explosions in the oxy-hydrogen blowpipe, and some of these you may find described in the older works on chemistry; but be sure not to trust to any of them; they are not safe; a mixture of oxygen and hydrogen is a thing not to be trifled with, as the explosion of a large quantity would very robably be attended with fatal consequences.

The heat of the oxy-hydrogen flame is of eculiar intensity; the most refractory ubstances yield to it with ease. Platinum, metal which resists the action of our trongest furnaces, melts in it like wax in he flame of a candle. A thick iron wire, held in the flame, melts and burns with great apidity, and a steel wire or a watch-spring burns with the most magnificent scintillaions. But though the heat of this flame 3 so intense, the light which it emits is ery feeble. If, however, a quantity of finely ivided solid matter, such as chalk powder, e sprinkled through it, the light immeditely becomes very strong, and if the jet be irected on a piece of solid quick-lime, a ght is produced which for whiteness and rilliancy is inferior only to that of the in; it is, in fact, used as a substitute for he sun's rays in the solar microscope, and so for night-signals in trigonometrical arveys. When placed in the focus of a arabolic reflector, it has been made visible t the almost incredible distance of sixtyine miles.

You see from these experiments that the ́onditions for producing heat and light are ot the same. To obtain the greatest intenty of heat, the essential conditions are at the combining bodies be mixed exactly the proportions required for combinaon, and every thing removed that might ke up a portion of the heat evolved, withat contributing to its production. But produce a strong light, it is necessary at a solid substance be present in the flame, id thereby raised to a very high temperaire. All substances become luminous when eated to a certain point; but at a given emperature, solids emit a much stronger ght than gases; hence it is that pure aseous flames, such as those of hydrogen nd alcohol, give very little light. Addional illustration of this principle may be und in the experiments described in the ist lecture. Compare, for example, the ombustion of sulphur and phosphorus in

oxygen. Sulphur forms a gaseous product, and therefore emits but little light; but phosphorus forms a solid product, and accordingly produces a light of almost insupportable intensity; charcoal, again, which forms a gaseous product, emits but little light, unless it is formed from the bark, in which case it throws out brilliant scintillations consisting of small particles of earthy matter in a state of intense ignition. The flame of a candle or a common oil-lamp consists of three parts, as represented in Fig. 7. Immediately surround. ing the wick is a non-luminous cone, formed of the vapour of the oily fuel, not in a state of combustion. Next to this is the bright part of the flame, and outermost of all, a thin blue envelope, so faint as scarcely to be seen without close inspection. In this outermost cone, the fuel, which consists of carbon and hydrogen, finds a full supply of oxygen, and is therefore completely burned, producing great heat, but little light. But to the portion immediately within this external envelope, the oxygen of the air has not free access ; and consequently, the carbon, which is the less combustible element of the two, is separated in the form of minute solid particles, which then become intensely heated, and give out a bright light. Some of this carbon is afterwards burned as it reaches the outside of the flame; the rest goes off in the form of smoke. A current of air blown through the flame by means of a tube of glass or metal having a fine aperture, facilitates the combustion of the carbon, and thereby increases the heat, but diminishes the light; this is the principle of the common blowpipe.

Fig. 7.

These general matters relating to combustion are of the utmost importance, both in a practical point of view, and as connected with the general laws of chemical action. They bear an important relation to the various modes of illumination by gas, or by lamps containing oil or other fluid. I cannot enter further into a consideration of them now, but we shall find ample opportunity of developing and explaining them as we proceed.

ZOOLOGY.-No. III.

CLASS ENTOZOA; OR, INTERNAL
PARASITES.

"Some get within him."-Shakspeare. THE word Entozoa means literally animals within, or inside of other animals. It is applied to those internal parasites by which the bodies of man, and of animals inferior to man, are infested. Eighteen distinct species are said to live within the cavities and tissues of the human body. Every known animal is believed to have one or more species peculiar to itself. If this be correct, the number of species belonging to the Entozoa would exceed that of all the other species of animals now living upon the earth.

Their structure is extremely varied. Some are so simple that they appear like little bladders filled with a watery fluid; as, for example, that one which, when abundant in the pig, gives to the flesh of the animal the appearance termed measly. Others are so different from this, that some naturalists are of opinion they might with greater propriety be arranged with the Annelids, or true worms of the class Articulata. This species has been found in the human body, in the eye, the brain, the substance of the heart, and the voluntary muscles.

I shall do little more than refer to two or three examples, illustrative of the variety of structure just referred to; but, as I would wish my readers to be assured that these despised creatures are worthy of investigation, I would beg their attention to what has been most truly and eloquently said by Professor Owen :

"In creatures surrounded by, and having every part of their absorbent surface in contact with the secreted and vitalized juices of higher animals, one might have anticipated little complexity and less variety of organization; yet the workmanship of the Divine Artificer is sufficiently complicated and marvellous in these outcasts, as they may be termed, of the animal kingdom, to exhaust the utmost skill and patience of the anatomist in unravelling their structure, and the greatest acumen and judgment in the physiologist in deter

mining the functions and analogies of the structures so discovered. What also is very remarkable—the gradations of organization that are traceable in these internal parasites reach extremes as remote, and connect them by links as diversified, as in any of the other groups of Zoophyta, although these play their parts in the open and diversified fields of nature."

