be named hereafter, live "full fathoms five" under the sea, and must be sought for occasionally at depths of twenty or thirty fathoms, or even more-it may naturally be asked, by what means are they to be reached? The instrument most effective for this purpose is cheap, simple, and easily managed. It is known as the Naturalist's dredge, or as Ball's dredge, so called from Dr. Ball, of Dublin, the gentleman by whom it has been brought to its present state of portability and usefulness. One of these, now lying on the table beside me while I write, weighs only four pounds. The upper part consists of an iron rim of four sides, to which a bag of fine net-work, or open canvas, is attached (Fig. 21). The two longest sides are twelve inches each in length, the shorter, or connecting sides, three inches. The longer sides are thick below, and "bevelled" away above. Fig. 21. Attached to the shorter sides are two arms, which are united together above by a screw passing through a ring, and can be separated at pleasure. These arms are attached by pivots to the side, so that they turn with ease, and thus adapt themselves to the varying position of the dredger, the greater or less length of his rope, and the quicker or slower movement of the boat. Possessing this simple means of adjustment, the dredge, attached by a rope, which the dredger holds in his hand, is slowly dragged along the bottom, and marine productions of various kinds collected in its net or bag. One or two small wire sieves of different degrees of fineness are convenient for receiving the contents of the dredge when pulled up, and giving facilities for washing them from mud and sand, and separating the more delicate specimens from those by which they might be injured. I have found, also, a small basket, with divisions, such as those used by wine merchants for keeping bottles from coming into contact, very useful and convenient for holding in safety a few open-mouthed glass jars, to which living specimens might be at once transferred, and kept alive in sea-water for examination. One single "haul" of the dredge will sometimes furnish occupation for successive days. And, as I am speaking of implements, I may mention another, which is even more portable, and more easily managed-a towing net. A hoop of stout brass wire, about equal in diameter to the crown of a man's hat, can be procured almost anywhere. A bag of open canvas, or any similar material, is sewed round this hoop, and three strings attached, as shown in the annexed sketch (Fig. 22). They are joined to a cord, which may either be held in the hand or tied round any convenient part of the boat. On a fine summer day such a net can be used without toil or trouble; its contents can be transferred to the glass jars filled with sea-water; and endless is the variety of lovely animated forms that will thus present themselves to the eye of the observer. This simple, inexpensive, and easily managed apparatus, I would venture to recommend to all who spend a portion of the summer months at the sea-side. It will open to them a new source of amusement, and tempt them onwards in the investigation of nature. I have known a lady to convert an old veil into a most effective towing-net; and have seen her chimney-piece decorated with living blessoms, not less beautiful than those of the Fig. 22. parterre. There are some of those sea animals so transparent that they appear like crystals endued with life, and moulded into beauty; and I trust a time is now coming when the knowledge of them will not be confined to the man of science, but extended to all who, with pure eyes and simple hearts, look abroad on the fair face of creation and, in the rippling wave of the summer sea, no less than in the daisy-spangled lawn, seek out the marvels with which they are replete, and feel while doing so, the deep significance of the words, "The sea is His, and He made it, and His hands prepared the dry land." ; Attention, we think, depends essentially on desire. Whatever the individual, whether child or adult, likes, engrosses his mind. The tutor, or parent's object, therefore, ought to be to create and foster a desire, or pleasurable sensation, in the mind of the pupil in connexion with the subject on which he wishes him to fix his attention. The means of doing this are very multiform, according to the disposition and circumstances of the individual. We must here, however, confine ourselves to generalities, and cannot profess to give rules universally applicable. Speaking generally, then, a desire is created by the application of some stimulus; but let it be observed, that the stimulus employed should be, so to speak, as little artificial as possible, and should never be stronger than is absolutely necessary to produce the intended effect. When violent excitements are resorted to they often defeat their intended end; for though they may be attended with a certain kind of pleasure, yet by their violence they disturb rather than fix the attention. Thus, suppose an excitable child is promised, as