Having examined many or all of the specimens we have mentioned let us turn our attention to some of the common foods, and learn some of the methods used in testing for impurities. Ordinary household bread, it is hardly necessary to state, is rich in starch and, by trying the iodine test, mentioned above its presence is easily shown. With a weak solution the deeper the blue colour produced, the greater the quantity of starch. Some parts of the bread will be stained yellow, this indicates the presence of another nourishing component of bread. Certain kinds of bread are supposed to contain no free starch, because this substance is not beneficial to some people. Iodine again will reveal whether the bread is as it is described, for, if there be no free starch there will be no blue colouration. Brown bread will show much more of the yellow colouration and less of the blue than white bread. Good, well-baked bread should keep for a considerable period without turning sour; we can easily see whether our sample is satisfactory by running a drop of litmus on to it and watching the effect under the microscope; if the litmus remains unchanged in colour the sample is not sour; if, on the other hand, the litmus turns red it shows us that acid is present and that our bread is not as it should be. Tea is difficult to prepare for microscopic examination and most of the tests call for expert knowledge, not only in the management of the microscope but of the plant itself. The structure of the leaves can be made out clearly in specimens which have been soaked for a time in water, but this is of little interest to the ordinary microscopist. One very pretty test may, however, easily be performed. We all know that it is not good to drink tea which has been standing for a long time. Some tea-drinkers are so particular that they cannot bear to see the teapot shaken before they have poured out their cup. All this trouble arises because tea contains a poison called “theine”; it is an alkaloid, one of a large class of chemical substances which are nearly all deadly poisons--cocaine and nicotine are alkaloids. Although theine is poisonous, tea which contained none of this substance would be tasteless and the absence of this substance shows that the tea leaves have been badly prepared. Tea after being gathered should be dried at once, sometimes it is re-dried and this process drives off the theine. For our test we require, in addition to our microscope, two watch glasses, a piece of copper wire gauze and a gas burner or a spirit lamp. Place a little tea in one of the watch glasses and cover with the other watch glass; then heat gently on the wire gauze. In a few minutes drops of moisture will appear on the upper watch glass; after about ten minutes’ heating beautiful, long, needle-shaped crystals will begin to appear, with a little further heating we shall obtain a good crop of lovely crystals on the upper watch glass and they make a