30. Q. What does the two-fold character of chemical study involve? A. First, the examination of elementary substances and their compounds. Second, a consideration of the many general and special laws and forces which determine the various possible combinations. 31. Q. How many elementary substances are there now generally recognized as such? A. Sixty-six. 32. Q. About how many of the elements possess names that are familiar to ordinary readers? A. About one sixth of them. 33. Q. Of what two elementary substances is it probable that three fourths of our globe is composed? A. Of oxygen one half, and of silicon one fourth. 34. Q. What general name is given to most of the elements? A. Metals. 35. Q. What symbol and what weight has each element? A. An atomic symbol and an atomic weight. 36. Q. How is an atom of each elementary substance designated? A. By a symbol, usually the initial letter of the native or Latin name of the substance. 37. Q. What are three properties an elementary substance accepted as a metal should possess? A. It must possess the property of existing in a solid condition; it should possess the metallic luster; and it should possess the power and tendency to readily form a chemical union with oxygen. 38. Q. What are called binary and what ternary compounds? A. Compounds having only two kinds of elements are called binaries. Compounds having three kinds of elements are called ternaries. 39. Q. What four binary compounds are given as examples? A. Hydric chloride, sulphur di-oxide, sulphur tri-oxide, and plumbic oxide. 40. Q. Under what two heads are the principal ternaries grouped? A. Acids and salts. 41. Q. What are the two principal ternary acids used by chemists? A. Nitric acid and sulphuric acid. 42. Q. What is meant by the term atom? A. It is that portion of any kind of matter that is to human beings indivisible in fact. 43. Q. With what invisible, occult power is each atom and each molecule endowed? A. A power called chemical affinity. 44. Q. What are three of the peculiarities of chemical affinity? A. Each kind of atom has its peculiar chemical affinities. Each atom has a certain equivalence or atom-fixing power. Chemical changes produce striking results. 45. Q. What is the most common way of producing hydrogen? A. By bringing together sulphuric acid and zinc. 46. Q. What are some of the properties of hydrogen as a gas? A. It is colorless, odorless, tasteless, and, bulk for bulk, it is the lightest substance known in nature. 47. Q. What is the most interesting chemical property of hydrogen? A. Its power to unite with oxygen. 48. Q. What is said of the uses to which hydrogen may be put? A. As an elementary gas it finds but few applications in the arts. 49. Q. For what standards is hydrogen used by chemists? A. As the standard of equivalence or atom-fixing power; the standard of atomic weight, and the standard of density for gases.