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nads. Here, therefore, a moving globule and a monad are considered as identically the same. In order, however, to establish this, they should have been seen to take in colouring matter, and to contain internal cavities, which Ehrenberg has done with regard to all the true monads. Ingenhous, Treviranus, Hornschuch and Nees Von Esenbeck, if I mistake not, have said that the Priestley's matter consists of dead infusoria, and that from this congealed vegetable mass, as it is called, Confervæ, Ulvæ, Tremellæ, nay even the Hypnum riparia, have all been formed. Here I do not doubt the fact; times without number have I seen the Priestley's matter take its origin from green infusoria, such as the Euglena viridis, and the green pellicle in pools and infusions very generally consists of the dead bodies of infusoria, but never have I seen plants formed from this green matter; it continued long fresh, and became afterwards yellow, brown, run together, and fell to the bottom. When Confervæ grew in the infusion, or in the midst of these masses, there was never a transition between the dead infusoria and the living plants. Whether these plants arose from the spontaneous development of globules, or from the presence of infinitely small seeds in the water when drawn, I will not pretend to determine. Still less can I believe in the observations of Wiegmann, that the Entomostraci, Cypris and Cyclops are generated in putrid water from Priestley's matter, and after their death are changed into confervæ. He even asserts that he has seen confervæ springing out of the feet and antennæ. It is very possible that these animals have arisen on confervæ, but not from them.

VI. Structure of the Cercariæ. The exact observations and inimitable drawings which have been given of the Cercariæ by Nitsch, must be known to every one who has occupied himself with researches into the infusoria. Subsequently Baer has made some highly important observations on their formation, after Bojanus had called our attention to the subject. During last summer I had an opportunity of observing the living Cercariæ in some water which had been drawn from a pool for the purpose of examining the infusoria, and which had been at rest in a glass. It contained also many planorbes, and as I think the Pl. carinatus. On examining the water in a few days, I observed with the naked eye some very active animalcules, which I immediately recognised as cercariæ by their very rapid motions. Their numbers increased on the following days. My observations were afterwards interrupted. On closer examination the animal appeared to be the Cercaria Lemna, Muller (Encyclop. Method. Pl. 8. fig. 8. 12.), a species which, as Nitsch supposes, is probably identical with the Cercaria major (Beytr. zur Infusorienkunde, Tab. ii. fig. 1-8.), for it resembles it exactly ; only I could detect none of the fine hairs on the tail, which, however, only appeared to Nitsch with very high powers, and with the field of vision half illuminated. I employed a power of 48. For a long time I was quite delighted with the singular movements of these Cercariæ, but I turned my attention principally to their internal structure, which is not sufficiently known.

The cercaria which I observed was about į" in length. The tail and the body were nearly of the same thickness ; the latter was longitudinally striated. The substance of the animal was homogeneous, transparent, and of a milk-white colour. At the anterior extremity was the round extensile opening of the mouth, from which it appears to be surrounded with a wreath which was notched. This mouth is succeeded by a smaller very distinct oesophagus, between which and the mouth the intestinal canal seemed to be somewhat narrowed. I saw no trace of the forked organ noticed by both Nitsch and Baer ; nor was the course of the intestinal canal here marked by any dark spot, but it appeared to me merely to resemble a broad band extending from the mouth to the anus.

The ovaria were very conspicuous, placed on the two sides of the body. When the animal was moderately extended, they made several convolutions in the form of an S, from the two sides of the oesophagus to the cloaca. The convolutions were less distinct when the animal was much extended. The hinder extremity was very short and broad, where the ovaria formed two irregular masses. Their texture appeared granular, and had a beautiful appearance, their dark colour being contrasted with the transparency of the rest of the body. Their exit into the cloaca I could not distinguish. Their origin towards the anterior part of the body was very distinct by two dark spots. Nitsch took these spots for

eyes, which they certainly are not, but the origin (if not the exit, against which, however, we have all analogy) of the ovaria, which are here destitute of the granular tissue. These spots sometimes assumed a spiral, sometimes a forked or double forked, form. With regard to the organs of motion, the mouth is large, more or less extensile, and surrounded with a notched or plain margin or ring; it is bell-shaped, and is attached to the body by a narrow pedicel. The tail has lateral indentations and longitudinal striæ or fibres towards its middle. I am inclined to suppose a union of transverse and longitudinal fibres, true muscular tissue, for I am convinced that these animals possess muscles quite analogous to those of the higher animals, and Ehrenberg has demonstrated it with regard to the hydatina. How the tail is fixed to the body, I have not been able precisely to determine, but it is probable that a sort of prolongation is fixed into a notch in the hinder part of the trunk.

