plication of red-hot iron, and a very painful sore is produced. Hydrofluoric acid unites with bases, and forms a class of salts called fluorides; the same salts are formed when metals dissolve in the acid. But the most remarkable property of this acid is its power of corroding and dissolving glass; even its vapour rapidly acts upon glass, and may be employed for etching upon this material. For this purpose the glass should be coated with a thin film of wax; the design being then traced upon the wax by a pointed instrument, the plate is afterwards exposed to the vapour of hydrofluoric acid. The glass becomes etched, wherever the instrument used to trace the design has cut through the coating of wax. Glass tubes and other scientific instruments are very conveniently and accurately graduated in this manner. E. FRANKLAND.

LESSON VII.

LIFE OF SIR HUMPHRY DAVY.

BY HUGO REID.

1. THE science of chemistry, which exhibits so many curious and striking phenomena, and is so rich in its applications to the arts and manufactures, made great progress during the middle and close of the last century and the early part of the present. During that time it was cultivated with great success by Black, Priestley, Cavendish, Davy, and Dalton, in England; by Scheele, in Sweden; and by Lavoisier, Fourcroy, Berthollet, and others in France. The great discoveries were made of the composition of air and water; hydrogen, oxygen, nitrogen, and several other gases were found out; new compounds and new elements were daily added to our previous stores, and mankind were deeply interested by the brilliant and important discoveries with which science was thus enriched.

2. In producing the various phenomena of chemistry, the chemist acts by two principal operations combination and decomposition. He effects combination when he causes two or more bodies to unite into one homogeneous compound, differing in properties from each of

its components; as when from sand, red lead, and soda, or potash, glass is formed. He effects decomposition when he separates a compound into its component elements; as when from water he extracts the two elements, oxygen and hydrogen. Previous to the close of the last century, the chemist effected combination and decomposition by simply bringing bodies in contact by dissolving them in some liquid, by beating them together, or sometimes by pressure: these were the only means within his reach for bringing about chemical changes. But at that time chemistry was armed with a new power-galvanism; and it is chiefly for the important discoveries he effected by means of this new agent that Sir Humphry Davy ranks as one of the greatest chemical philosophers the world has yet seen.

3. Sir Humphry Davy was born at Penzance, in Cornwall, in the year 1778, of a respectable family in that town. He was sent to a good school at Truro, where he acquired the usual rudiments of education, including the classics. He possessed a great facility in composing Latin and English verse, and was frequently engaged by the other boys to assist them in their exercises. But his facility in composition enabled him to assist them or amuse them in very different оссираtions from school exercises; he used to aid them in writing valentines, and entertain them with stories which he improvised, that is, composed as he went along. His tales were full of incidents of wonder or terror, and so interesting did they prove, that numbers of boys would collect by appointment, under the balcony of the 'Star' inn, to hear him. Fishing was, very early, a favourite amusement of young Davy, and continued so during his maturer years: when a mere child, he is said to have played at angling with a stick, and a thread with a crooked pin tied to it, with which primitive implements he angled in the gutter! In the year 1795 he was apprenticed to Mr. Borlane, a surgeon and apothecary in Penzance; there, in the preparation of various medicines, he had his attention turned to the science of chemistry, in which he afterwards became so distinguished. In 1797 he first made chemistry a special

subject of study. About that time he had the good fortune to become acquainted with Mr. Gregory Watt (the son of the eminent James Watt), a man of ability and superior scientific attainments. Mr. Watt was much interested by the ardour and genius of young Davy; a warm friendship grew up between them; they studied together the mineralogy and geology of the district; and, partly through the friendship of Watt, Davy obtained an appointment which afforded him a field for the development of his genius and the cultivation of his now favourite science-chemistry.

4. Soon after the gases had been discovered, and their properties investigated, it was conjectured that the respiration of several of these gases, or of mixtures of them with air, might be beneficial in various diseases, and an institution was established at Clifton, near Bristol, with the view of experimenting on the subject. It was founded, mainly, by Dr. Beddoes, the elder Watt, and the well-known Mr. Wedgwood, and was called the Pneumatic Institution. In 1798 Davy was appointed to superintend it—to direct the preparation of the various gases, to administer them, and investigate their effects. There he prosecuted diligently the study of chemistry; published in 1799 a volume entitled 'Researches on Heat and Light,' and in 1800, the volume which laid the foundation of his reputation, 'Researches, Chemical and Philosophical, chiefly concerning Nitrous Oxide and its Respiration.'

