you most conveniently by inspecting a diagram wherein

the demonstration is made clear.

Time presses, or I would expatiate on several of the functions in which oxygen plays so important a part. I would have entered more extensively into the beautiful function of respiration, and have shown the beneficent economy by which the oxygen of the air inspired is made to remove that very substance from the animal economy which vegetables require: I would have traced the analogies between combustion and respiration, but for all this there is not time: probably, too, these considerations may be postponed with some advantage, until we come to speak of the element, carbon. A few simple experiments, however, illustrative of the nature of respiration, I must not omit.

The first has for its object to demonstrate, that air which has been taken into the lungs and expired, will no longer support combustion. A similar effect then appears to have been produced upon it to that incident

on the burning of a combustible. We can go further, and demonstrate the nature of the substance which air respired has been the agent of consuming. We can prove it to have been carbon, by agitating air thus respired with lime water, when the peculiar milkiness so characteristic of carbonic acid, will be made apparent.

And now, having cursorarily glanced over the leading points of oxygen-having drawn your attention to its enormous distribution in nature; to its existence in the quiescent state; to its condition of intense activity; and, lastly, to the peculiar ozonic modification of it-I will conclude this imperfect sketch with some remarks on the strange condition of allotropism, of which the condition of ozone is only one out of several manifestations. There was a time, and that not long ago, when it was held amongst the fundamental doctrines of chemistry that the same body always manifested the same chemical qualities, excepting only such variations as might be due to the three con

ditions of solid, liquid, and gas. This was held to be a canon of chemical philosophy as distinguished from alchemy; and a belief in the possibility of transmutation was held to be impossible, because at variance with this fundamental tenet. But we are now conversant with many examples of the contrary; and, strange to say, no less than four of the non-metallic elements, namely oxygen, sulphur, phosphorus, and carbon, are subject to this modification. The train of speculation which this contemplation awakens within us, is extraordinary. If the condition of allotropism were alone confined to compound bodies, that is to say, bodies made up of two or more elements, we might easily frame a plausible hypothesis to account for it; we might assume that some variation had taken place in the arrangement of their particles. But when a simple body such as oxygen is concerned, this kind of hypothesis is no longer open to us, we have only one kind of particle to deal with, and the theory of altered position is no longer applicable. In short, it does not seem possible to imagine a rational hypothesis to explain the condition of allotropism as regards simple bodies. We can only accept it as a fact not to be doubted, and add the discovery to that long list of truths which start up in the field of every science, in opposition to our most cherished theories and long received convictions.

INTRODUCTION TO LECTURE II.

CHLORINE-ITS

SYNONYMES, ETYMOLOGY, AND

HISTORY

NATURAL HISTORY, PREPARATION, AND QUALITIES.

SYNONYMES, &c.-Chlorine, from xλupòs, green.

(Davy.)

Dephlogisticated muriatic acid. (Scheele.)

Oxymuriatic acid.

(Lavoisier.)

HISTORY.-Discovered by Scheele in 1774, and called by him dephlogisticated muriatic or dephlogisticated marine acid. If we regard the term phlogiston as synonymous with hydrogen, then Scheele's appellation is quite accordant with our present notions of the chemical constitution of chlorine. Lavoisier and his colleagues, when arranging their chemical nomenclature of chemical bodies, termed it oxymuriatic, or oxygenised muriatic acid, under the impression that it was a compound of hydrochloric acid plus oxygen. In giving to chlorine this name, there can be no doubt that Lavoisier and his colleagues were influenced by a preconceived notion of oxygen being the universal principle

of acidity. It remained for Sir H. Davy, in 1810, to demonstrate that the gas in question could not be proved to contain oxygen, nor had it been resolved into simpler elements; hence, according to the logic of chemistry, it must be regarded as a simple body. He applied to it the expressive appellation chlorine, on account of its yellowish-green tint.

DISTRIBUTION AND NATURAL HISTORY.-Chlorine, although never existing naturally in an uncombined state, is, when united to other elements, a large constituent both of the inorganic and organic kingdoms. In the former it exists combined with sodium, constituting enormous beds of table salt. In the ocean it not only exists combined with sodium, but also with calcium, magnesium, and potassium. In the organic kingdom it is found as a constituent of both animals and vegetables; existing in the greater number of animal liquids, and in various fluids and secretions of plants.

Process I. By

PRODUCTION OF CHLORINE. mixing together, and heating in a retort, one part, by weight, of coarsely powdered binoxide of manganese, and two parts, by weight, of hydrochloric acid.

Theory of the process.—Peroxide or binoxide of manganese is composed of two equivalents oxygen plus one equivalent of manganese. Protoxide of manganese, of one equivalent of manganese and of oxygen respectively.

Hydrochloric acid will not combine with binoxide