30. By this means it has been demonstrated that--first, when any one of these substances is eaten exclusively, the body is imperfectly nourished, and life is shortened. Dogs fed exclusively upon either albumen, fat, or sugar, soon die of starvation. Second, a diet long deprived of either of these principles, is a fertile cause of disease; for example, on ship-board, where fresh vegetables are not dealt out for a long period, _scurvy_ becomes prevalent among the sailors. They are, however, to a certain extent mutually convertible, and thus the missing article is indirectly supplied. For {63} instance, sugar changes to fat in the body; and hence, as is well known, the "hands" on a sugar plantation grow fat during the sugar season, by partaking freely of the ripened juices of the cane. 31. That is the best diet therefore which contains some of each of these principles, in due proportion; and that is the worst which excludes the most of them. The cravings and experience of man had unerringly guided him to a correct regulation of his diet, long before the chemistry of food was understood; so that his ordinary meals long ago combined these various principles, the necessity and value of which are now explained. QUESTIONS FOR TOPICAL REVIEW. PAGE 1. What is understood by the term food? 53 2. What can you state in relation to the source of food? 53 3. What discriminations and selections are necessary? 53, 54 4. How can you tell the organic from the inorganic substances? 54 5. What relative position does water hold as an article of food? 54 6. In what parts of the body is water found? 54, 55 7. In what articles that we eat is it found? 55 8. If you were required to go without water or solid food for a number of days, which would you prefer to have, and why? 55 9. What can you state of the importance of salt as an article of food? 55, 56 10. How abundant is salt, and how does it find its way into the human system? 55, 56 11. What can you state of the importance of lime in the body? 56 12. What, of the importance of iron? 56, 57 13. What further is stated of other inorganic substances? 57 14. What in relation to organic substances? 57 15. What can you state in relation to the albuminoids? 57, 58 16. What, in relation to albumen? 58 17. What, in relation to casein? 58, 59 18. In relation to the fats or oils, and how generally consumed? 59, 60 19. What do we understand by the sugars or saccharine substances? 60 20.