This

remarkable gas is now well known under the name of laughing gas, or intoxicating gas; soon after its properties were discovered, it was called pleasure-giving air.

In this work Davy unfolded the best methods of procuring this compound and of developing its singular properties. He had the boldness to ascertain its effects on man by breathing it himself a hazardous experiment -for some of the gases are extremely acrimonious and irritating, as chlorine; others are highly poisonous, as sulphuretted hydrogen and carbonic acid. He breathed it first mixed with air in various proportions, and then pure or unmixed, and found, to his surprise, that it possessed an exhilarating or intoxicating effect; stimu

lating the circulation, exciting the imagination, raising the spirits in a remarkable degree, and causing most agreeable sensations. It has been found, however, not to affect all alike; it moves some to uncontrollable and violent laughter, whence one of its names; others break forth into animated speech, declaiming with unusual energy and volubility; while there are some who are rendered pugnacious by it, and after inhaling it, immediately begin to strike right and left at all around them. It endues them with remarkable strength for a time, but its effects soon pass away. This singular gas consists of the same components as atmospheric air, but in different proportions.

5. In 1801 Davy left the Pneumatic Institution, and I went to settle in London. The anticipations that a powerful aid in the cure of diseases would be found in the various newly-discovered gases turned out fallacious; but Davy had established his reputation, and in 1802 he was appointed professor of chemistry at the Royal Institution in London, having acted as assistant-professor in the previous year.

In his new career Davy was eminently successful. His lectures, full of interest and eloquence, attracted crowded audiences, and his papers in the 'Philosophical Transactions' extended his fame. In the year 1800 Nicholson and Carlisle had discovered the decomposition of water-its separation into its elements, hydrogen and oxygen--by the voltaic battery. Davy applied himself to the investigation of the chemical effects of the new power. He soon proved the remarkable connection between chemical action and that of the battery, showing that the power developed is caused by the chemical changes going on between the metal and the acid solution. But his greatest discovery was made in 1807: this was proving the metallic composition of the alkalis, potash and soda.

6. It had long been known that various metals—as iron, lead, quicksilver-rusted, that is, became converted into an earthy matter on exposure to air, losing their lustre and other metallic properties. Since oxygen and the composition of the air had been discovered, it was

ascertained that this earthy matter was a compound of the metal with oxygen-a metallic oxide as we now term it; and it was conjectured that, perhaps, other earthy bodies, as lime, potash, soda, &c., might also be composed of some metal in union with oxygen. It was reserved for Davy to prove that this really was the case; and he first effected the decomposition in 1807, by means of a powerful galvanic battery. He subjected a piece of moistened potash to the action of the battery, and, "to his great surprise, found metallic matter formed." "This matter," he says, "instantly burnt when it touched water-swam on its surface, reproducing potash. In dry oxygen gas, likewise, it burnt into perfectly dry potash."

7. This great discovery marked a new era in chemical science; and soon soda, and lime, and other earths were found to be metallic oxides. The remarkable metals, potassium and sodium, were thus discovered, presenting the unusual circumstance of metals being lighter than water, and at ordinary temperatures, so soft as to be easily cut with a knife, but still having the metallic lustre, and other properties, entitling them beyond question to be classed as metals. Thus, in time, we came to the knowledge that the minerals which compose the crust of the earth are mostly metallic oxides, that is, compounds of some metal with the widely-diffused element, oxygen. This was the last of the brilliant series of discoveries, regarding the composition of bodies which characterized that period. Air had been analysed; water had been analysed; Sir Humphry Davy's discovery led to the analysis of earth; and though chemistry has made rapid advances since the time of Davy, there has been none equally important and extensive in its applications as his great discovery, that the alkalis and earths are compounds of oxygen and a metallic basis.

8. In the year 1811 Davy was knighted, and soon after he was created a baronet, honours which had seldom been bestowed on men of science, and which, therefore, indicated the high estimation in which he was held. About the same time he married a widow lady, Mrs. Apreece. Amongst other less important of his