THE PLANT WORLD 77 



has been devised by Professor H. M. Richards, of which he has very- 

 kindly handed me the brief description found below. 



For a suitable plant from which to collect oxygen, Cabomba answers 

 well, and may usually be purchased from dealers in gold fish, etc. If 

 this is not to be obtained, however, any submerged plant will serve with 

 varying degrees of efl&ciency, and Elodea has most frequently been used. 

 It is of distinct advantage to enrich the CO 2 "content of the water 

 by passing small bubbles through it from a generator, being careful, 

 however, not to collect them ! The gas from the plant may be collected 

 in the usual way, by means of an inverted filter, by displacement of water. 



''Operation I. — Collect gas sample from plant in a small (2 dram) vial. 



''Operation II. — Fill a glass tube closed at one end, say of 3-4 mm. 

 diameter, and 30 cm. long, with mercury, and invert over a dish of 

 mercury. 



Operation III. — Bring the mouth of the vial containing the gas sample 

 to the lower end of the tube, under mercury, and dip it so as to allow the 

 gas to enter the tube. Some of the gas will probably be lost, but this 

 will not affect the result. 



"Operation IV. — Remove the tube from the mercury bath, closing it 

 with the finger-tip and cautiously allow the mercury to escape while the 

 open end of the tube is plunged in a solution of Pyrogallol and KOH, 

 with the result that the mercury is now replaced by the testing solution. 

 If this tube is now allowed to stand, the level of the solution will 

 rise within the tube until all the oxygen has been absorbed, and in a 

 successful experiment this should be more than \ (approximately) of the 

 original volume of gas in the tube. If the end of the tube be tightly 

 closed with the finger-tip it may be shaken to hasten the absorption of 

 oxygen. 



This experiment may be done on a scale large enough for a whole 

 class to see it." 



It may be added that the method of determining the amount of oxygen 

 present by an absorbent has in my experience been found much more sat- 

 isfactory than that of attempting to increase combustion, as of glowing 

 charcoal. The greater weight of the mercury column introduces a slight 

 error, which however may be disregarded. 



REVIEWS OF EDUCATIONAL LITERATURE. 



Some interesting results have been obtained by Dr. D. T. MacDougal* 

 from the study of soil temperatures by means of a new type of soil ther- 

 mometer known as the " Hallock Thermograph." Essentially, this 

 instrument consists of a thermometer made of a large metal bulb and a 

 long fine-bore metal tube, the upper end of which is attached to metal 



• " Soil Temperatures and Vegetation," Mo. Wtalher Review, August, 1903 



