284 



EXPERIMENTS WITB PLANTS 



into the aquarium, while the water falls straight down 

 into a receptacle below. The water outl-et tube must 

 dip a little deeper under water than the air outlet 

 tube ; otherwise the air, following the path of least 

 resistance, will escape by the same outlet as the water. 

 In order to keep the latter submerged to a constant 

 depth, use a small jar as shown in the figure : the 

 shallower the depth the greater the amount of air which 

 will pass over, since enough drops must accumulate 

 in the tube to overcome the pressure of the water at 

 the outlet where it escapes. The drops should be made 

 as large as possible and should strike the tube nearly 

 in the center ; if the bore of the tube is too large the 

 drop will not fill the cross -section and consequently 

 will not act as a piston. In order to regulate the rate 



of flow of the siphon, it may be 

 drawn out to a fine point (as 

 shown in the figure) , which can 

 be broken off to the requisite 

 degree and covered with cotton 

 (held in place by an elastic 

 growing in water, baud) to filter the water. 



A very successful way of growing algse and other 

 water-plants is shown in Fig. 159. A jar or bottle is 

 filled with water and inverted over water : carbon 

 dioxide is conducted into it from a bottle containing 

 fragments of marble (or marble dust or whiting) , into 

 which we slowly pour weak sulphuric acid by means of 



l.'i!). Arrangement for supplyins 

 ■lion dioxide to plants 



