All the metals are fusible, but some require very different degrees of heat to render them fluid--platinum requiring the heat of the oxy-hydrogen blowpipe, while tin melts in the flame of a candle, and mercury is fluid at all temperatures in this climate, but becomes solid at 40 degrees Fahrenheit below 0--a temperature occasionally experienced in the Arctic regions, where the mercurial thermometer is useless, the mercury becoming solid. They are all excellent conductors of heat and electricity, and have the property of reflecting light and forming mirrors; for looking-glasses owe their power of reflecting objects principally to what is called the “silvering;” that is, a mixture of mercury and tin spread over the back of the glass, which being transparent, allows the image reflected from the metal to pass through it. The following classification is most instructive, because it suggests to the young student that there must be identical properties in the metals thus placed together: _Class 1._ Ammonium, cæsium, lithium, potassium, sodium. _Class 2._ Calcium, barium, strontium. _Class 3._ Aluminium, cerium, didymium, erbium, glucinium, lanthanum, thorium, yttrium, zirconium. _Class 4._ Zinc class: cadmium, magnesium, zinc. _Class 5._ Iron class: cobalt, chromium, indium, iron, manganese, nickel, uranium. _Class 6._ Tin class: niobium, tantalum, tin, titanium. _Class 7._ Tungsten class: molybdenum, tungsten, vanadium. _Class 8._ Arsenic class: antimony, arsenic, bismuth. _Class 9._ Lead class: lead, thallium. _Class 10._ Silver class: copper, mercury, silver. _Class 11._ Gold class: gold, iridium, osmium, palladium, platinum, rhodium, ruthenium. Potassium. Potassium was discovered by Sir H. Davy in the beginning of the present century. It is a brilliant white metal, so soft as to be easily cut with a penknife, and so light as to swim upon water, on which it acts with great energy, uniting with the oxygen, and liberating the hydrogen, which takes fire as it escapes. Experiment. Trace some continuous lines on paper with a camel’s-hair brush dipped in water, and place a piece of potassium about the size of a pea on one of the lines, and it will follow the course of the pencil, taking fire as it runs, and burning with a purplish light. The paper will be found covered with a solution of ordinary potash. If turmeric paper be used, the course of the potassium will be marked with a deep brown color.--_Corollary._ Hence, if you touch potassium with _wet_ fingers you will burn them. If a small piece of the metal be placed on a piece of ice, it will instantly take fire, and form a deep hole, which will be found to contain a solution of potash. In consequence of its great affinity for oxygen, potassium must be kept in some fluid destitute of that element, such as naphtha.