paratively small; and those animals, as articles of food, may be considered rather as luxuries than necessaries.

Of the animals which supply us with food, the flesh or muscular fibre is that part which is most acceptable to the palate and it is worthy of consideration that the flesh of those animals, of whose living services we stand hourly in need, as the horse and the dog, are so unpalatable that we are not tempted to eat them unless in cases of dreadful necessity. Many individuals however, through poverty, are content, and some by peculiarity of taste are inclined, to feed on the lungs or liver, or other of the viscera of animals. And modern researches and experiments have taught us that even the bones may be rendered digestible, either by the effect of long boiling under a high degree of artificial pressure, as in the apparatus called Papin's Digester, or in consequence of the removal of their earthly basis by means of any convenient acid; and we have also learned, from similar sources, that common saw-dust, by certain chemical processes, may be made nutritious: but we may fairly argue, from the provisional care of nature, that mankind will never be generally reduced to such circuitous means of obtaining their necessary food. In the mean time we may console ourselves with the reflection, that in the event of any temporary or local difficulty, we may find a supply of food where antecedently to the researches above-mentioned we should never have dreamed of looking for it. Vitruvius mentions, in speaking of the construction of garden walks, that the fragments of charcoal, which were a common substratum of such walks, had occasionally afforded a most important magazine of fuel in a protracted siege and in such an emergency the bones of animals might continue a supply of food, after the flesh had been

eaten.

SECT. VI.

Manufacture of Sal Ammoniac.

EVEN in the present abundance of animal food the refuse is not wasted; and all that is thrown aside, as unpalatable or indigestible, is subsequently collected, for the purpose of obtaining a material, very extensively employed and of considerable value in the arts, known familiarly under the name of sal ammoniac. Perhaps in the whole circle of the arts there is scarcely any process more interesting, if all the attendant circumstances be considered, than the fabrication of this substance: and the interest principally arises from this peculiarity in the nature of the process, that, among the numerous products which are evolved in its different stages, there is scarcely one which is not sufficiently useful to prevent the necessity of its being thrown away.

Any one, who is in the habit of walking much in the streets of London, will frequently see some half-clothed wretched individual stooping down and holding open an apron, into which he throws from time to time pieces of broken bone and other offal, which he has disengaged from the interstices of the stones that form the carriage pavement. The unsightly load thus obtained is conveyed to the sal ammoniac manufactory; and when a sufficient mass of bones has been accumulated from this and other sources, they are thrown into a cauldron of water, and are boiled for the purpose of clearing them of the grease with which they are enveloped: which grease, subsequently collected from the surface of the water on which it floats, is employed in the composition of soap.

The bones thus cleaned are thrown into large retorts, surrounded by burning fuel, and submitted to the process called destructive distillation; whereby, in consequence of the application of a sufficient degree of heat, the matter of the bone is resolved into its constituent elements, from which new compounds are formed. Of these, some pass off in the state of vapour or gas, while the fixed principles remain in the retort.

Among the more remarkable products which pass off are carbonic acid gas, commonly known by the name of fixed air; and various combinations of hydrogen and carbon, forming different kinds of inflammable air; together with water holding carbonate of ammonia (salt of hartshorn) in solution; and a peculiar oil. Of these products, the fixed air and inflammable air are disregarded, and suffered to escape. The oil is employed to feed lamps placed in small chambers, the sides of which become incrusted with the smoke arising from the combustion: which smoke being collected, becomes an article of sale under the name of lamp black; a substance of considerable importance as the basis of printing ink, &c.

It would be tedious, and uninteresting to the general reader, to describe all the intermediate steps of the process: and it is sufficient for the present purpose to state that, towards the conclusion of it, two new compounds are formed, namely muriate of ammonia and sulphate of soda: of which the sulphate of soda is separated by the process of crystallization, and is sold to the druggists under the common name of Glauber's salt; and the muriate of ammonia, (sal ammoniac,) the great object of the whole manufacture, is finally obtained in a separate state by the process called sublimation.

The form of the bones, submitted to destructive distillation in this process, is not altered; and the unvolatilized mass, remaining in the retorts, consists of the earthy and saline matter of these bones, blackened by the carbon which is evolved from their animal matter. Exposure to an open fire drives off this carbon, and leaves the bones still unaltered in form, but nearly blanched: and these bones, subsequently reduced to powder, and mixed with a

sufficient quantity of water to give them the requisite degree of consistence, are formed into vessels, which are employed in the process of refining gold and silver.

It was stated that, during the destructive distillation of bone, the carbonic acid and inflammable gases are suffered to escape: but of these the latter might be employed in supplying light to gas burners; and then, out of the numerous products of the complicated process which I have been describing, the carbonic acid would be the only substance not employed for some useful purpose.

SECT VIL

Animals as a Source of Clothing, &c. for Man.

THE utility of many of those animals which supply us with food does not terminate in merely that adaptation of them to human wants. From the same animals we are supplied with clothing also; (but this service, indeed, they render to us in common with various other animals which are unfit for food); and, according to the different states of civilization in which mankind exists, that clothing is more or less artificially prepared. Thus while the African or Australian savage scarcely protects his body from exposure by a partial covering of leaves, or the inner bark of trees; and the Esquimaux envelopes his body in the undressed skin of the seal which he has recently killed, supplying also the separate coverings of his head and feet and hands from the same source; the poorest peasant of any civilized part of Europe derives his clothing not only from one but many different species of animals; to say nothing of those occasional parts of his dress which are obtained from the vegetable and mineral kingdom. The ox, the dog, the sheep, the beaver or the rabbit, and the silk-worm, in almost every instance contribute their direct contingent to the apparel of the humblest individual of Europe, who is not absolutely a mendicant: and, with reference to the dress and ornamental appendages of individuals of more elevated rank, to the animals already mentioned may be added the deer, the goat, the camel, the elephant, the ermine, and numerous other animals which supply the various and rich furs of commerce; the ostrich, and many other birds; and even the tortoise, the oyster, and the puny architect of the more beautiful species of coral.

