"Recent researches, especially those of Professor Mantegazza, communicated during the year to the Institute of Lombardy, would seem to show that the ancients, after all, were by no means following merely imaginative or superstitious speculations in the practice they adopted of attempting, by the free use of odoriferous substances, to guard themselves against the attack of infectious diseases. This subject was deemed of sufficient importance to be referred to in the opening remarks of the presiding chairman of the public health section of the recent British Medical Association, as one deserving of careful study. Whether we shall, however, ever recur to the practice of Acron of Agrigentum, and other followers of Empedocles, the physicist, who not only used aromatic and balsamic herbs as preventives of pestilence, oftentimes planting them in abundance, for that purpose, round their cities, or adopt a similar course to that followed in a plague that once devastated Italy, when, acting on the advice of the faculty of the day, strangers crowding into Rome retreated to Lauretum, now San Lorenzo, that by a cooler atmosphere and by the odour of laurel they might escape the chance of infection, we cannot pretend to say.

But it would really seem that we may, with increased confidence, rely upon our camphor bags, our lavender bundles, and the like, for Mantegazza says that in the oxidation of the essences ot odoriferous plants a large quantity of ozone* is evolved, at least as much as is generated by electricity or phosphorus; the ozone being developed by the direct action of the sun's rays, and in some cases, whilst this commences in solar light, it continues in the dark. The plants which give ozone most readily are cherry-laurel, clove, and lavender, among herbs; the narcissus, hyacinth, and mignonette, and amongst perfumes eau-de-Cologne, oil of bergamot, and certain aromatic tinctures. The cultivation of herbs and odorous flowers ' in marshy districts and in places infected with animal emanations,' is the advice which Mantegazza gives to those who pin any faith to his experiments. Verily, if Apollo the Healer, by his life-inspiring and health-restoring ways, can draw from the fairest flowers of nature so mysterious and yet so mighty a purifier, there is hope for the slums and pest-stricken nooks of our towns and cities, and so grateful and attractive a mode of disinfection will surely soon become universal".

We cannot leave the subject of air without pointing out the danger of a charcoal fire. - Charcoal is carbon; when burning, it readily unites MM. de Babo, Claus, and Soret all maintain that ozone is oxygen denser than common oxygen gas. The latter philosopher especially declared that ozone as a molecule consisted of three atoms of oxygen, and he therefore called it binoxide of oxygen. M. Soret, in continuation of his researches on the density of ozone, and employing the absorptive powers of essences of turpentine and cinnamon, has come to the conclusion that the density of ozone is 1 1/2 times that of oxygen. Other learned chemists appear to concur in this opinion.

* Ozone. Although oxygen has never been changed from the gaseous state to a liquid or solid condition, it seems to be capable of assuming a peculiar condensed form called Ozone, - a word taken from the Greek , to give out an odour.

Ozone is produced in various ways.

1. By passing a series of electrical discharges - silent ones - through dry oxygen gas ; the latter diminishes in bulk to the extent of one-twelfth, showing condensation ; and if subsequently heated to a temperature of 5500 F., recovers its former bulk and loses its peculiar ozone qualities. The reason the electrical discharge should be a silent one is because the electrical spark produces too much heat, which destroys a considerable portion of the ozone, and thus prevents any considerable accumulation.

2. This peculiar condition of oxygen is obtained by acting either on potassium permanganate, or upon baric dioxide with strong sulphuric acid.

3. A stick of phosphorus scraped clean under water, and then exposed in a bottle containing moist air, produces ozone.

4. When water is decomposed in the apparatus called a voltameter, the mixed gases give the peculiar odour, and are found to contain a certain quantity of ozone.

It is known that sea-air contains ozone, whilst the same air, having passed over or through a town, is supposed to be deprived in a great degree of this agent, which is considered to have purifying and health-giving powers. The absence of ozone from the air is said to be prejudicial to health, and during the prevalence of cholera it was thought to be due in some degree to the absence of this condition of oxygen.

Ozone was first discovered by Van Marum in 1785 ; but it was not till Schonbein experimented upon it, in 1840, that it received its name with the oxygen of the air and forms the deadly carbonic acid. If a charcoal fire be shut into a bedroom, it will deposit this poisonous gas at the bottom of the room, and as all gases diffuse themselves through each other - just as wine will through water - it will gradually permeate the air of the chamber, and a person sleeping in it for six hours would probably never wake again. He would be suffocated.

