PICTURESQUE EXPERIiMENTS 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 ser^'es 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 



