Note 2 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

Therefore in former times sulphur was known as an amphid element. Although the analogy between the compounds of sulphur and oxygen has been recognised from the very birth of modern chemistry (owing, amongst other things, to the fact that the oxides and sulphides are the most widely spread metallic ores in nature), still it has only been clearly expressed by the periodic system, which places both these elements in group VI. Here, moreover, stands out that parallelism which exists between SO2 and ozone OO2, between K2SO3 and peroxide of potassium K2O4 (Volkovitch in 1893 again drew attention to this parallelism).