on 

 the American market. It somewhat resembles soft filter paper, 

 but is much thinner. It is very uniform in quaUty and thickness, 

 is very bibulous and sufficiently opaque. 



The paper is saturated with a 3% solution of cobalt chloride 

 and suspended by one edge to dry. The most uniformly colored 

 portions of the sheets are then selected, cut into small squares 

 (about 7 mm. on a side) and these preserved in a suitable stop- 

 pered bottle. Each paper slip is dried just before use, by passing 

 it several times over a smaU flame, as of a candle, the sUp being 

 held in a pair of forceps and the position of the forceps being 

 changed during the operation, so as to give a uniform blue color 

 throughout. An acetylene lamp such as is in use for bicyles 

 has proved convenient for drying the papers, as well as for night 

 observations. 



The standard of water supplying power first adopted was that 

 of the air at a distance of approximately one millimeter from a 

 free surface of pure water. A small glass vial, with the top 

 ground plane, is filled with water and the excess is carefully 

 removed with absorbent paper, till the margin of the meniscus 

 is definitely below the free edge of the glass. A square of dry 

 cobalt paper is laid over the mouth of the vial, thus coming to 

 lie within about a millimeter of the liquid water, and a plain 

 glass plate is quickly superimposed. Observation is made of the 

 time required for the complete disappearance of blue color. A 

 second vial like the first, over which a square of paper and its 

 glass plate are left continuously, furnishes the eye with a color 

 standard for comparison. The test is repeated, with the same 

 and with other paper slips, and a uniform lot are thus selected. 

 It is found that there is but little variation in the rapidity of 

 response of the various slips at the same temperature. The 



