PICTURESQUE EXPERIMENTS 217 



mann, which I was never tired of seeing year after 

 year in my Cambridge class. Defibrinated bullock's 

 blood is freed from air by means of an air pump 

 and charged with CO,. In the course of this 

 process it acquires the dingy tint of venous blood. 

 A single leaf of the American weed (Elodea) is 

 mounted on a glass slide in a drop of this blood and 

 covered by an ordinary cover slip. Then comes 

 the dramatic moment : the preparation is exposed 

 to sunshine, and in 3 or 4 minutes a delicate scarlet 

 border begins to appear round the leaf and grows 

 rapidly, making a curious sunset effect in contrast 

 with dingy purple of - the venous blood. The 

 meaning is very clear ; the Elodea leaf in sunshine 

 took the carbon from the CO^, and the oxygen 

 thus set free gave the venous blood the scarlet hue 

 characteristic of the arterial condition. Professor 

 Farmer has designed a striking method based on 

 another well-known experiment of Engelmann's. 

 A drop of water containing the products of decay, 

 and therefore swarming with bacteria, supplies 

 the test. A drop of this fluid is placed on a glass 

 slip, one or two delicate leaves of a green water 

 plant (Elodea) are added, and a square of thin glass 

 is placed on it. Round the edges of the cover-slip 

 the preparation must be sealed with a preparation 

 of wax, which melts at a low temperature, and 

 when cold serves to prevent the preparation drying ; 

 it also isolates it from the surrounding atmosphere. 

 After making sure under the microscope that the 

 bacteria are in active movement, the glass slip is 

 placed in the dark for some 3 or 4 hours. It is then 

 examined, and the bacteria will be found to have 