Observations made on some species of Entozoa prove that their tenacity of life is not less than that already mentioned in the Rotifera. A minute worm that attacks wheat, and is essentially the same in point of structure with some of the internal parasites, has been dried, and, after periods of from four to seven years, revived by the application of moisture. Their power of enduring the extremes of heat and cold is very remarkable. A worm has been seen to exhibit strong contortions-evidently vital motions—after having been subjected for an entire hour to the heat of boiling water, along with a cod-fish, which it had infested. Rodolphi states that other Entozoa, which attack herrings, are annually sent with these fish to Berlin, hard frozen and packed in ice, and, when thawed, exhibit unmistakeable signs of restored vitality. These observations, it should be borne in mind, are made on the mature Entozoa; still, greater capabilities of endurance must naturally belong to the eggs, or ova.

This circumstance, and the extreme abundance in which the ova are produced, will enable us to account for their retaining dormant powers of life until placed in circumstances suitable for their development. Let us now turn to an example of one of their diversified modes of reproduction.

There is a kind of parasitic worm, known in these countries as the Tape-worm, Tania solium. It is found in the human intestines, and attains the length of three or four yards; occasionally even more than this. Now one curious fact about it is, that the very same species that infests the natives of Great Britain, infests also the Dutch and Germans. Another species, Botriocephalus latus, is peculiar to the Swiss and Russians; while the inhabitants of the French provinces adjoining Switzerland possess the unenviable distinction of being infested with both. It is strange that these creatures should thus be re

attaching themselves, by means of their sucker, to the body of the Mollusca. The tail now falls off, the worm buries itself in the mucous substance of the snail, and there remains, nearly motionless, like a caterpillar on its transformation into the Pupa. If we remove the little creature from its retreat, we find it now changed into a Distoma. The former animal was but the larva of the present one.

What now is the origin of the Cercaria? If, at certain seasons of the year, the viscera of one of the Mollusca referred to be opened, we find a quantity of worms of a peculiar form. The cavity of their bodies is filled with a mass of other little worms which a practised eye easily recognises as young Cercaria. The worm that contains them is but their living envelope; and on this account has been called the Nurse. These nurses, strange to say, are the offspring of other nurses, which have been produced directly from the eggs of the Distoma. If this be so, it takes four generations and one metamorphosis, to bring round again the likeness of the perfect animal from which they all originated; in other words, the parent would find no resemblance to himself in any of his progeny until he arrived at the great-grandson!

It would be rash to affirm that every species of fluke passes through a similar series of changes, for we must not venture in Natural History to reason from analogy further than to say that such an inference is probable. Every species must be carefully investigated by itself before we can be said to know its history. That this family (Trematoda) exercises a wide-spread influence is certain. They are found in the eyes of many animals, as well as in other organs. In the eyes of fishes they are particularly abundant. The little white specks which may sometimes be seen in the eyes of the common fresh-water perch, are in fact minute animals belonging to this group of zoophytes.

"Uncle Robert" is unwilling to bring this paper to a close without mentioning another creature, that to his younger readers may seem more interesting than those which have been treated of; and which, though classed among the Entozoa, does not pitch its tent among their habitations. Look well about the moist stones. on the sea-shore, and you may possibly

notice a little animal, about an inch long and a quarter of an inch broad, gliding over their surface, and looking like a bit of leather cut from a lady's glove, and endowed with life. It is a Planaria; some of these are found in fresh water, some in salt; they are carnivorous in their habits, and vary in colour from time to time, according to the colour of their food, so that the same individual may appear red one week, and green the next.*

We cannot say of them that they eat until they are "like to burst," for so voracious are they, that they actually do burst from their excessive greediness. The strangest thing, however, about them is the manner in which the body breaks into fragments, and each part becomes a perfect animal. Sir John Dalyell, in speaking of one of them, says" But, independent of propagating by eggs, the black planaria is privileged to multiply its species in proportion to the violence offered to its otherwise delicate frame. It may almost be called immortal under the edge of the knife. Innumerable sections of the body all become complete and perfect animals; if the head be cut off, a new head replaces it-if the tail be severed, a new tail is acquired." On a summer evening, when Sir John was looking at one of these animals, he saw a strange performancethe head was made to separate itself from the body, and crawl away-a feat equal to that of the "Headless Horseman of Sleepy Hollow," when he flung his head at the terrified Ichabod Crane. Thus it is that the facts of Science are more wonderful than the creations of Fancy. The truth is stranger than the fiction.

And now, reader, my promise to you has been fulfilled. I have introduced you to the unseen world of animal life. When next we meet, we shall survey the humblest of those creatures that reveal themselves to the unassisted eye, and assume a resemblance to plants and flowers. When we have made another step or two forwards, we shall pause, take a look at the ground we have gone over, and rest awhile ere we proceed.

Planariæ from within the tropic to lat. 47 deg. * Darwin found eight species of terrestrial south. They were striped, with bands of gay colours, and were found about decaying timber, on which they appeared to feed. + Observations on Planariæ, page 31.-Edinburgh, 1814.