a small quantity of nitrate of potash, and fashioned into cones or cylinders of about an inch and a quarter in length, which should be thoroughly dried. The egg thus obtained can be hatched by the simple application of a lighted match, and gives rise to the phenomenon. The sulpho-cyanide slowly expands, the cylinder increases in length, and changes to a yellowish substance, which dilates and extends to a length of twenty or five-and-twenty inches. It has the appearance of a genuine serpent, which has just started into existence, and stretches out its tortuous coils, endeavouring to escape from its narrow prison (fig. 304). The residue is composed partly of cyanide of mercury and of para-cyanogen; it constitutes a very poisonous substance, which should be immediately thrown away or burned. It can be easily powdered into dust in the fingers. During the decomposition of the sulpho-cyanide of mercury, quantities of sulphurous acid are thrown off, and it is to be regretted that Pharaoh’s serpent should herald his appearance by such a disagreeable, suffocating odour. [Illustration: Fig. 303.—Iodide of cyanogen.] [Illustration: Fig. 304.—Pharaoh’s serpent.] After these few preliminary experiments, we will endeavour to show the interest afforded by the study of chemistry in relation to the commonest substances of every-day life. We will first consider the nature of a few pinches of salt. We know that kitchen salt, or sea salt, is white or greyish, according to its degree of purity; that it has a peculiar flavour, is soluble in water, and makes a peculiar crackling when thrown in the fire. But though its principal physical properties may be familiar enough, many people are entirely ignorant of its chemical nature and elementary composition. Kitchen salt contains a metal, combined with a gas possessing a very suffocating odour; the metal is _sodium_, the gas is _chlorine_. The scientific name for the substance is _chloride of sodium_ (salt).[19] The metal contained in common salt in no way resembles ordinary metals; it is white like silver, but tarnishes immediately in contact with air, and unites with oxygen, thus transforming itself into _oxide of sodium_. To preserve this singular metal it is necessary to protect it from the action of the atmosphere, and to keep it in a bottle containing oil of naptha. Sodium is soft, and it is possible with a pair of scissors to cut it like a ball of soft bread that has been kneaded in the hand. It is lighter than water, and when placed in a basin of water floats on the top like a piece of cork; only it is disturbed, and takes the form of a small brilliant sphere; great effervescence is also produced as it floats along, for it reduces the water to a common temperature by its contact. By degrees the small metallic ball disappears from view, after blazing into flame (fig. 305). [Illustration: Fig. 305.—Combustion of sodium in water.]