the Transpiration-Current . 415 
under examination is completely surrounded by the cooling 
liquid, which also flows round the bulb of the thermometer. 
The temperature is therefore very accurately known. The 
bottom of the cell is of glass ; a ring screwing out upon the 
top serves to permit the lifting of a cover-glass acting as 
a water-tight window, this being luted on the edge with 
a little white lead. The object is luted between two cover- 
glasses and carried upon an open support within. It is 
necessary to protect the upper window from moisture pre- 
cipitated from the atmosphere ; this is done by the loose 
metal ring surrounding the object-glass and packed round 
with a little cotton wool. The thermometer enters by a 
tubulure in front ; its bulb appears in cross-section at t in the 
Woodcut. The regulation of the temperature is very simply 
effected by retarding or accelerating the current of cold liquid 
(brine) by means of the pinch-cock. 
The section of Taxus is cut with as little addition of 
moisture as possible, so that when luted up between cover- 
glasses it is surrounded by air while containing water within 
its substance. The close proximity of the section to the 
upper window, some 1*5 millimeters, allows of considerable 
magnification. 
The cold cell, after the introduction into it of the section 
sealed up between the cover-glasses, is placed on the stage of 
a microscope, and then, by the arrangement already described, 
the temperature is caused to fall gradually, while the water 
within the section is carefully observed. 
The phenomena attending freezing were perfectly definite, 
G g 2 