These few observations on the infusoria cannot be compared with those of Nitsch, Baer, and Ehrenberg; they are only a fragment towards the completion of the history of this vast kingdom of nature. In conclusion, I must add a word to explain why I have taken no notice of two recent observers of infusoria—Bory St Vincent and Muncke; but so much do I value the labours of O. F. Muller and Schrank, that I owe it to truth to assert that I consider the communications of the former as quite lost for science. When Muncke says that he has devoted from three to four hours every day for three weeks to the observations which were laid before the meeting of Naturalists at Hamburg, we can only lament that a man of acknowledged reputation as a natural philosopher has spent so much time uselessly upon matters with which he was quite unacquainted. It is worse with Bory; for it is quite incomprehensible how any man can be considered as a naturalist of eminence, who displays in his writings an inaccuracy, superficiality, and ignorance, that must be frightful' to every one who examines closely his works, and as revolting as the frivolity of which his Histoire Naturelle de l'Homme is so striking an example.

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An Exposition of some of the Laws and Phenomena of Mag

netic Induction, with original Ilustrative Experiments. By the Rev. WILLIAM SCORESBY, F.R.S. Lond. & Edin., Correspondent of the Institute of France, &c. &c. &c. Com

municated by the Author. THE

he magnetic principle, like the electric, to which it is nearly allied, is not a mere attribute of a particular class of bodies, but a principle or influence pervading, doubtless, the whole of the terrestrial creation. For it is not likely that such an influence has been ordained by Infinite Wisdom for the comparatively minor purposes to which men are able to apply it; but rather that it is an essential constituent in the economy of the globe,-and not of this globe only, but of the entire system, perhaps, of created nature. Wherever the exploring traveller has urged his way, there its influences have been marked ; wherever the adventurous mariner has traversed the ocean, there its agency has availed him; wherever the laborious miner has penetrated the earth, there its energy has been found undiminished ; and wherever the daring aeronaut has ascended into the atmosphere, thence its power has been extended.

Though a universal agent, however, and a part of the constitution of our globe, it is chiefly in ferruginous bodies, and in bodies in a peculiar electric condition, where its phenomena become sensible, and its influences capable of being controlled. In ferruginous bodies, its strongest and most permanent energies are exhibited.

In iron, the magnetic principle has evidently permanent residence--capable, indeed, of exerting external influences, but not capable of being abstracted or increased. Each portion and description of iron has its own constant and unalterable quantity, abiding apparently in its individual particles,--the two qualities possessing northern and southern polarity existing in every particle. The usual condition of the magnetisms is generally neutral, or nearly so, so that but a slight and imperfect energy is naturally evinced. Yet the latent energies, especially in the softest kinds of iron and steel, are readily developed by electric influence, or by the touch, or even the mere juxta-posi

VOL. XIII. NO. XXVI.-OCTOBER 1832.

R

tion of an active magnet. For though the natural quantity of inherent magnetism cannot be altered, yet an active and constant influence may be, and is, externally exerted, without in the smallest degree diminishing the original power of the actuating magnet. That this is the case, a well known fact sufficiently proves,—that a magnet may elicit the magnetic condition in ten thousand bars of steel, and yet retain its original strength perfectly unimpaired. And that the magnetisms reside inalienably in their own particles of the metal, is equally evident from another familiar fact,—that if a bar in a magnetic condition be broken in the centre, so as to separate apparently the northern and southern polarities, each portion, instead of comprising one quality only, will be found to be a distinct magnet, exhibiting, like the original mass, the two different polarities at the extremities.

These general principles being premised, we are prepared for the consideration of the particular objects of this Essay.

CHAP. I.-EXPOSITION OF SOME OF THE LAWS AND PHENOMENA

OF MAGNETIC INDUCTION. INDUCTION is that well known property of magnets, of producing in contiguous ferruginous substances the magnetic condition. It is not, however, strictly speaking, the communication of any thing previously foreign to the ferruginous bodies, as nothing (as we have shewn) is abstracted from the inducing magnet, and nothing in reality infused into the iron thus magnetized. Induced magnetism, therefore, may be defined, the development of the latent magnetism in iron or steel, by the juxtaposition of any substance in a magnetic condition. For this property is elicited not only by actual magnets, but by all electro-magnetic or thermo-magnetic arrangements. And the inductive effect is produced upon all substances capable of a magnetic condition, according to their respective susceptibility. The degrees of capacity of different ferruginous bodies for magnetism by induction may be ranged, beginning with the least susceptible, in the following order ; iron-ores, hard cast iron, hard cast steel, hard blistered steel, soft steel, common malleable iron, best Swedish iron,--the most susceptible of all being the softest and most ductile iron. Hence it is probable, that the measure of

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