initial letter dashed over. On the other hand, vegetable alkalis may be indicated by placing a cross over their initial letter, and other literal representative. Thus, Quina may he indicated by ch (i.e. China); morphia by ; oxide of lead, PbO, i.e., plumbum and oxygen; potash, KO, i.e., kalium, or potassium, and oxygen.

+

In these and other cases, where single equivalents enter into combination, the usual algebraical rule is followed of appending no numeral of unity, which is always implied. When, however, more than single atoms enter into combination, the employment of numerals becomes indispensable. Thus sulphuric acid, which is a combination of 1 equivalent of sulphur with 3 of oxygen, is thus expressed, SO3, or SO,; some chemists preferring to write the 3 inferiorly, so as to make a distinction between it and an algebraic power. In this form of expression the 3 only refers to the quantity of one of the elements in the binary compound, namely, oxygen. Had we occasion to indicate symbolically 3 equivalents of sulphuric acid, it would be accomplished thus, 3SO,; and, generally, all large figures on the left of a symbolical expression affect the whole constituents of that expression; whereas, small figures on the right affect merely the element to which they are immediately attached.

As binary compounds of elements are indicated by putting symbolical initials in juxta-position, so the comma and the plus sign + are employed to designate ultimate combinations arranged in proximate series.

Thus, PbO indicates lead; SO, sulphuric acid; and PbO,SO, or PbO+SO, indicate a proximate union of sulphuric acid with oxide of lead, to form sulphate of oxide of lead, commonly termed sulphate of lead. The bracket, or vinculum, in accomplishing symbolical notation, is similar in its use to the application of the same in algebra. Thus, 3PbO+A indicates a compound of 3 equivalents of oxide of lead with 1 of acetic acid, the tribasic acetate of lead: whereas, 3(PbO+Ã) means 3 atoms of neutral acetate of lead.

Such is the principle on which the symbolism or chemical combination was effected by Berzelius. This great chemist, however, effected still further abbreviations, which have only been partially disused on account of the practical difficulty of printing them, the signs having to be made specially as an addition to the printer's fount. Nevertheless the system is eminently convenient and expressive. It is still employed by many chemical writers. It consists in representing sulphur and oxygen (the two most frequently occurring constituents in compound bodies) by means of commas and dots placed over a letter representing the other constituents. Thus, i would represent water, according to the usual English theory of regarding it as a binary compound of oxygen and hydrogen, although Berzelius represented it as =,; inasmuch as he adopted. the theory of its being a compound of two atoms hydrogen plus 1 of oxygen. Sulphuric acid would be represented as S, and so on for other bodies.

=

INTRODUCTION TO LECTURE 1.

OXYGEN-ITS SYNONYMES AND ETYMOLOGY, HISTORY, NATURAL HISTORY, PREPARATION, AND QUALITIES.

SYNONYMES.-Oxygen, oğùç, acid, yévvaw, I produce.

(Lavoisier).

Dephlogisticated air. (Priestley).
Empyreal air.
Vital air.

HISTORY.-Oxygen gas was discovered by Priestley, August 1, 1774, who called it dephlogisticated air. In the following year it was again discovered by Scheele, without cognizance of the previous discovery by Priestley.

DISTRIBUTION AND NATURAL HISTORY.-Oxygen, of all substances, is most largely found distributed throughout nature, constituting at least three-fourths of the known terraqueous globe. It exists both in the inorganic and organic kingdom. In the former it constitutes eight-ninths of water, and water covers about

three-fourths of the surface of the earth. Of the solid crust of the globe, at least one-third is oxygen, as will be rendered manifest on considering the materials of which the crust is formed, namely, for the most part of silica, carbonate of lime, and alumina. Silica alone constitutes 45 per cent. of the earth's mineral crust. Of the dry atmosphere oxygen constitutes twenty-three parts in one hundred, and eight-ninths of atmospheric

vapour.

In the organic kingdom oxygen is an essential constituent of all living beings.

PREPARATION OF OXYGEN GAS.-Process I.-By exposing to heat in a small glass retort the peroxide of mercury, when oxygen gas is eliminated, and metallic quicksilver condenses in the upper part of the tube. Oxides of gold, silver, platinum, and mercury are those only which yield up the whole of their oxygen on the application of heat. Many other oxides yield up a portion of their oxygen by similar treatment. The method of generating oxygen gas from the peroxide of mercury is seldom had recourse to at this time, seeing that it is more expensive and less convenient than many

other processes. It possesses great interest, however,

in consequence of its having been the original process by which oxygen gas was developed by Priestley, and as beautifully illustrating by analysis the composition of oxides.

Α

very curious circumstance in relation to the oxide

of mercury may here be indicated. At a certain temperature, this material, as we have seen, yields up its oxygen, and leaves metallic quicksilver; but if metallic quicksilver be exposed to a lower temperature, oxygen is absorbed and peroxide of mercury formed. Taking advantage of this principle, M. Boussingault has recently proposed to abstract large stores of oxygen gas from atmospheric air, and to liberate them for economic uses. He does not employ mercury for the purpose, but caustic baryta, which at one grade of temperature absorbs an additional dose of oxygen, becoming peroxide of barium, and at an additional grade evolves the same. Thus, he causes one charge of caustic baryta to alternately absorb oxygen from the atmosphere, and evolve it into a reservoir.

Process II. By exposing to a red-heat binoxide or peroxide of manganese in an iron retort.

Theory of the Process.-Manganese in acid, to its acid combinations, forms with oxygen three oxides, viz.:

In the binoxide half an equivalent of oxygen is so loosely combined that it is evolved on the application of heat, and sesquioxide of manganese remains. This is