THE SEED AND GERMINATION 29 
vinegar on washing-soda in a test tube. Effervescence 
takes place owing to the bubbles of carbon-dioxide 
given off. 
Carbon-dioxide is a very heavy gas, and can be 
readily poured from one vessel into another. Before 
pouring the vinegar on the soda have ready another 
test-tube containing about an inch of lime-water. 
Without allowing any of the liquid to pass over the 
lip, pour the gas from the first test tube into the 
second. On shaking the tube the lime-water becomes 
milky, and this milkiness is the recognised sign that 
carbon-dioxide is present. Now blow through a glass 
tube into clear lime-water, which, by becoming milky 
shows that the breath is heavily charged with carbon- 
dioxide. Next pour some lime-water into a shallow 
dish and expose it to the air. A milky seum soon 
forms on the top, showing that the air contains a 
certain amount of the same gas. 
Now pour some lime-water into a jar of air, or 
better still, of oxygen. The liquid does not become 
milky, for in the small quantity of air enclosed in the jar 
there is not enough earbon-dioxide to produce an 
appreciable result. Now lower into the jar a burning 
candle, and withdraw it, when, having used up almost 
all the oxygen, it is about to go out. Then shake up 
the lime-water: it will become milky, showing that 
the carbon of the candle has combined with the oxygen 
to form ecarbon-dioxide. In the same way the carbon- 
dioxide of the breath is formed by the oxidation of 
the carbon in the food. It will be shown that a similar 
process of oxidation, producing carbon-dioxide, goes 
on in the plant. 
Iodine in the dissolved state is used as a test for 
Starch, and may be prepared as follows :— 
Dissolve a few crystals of potassium iodide in 
water, (about 1 gram to 75 ¢.c. of water), and add to 
this iodine crystals till the liquid is a dark brown. 
