As soon as the diphtheria germ begins to grow in a throat, the little cells of the body begin to make a certain substance and to pour it into the blood. This substance we call _antitoxin_, which means opposed to the toxin in the blood. If the little cells make the substance fast enough, the germs will stop growing, or in some cases they never really get started growing, because they cannot exist where there is much antitoxin. Antitoxin looks like clear water. The following experiment will show you something that acts in very much the same way that antitoxin does. If you take a solution of litmus that is made alkaline, it will be very blue, like indigo; but if you drop a few drops of lemon juice into this solution it will turn red.[3] Lemon juice is acid, and is just the opposite of alkali. Now, if you put a few drops of ammonia, which is alkali, into the red solution, it will turn blue again. If you put a little more lemon juice, very carefully, drop by drop, into the blue solution, it will gradually turn lighter, until it is entirely clear. [Sidenote: How antitoxin acts] We will suppose that the blue is due to the toxin produced by the diphtheria germ, and that the lemon juice is the antitoxin produced by the cells in the body. If the antitoxin is made fast enough, the blue disappears; but if the toxin is made faster than the antitoxin, the blue remains. It is the same way in the body, only it is not litmus and acids and alkalies that we have to deal with. If the toxin is made faster than the antitoxin, the germs grow, and we get sicker and sicker; but if the antitoxin is made faster than the toxin, then the germs cannot grow, and we soon get well, or perhaps do not get sick at all. [Sidenote: How antitoxin was discovered] Doctors knew that this was what happened, but for a great many years they could not discover the composition of the antitoxin that is made in the body. One day a doctor suggested, "If we cannot find out the chemical nature of this thing that is made in the body, why can we not make it in the body of some animal and then use the blood of the animal?" And that is just what they did. They put diphtheria germs into beef tea, and let them grow very fast and make all the toxin they could. Then the doctors strained the germs out by passing the beef tea through a fine filter, in this way getting the poison, and not the germs. Then they gave a strong, healthy horse a small quantity of this poison; they did not feed it to him, but injected it into his blood. Of course the horse was sick for a while, but soon he began to get well again, for the cells in his body immediately went to work making antitoxin.