We must guard carefully in our houses against the presence of this insidious foe. As the breath of animals, and combustion of any kind create it, we should frequently renew the air, by letting the carbonic gas escape from our rooms and admitting the fresh out-door atmosphere. The decay of animal or vegetable matter causes it also. Our dust bins therefore require attention; the contents should not be let accumulate, or if they cannot be frequently cleared out, fresh slaked lime must be thrown into them, which will absorb the carbonic acid, and render it harmless.

As oxygen creates the animal heat which is life, but which also consumes the body, it follows that very pure air by increasing combustion will create appetite - hunger being the craving of the human frame for fuel to supply the hourly consumption. Therefore good air will give a good appetite. Exercise, also, increases animal heat, by causing man to consume more oxygen than when he is still and sedentary, for in exercise the respiration is quicker, and more air is inhaled. The heat in the capillary vessels is acted on by running, jumping, etc., as a fire is by a pair of bellows.

Hard work, reading aloud, singing - all increase combustion by introducing a larger amount of oxygen into the lungs, and cause the persons working, reading, or singing, to require food to sustain the waste of the frame.

In sleep we breathe more slowly than when awake, consequently we require no food during the hours of the night (if we slumber), for less oxygen is introduced into the lungs, and our food-fuel consumes slowly till we wake.

Want of exercise and over-feeding produce over-fat and ill-health, because more hydrogen and carbon are taken into the blood than can be consumed by respiration. These, therefore - remaining unburnt up - turn to superfluous fat, or cause disease.

In cold weather flesh-food and fatty matters are required, because they contain more carbon and hydrogen, which when burned in the blood produce a larger amount of heat than any other kind of food. Hence the inhabitants of the Arctic regions eat train oil, whale blubber, etc. And in winter it is well to supply a fuller meat and fattier diet than in summer heat. In cold weather more food is required, to keep up the animal heat, because the air contains more oxygen than in warm weather, and therefore burns up the food more fiercely; besides which we are generally more active and our faster respiration fans the heat in the blood and consumes the fuel faster.

In summer we naturally dislike animal food, because the carbon and hydrogen in it make us too hot; - we prefer fruit and vegetables, for they contain less carbon and hydrogen than meat, and also a very large amount of water, and therefore produce less blood, and are not of so combustible a nature. On this account the natives of tropical climates, by a wise instinct, live on rice and fruit or vegetables.

The frequent use of baths tends to improve health and appetite in consequence of their freeing the pores of the body from obstruction, and thus promoting combustion. Ventilation, also, increases combustion by means of the introduction of more oxygen to the house. Therefore the badly-fed poor are instinctively averse to washing and to pure air, both of which increase hunger by diminishing warmth.

The cold-blooded animals - frogs, fishes, snails, lizards, etc. - have cold blood because they consume very little air; without a plentiful inhaling of which combustion is slow and animal heat small.

But while carbonic acid, when inhaled, is so injurious, it is, in creation, an agent of great good. In water it is beneficial to us; vegetation would perish without it, and chalk could not be formed without carbonic acid. There is nothing in nature which does not in some way tend to good.

"When we consume wood and coal in our fires, or bread and wine in our bodies," says Dr. Odling, "we merely effect a combination whereby their potential is converted into actual energy; this potential energy* having been stored up in them at the period of their formation, this energy being, in fact, the robbing of the sun's rays, and the storing up the heat of these rays in these articles of fire fuel. Under the action of the sun's rays the decomposition is effected of the carbonic acid and water into oxygen gas restored to the atmosphere, and of carbon-hydrogen, which is accumulated in the vegetable tissue. When we burn these tissues in our fires or bodies, we are simply restoring, in the form of actual energy, the potential heat of the sun's rays or its mechanical equivalent. We have all read of the Bourgeois Gentilhomme who had been talking prose all his life without knowing it. We have all our lives, and some of us without knowing it, been realizing that celebrated problem of extracting sunbeams from cucumbers".

This mighty agent of Life possesses also tremendous power in other respects. In can overcome the power of cohesion - that is, the attraction which by drawing particles of matter together makes bodies solid. Heat will rend these atoms asunder and change solids into liquids, liquids into vapour, and expand even the air itself, and the fine gases to a marvellous extent. We are all aware how heat expands our bodies in hot summer weather; how furniture expands in the day, and contracting at night by the colder atmosphere, makes night hideous to a wakeful person by divers mysterious cracks and sounds which have no apparent cause. It is the expansion of heat which bursts a roasting chestnut if an opening has not been made in the skin or shell, and which causes breakages of glass and china from hot water being poured into them.

Liquids expand still more readily with heat - some fluids expand more than others.