glass, when their operations are carelessly conducted, discharge from their cones unpleasant-it may be injurioussmells.

There is scarcely a manufactory, indeed, which involves the immediate application of chemical principles-and this includes by far the greatest number-which, if carelessly conducted, may not become a source of real annoyance, or even of injury to its neighbourhood. I speak from a very wide experience, however, when I say that the escape of injurious substances into the open air, from such works, is rarely necessary to the prosperity of the several branches of manufacture. For the comfort of common life, therefore, the intentional discharge of them into the atmosphere ought not to be permitted.

CHAPTER XXVIII.

THE SMELLS WE DISLIKE.

THE PREVENTION AND REMOVAL OF SMELLS.

Wide diffusion of evil odours.-Prevention of smells.-Decay prevented by freezing, by drying, by excluding the air, by salting, and by smoking.-Effects of charcoal. Smell-disguisers or perfumes-Smell-removers or deodorisers.-Charcoal; cause of its remarkable action.-Dr. Stenhouse's charcoal respirator; where it is likely to be useful.-Peat, vegetable soil, and burnt clay.-Smell destroyers or disinfectants.-Nitric oxide, sulphurous acid, inuriatic acid, and chlorine gases.The chlorides of lime, iron, and zinc.-Sulphate and pyrolignite of iron.-Iodine and iodoform.-Quicklime; its unlike action on fermenting and unfermenting matters. Summary.

EVIL odours are equally penetrating with sweet smells. They diffuse themselves through the air, and affect the senses unpleasantly, even when the absolute quantity of matter present is too minute to be detected by our most refined methods of chemical analysis. Unlike the sweet odours, however, they are produced everywhere around us, and are therefore a universal source of more or less perceptible irritation and annoyance. To prevent the introduction of evil. smelling substances into the atmosphere which surrounds us, and when present to remove them, has consequently been at all times an object of desire. The attainment of this object

has been rendered both more easy and more perfect by the discoveries of modern chemistry.

I. THE PREVENTION OF SMells. The smells which usually arise from the decay or decomposition of the bodies and droppings of animals can often be either arrested or altogether prevented. Extreme cold, for example, such as is sufficient to freeze and harden the dead body of an animal, will preserve it in a state of absolute freshness, even for thousands of years. In northern winters the freezing of flesh and fish is the common way of preserving it; and in the ice cliffs on the banks of the Siberian rivers, the entire body of an extinct species of elephant has been met with, so little decayed as to be still greedily devoured by dogs. Even moderate cold, if accompained by a drying wind, will prevent decomposition, the former retarding the decay till the latter removes the moisture which is necessary to its continuance. Or the total exclusion of air will have the same effect, as is seen in the preserved meat, now so useful in long voyages and in remote parts of the earth.

These modes of preventing decay illustrate what has been said of the agency of heat, air, and moisture (p. 523), in promoting the putrescent fermentation of animal and vegetable substances. When we freeze them, we arrest decay by removing the necessary heat; when we dry them, by removing the necessary moisture; and when we shut them up in sealed vessels, by excluding the necessary

air.

But decay can also be prevented by the direct application of chemical substances. Such is done when flesh meat is immersed in sugar, or when it is impregnated with common salt, or with a mixture of common salt and nitre. These substances fill the pores of the flesh, and thus preserve it by excluding the air. They form also, and especially the two

PRESERVATION OF MEAT.

539

latter substances do, a species of chemical combination with the fibre of the meat, and with the substances contained in its natural juices, which are less liable to decay than the substances themselves, and thus retain the whole in a state of sweetness for an indefinite period.* Volatile tarry matters, such as creosote and others, which are contained in the smoke from peat and coal, in wood vinegar, and in the spirit which is distilled from coal or wood tars, act in a similar way. They combine with the fibre of flesh or fish, and retard its decay, until the removal of moisture by evaporation renders decay both slow and difficult. It is in this way that the smoking of fish or flesh contributes to a speedy cure, saving both time and salt, rendering the cure more certain, and adding at the same time an artificial flavour, which to many is very grateful.

Substances which thus retard decomposition are called. antiseptics. Besides those I have mentioned, white arsenic, corrosive sublimate, the chloride of zinc, pyrolignite of iron, alcohol, camphor, and many essential oils, possess antiseptic virtues. In common life, however, these substances are rarely employed, though in museums. museums of natural history alcohol is much used for bottling up anatomical and other preparations, and arsenic, corrosive sublimate, and camphor, for preserving insects and the skins of animals.

Charcoal, when recently burned, has much efficiency in preventing the offensiveness of animal decay from becoming sensible to the smell. Sprinkled in the state of powder over the parts of dead animals, it preserves them sweet for a length of time. Placed in pieces beneath the wings of a fowl, it keeps away much longer than usual any appearance of taint. Or if strewed over substances already tainted, or mixed with

* See THE BEEF WE COOK, p. 124.

liquids which have acquired the unpleasant smell of decaying organic matter, it removes the evil odour, and makes them sweet again. It is for this reason that pieces of fresh charcoal are now and then introduced into our common waterfilters.

In all these cases, charcoal appears to act rather as a smell-remover than as a decay and smell preventer. In what way it acts as a remover of smells will be explained in a future part of the present chapter.

Quicklime also possesses the property of retarding, and to a certain extent preventing, the decay of animal and vegetable substances. Its action, however, as we commonly use it, is of a complicated kind, and will be explained when we come to treat of the smell-destroyers.

II. THE DISGUISING OF SMELLS.-Where evil-smelling decay of any kind commences, or where volatile substances which disagreeably affect the organ of smell escape into the air from any source, we naturally desire to rid ourselves of the unpleasant sensation. This we generally wish, and always ought if possible to do, by removing the substance to which the noisome smell is owing. In the great majority of cases, however, we merely overpower or disguise it. We are content to mingle with the smell we dislike some odour we can enjoy, and to leave floating in the air around us the evil and the good together, to produce unheeded their natural effects upon the system.

Sweet odours are thus the natural disguisers of evil smells. They are the only resource of rude and dirty times against the offensive emanations from decaying animal and vegetable substances, from undrained and untidy dwellings, from unclean clothes, from ill-washed skins, and from illused stomachs. The scented handkerchief in these circumstances takes the place of the sponge and the shower