Nor are the advantages which mankind derive from the animal kingdom, with reference to general commerce and the arts and economical purposes of life, of less importance than the foregoing. How many different substances, as leather, and parchment, and glue; and what various instruments, either for common use, or ornament or amusement, are manufactured from skin and horn, and bone and

ivory! With respect to the last-mentioned of which substances indeed, it is a highly interesting fact, 'that the world has not been supplied with it solely from the two still existing species of elephant, but also, and in a very large proportion, from the extinct and fossil species. Under the name of licorne fossile, the tusks of the extinct species have for ages been an object of commerce in the Russian dominions and M. Pallas describes the abundance of these fossil tusks to be such, that they are found in every direction throughout the greater part of north-eastern Russia.

If we only consider the amount of the consumption of wax and honey, of what importance is not that little insect the bee: and the same observation may be made with reference to the silk-worm and the cochineal!

Lastly, for it is necessary to bring the present subject to a close, what immense advantages accrue to commerce and navigation from the traffic in even a very few species of fish, as the whale, the cod, the herring, and the pilchard! so great indeed are those advantages, that the question of the right of fishery on a particular coast has sometimes been the occasion of involving the most powerful nations in expensive wars: for these fisheries, at the same time that they are a source of immense riches to individuals, constitute, as it were, a nursery for the hardiest race of sailors, and thus become of the highest importance in a national point of view.

CHAPTER X.

ADAPTATION OF THE EXTERNAL WORLD TO THE EXERCISE OF THE INTELLECTUAL FACULTIES OF MAN.

SECTION I.

On the Rise and Progress of Human Knowledge.

In the preceding part of this treatise the physical character and condition of man were first considered; and, afterwards, the adaptation of external nature to the supply of his bodily wants. It remains for us to consider the adaptation of the various objects of the material world to the exercise of his intellectual faculties.

But, in contemplating the connexion which exists between the external world and the exercise of the mind of man, who shall attempt to describe the nature and boundaries of that yet unmeasured plain of knowledge, in which man is constantly either intellectually expatiating, or practically exerting himself? who, without wandering into

the mazes of metaphysical speculation-always amusing in the pursuit, but never, perhaps, satisfactory in the result-who shall develope the obscure steps by which science first finds access to the mind? In reflecting indeed on the state of civilized society during its earlier periods, there is nothing more wonderful in the intellectual history of mankind, than the skilful management of many processes in the arts, the true nature of which was not understood till ages and ages afterwards. Thus, although zinc was scarcely known as a distinct metal till about a century since; and, almost within the same period, one of its commonest ores, calamine, was held in so little estimation in Great Britain that it was frequently used merely as ballast for shipping, (Watson's Essays, vol. iv. p. 6.); yet that same ore was used before the time of Aristotle for the purpose of making brass, and to that purpose is principally applied at the present day. The process also of making wine was known in the earliest periods of history; although the principles on which it is produced were not well understood till a few years since.

Another remarkable fact in the history of human science, which, though frequently observed, has not yet been explained, is the occasional arrest of its progress at a point immediately bordering on discoveries which did not take place till many ages subsequently.* This may be affirmed, in a certain sense at least, with respect to glass for this substance, though very early discovered, hardly came into general use for ordinary purposes till comparatively a very late period. But a more remarkable instance occurs with respect to the art of printing and whoever looks at the stereotype stamps, as they may be called, which have been discovered at Herculaneum, and other places, will be disposed to allow that the embryo of the art of printing died, as it were, in the birth.f

In order that the external world may be fitted to the just exercise

The substance of the following note, though not directly illustrative of the subject now under consideration, is not irrelevant to it; and is sufficiently curious in itself to justify its introduction to the notice of the reader.

In Dr. Thomson's Annals of Philosophy for 1817, p. 149, is an account of a paper read at the Royal Society, relative to some experiments made on torpedoes at Rochelle, in which it is stated that, where torpedoes abound, boys are in the habit of playing the following trick to those who are not in the secret. They persuade the ignorant boy to pour water in a continued stream upon the torpedo; and the consequence is, that an electrical shock is conveyed, along the stream, to the body of the boy.

Plutarch notices the same fact in almost the same terms. "It is affirmed by those," he says, "who have often made the experiment, that, in pouring water on a live torpedo, the hand of the person who is pouring the water will be sensible of a shock, which has apparently been conveyed through the water to his hand." Ἔνιοι δὲ ἱστοροῦσι, πεῖραν αὐτῆς ἐπιπλέον λαμβάνοντες, αν εκπέση ζῶσα (Νάρκο, the Torpedo,) κατασκεδαννύντες ὕδωρ ἄνωθεν, αἰσθάνεσθαι τοῦ πάθους ἀνατρέχοντος ἐπὶ τὴν κεῖρα, καὶ τὴν ἀφὴν ἀμβλύνοντος, ὡς ἔοικε δια τοῦ ὕδατος τρεπομένου καὶ προπεπονθότος.

PLUT. MORALIA, Oxon. 4to, 1797, tom. iv. p. 643, 644.

A very interesting conjectural account of the origin and progress of the arts, and of social life, occurs in the last part of the fifth book of Lucretius.