a reward, on condition of getting his lesson by a specified time, some indulgence of which he is known to be particularly fond, the perturbation thus created is likely to prove a hindrance rather than a help to him in accomplishing his task. ON FIXING THE ATTENTION. THE power of fixing the attention is a most important point in the business of education. By fixing the attention, we mean the concentrating of the mind upon any particular subject, or object, to the exclusion, for the time, of all others. Some, indeed, have gone so far as to say, that the fixing of the attention implies the exercise of a peculiar intellectual power. But we believe that all, who have carefully watched the workings of their own mind, or the mental processes of others, will agree with us that this is by no means a true representation of the case. It is not, how- A good way of rendering the instruction ever, our object here to treat the subject in pleasing, is for the tutor himself to evince a speculative, but in a practical point of pleasure in the subject he teaches. He view. It is, therefore, more to our purpose should never forget that not only does intelto observe, that all persons, not labouring lect act upon intellect, but feeling also under any mental idiosyncrasy, possess to acts upon feeling. On the other hand, a greater or less extent the power of fixing there is nothing more likely to distract the the attention. It is important, too, that attention, and create a distaste in the mind every tutor and every parent should be of a pupil in connexion with any branch of thoroughly convinced of this fact. How instruction, than for the tutor to evince peevishness and impatience in teaching it. Let him especially beware of employing scolding as a cloak for his inability to explain. To avoid a scolding, the child will be ready enough to say he understands an explanation, perhaps really unintelligible. But let not the tutor fancy that in such a case he has gained the pupil's attention. He must first remove the timidity under which he labours, before it is possible he can really do so. often have we heard it said-"My little boy is of very quick parts, and can learn anything that he really tries to learn; but he is so listless and fickle that I cannot fix his mind to anything for any length of time together; and he seems to me as if he had not the power of doing so." We, as we have already observed, think he has; and that, in such cases, the fault is frequently to be attributed more to the want of proper tact or skill on the part of the tutor or parent, than to any mental peculiarity belonging to the pupil. Some persons seem naturally to possess, to a much greater extent than others, the power of winning and preserving the atten-notice of the pupil only one thing at a tion of children and young persons; but time. Attention to this point is a great it is, perhaps, not too much to say, that all secret in successful teaching. Every expemay, if they strive after its acquisition, rienced tutor is familiar with the superiority attain this power to a greater or less extent. of those class-books which make such an It is an acquisition, too, well worth the analysis of their subjects, that they present attention of him who takes upon himself only one difficulty to be mastered in each the office of teaching and training young step of the progress-take nothing for and immature minds, as his efficiency in granted of which the pupils can be ignohis sphere will depend in no small measure rant, and anticipate nothing which ought upon his possessing it. to come in at an ulterior stage. To pursue a course opposed to this, is an error into which all tutors are liable to fall. For instance, all, unless much upon their guard, are liable to make use of terms and employ language not level to the capacity of their pupils are liable to give their pupils credit for knowing things of which they may be ignorant. FAMILIAR LECTURES ON CHEMISTRY. LECTURE VII.-CHLORINE. The tutor should, in some measure, adapt his manner to the dispositions of his pupils. Many children are naturally of quick and lively habits of mind; and when the instruction is imparted in a slow and monotonous way, such children are apt to fall into a kind of sluggish quiescence, which shows that they are not thinking at all, or perhaps (which is more frequently the case) that their thoughts are wandering off to some other subject, quite foreign to that on which the tutor is engaged, and on which he wishes to fix their attention. On this subject Locke has a sensible remark, which seems worthy of quotation. "He that will observe children," he remarks, "will find that even when they endeavour THIS element exists in nature in great their utmost, they cannot keep their minds abundance, but is never found in the free from straggling. The way to cure it, I state. It forms an extensive class of comam satisfied, is not angry chiding or beat- pounds called CHLORIDES, many of which ing, for that fills their heads with all the are found ready formed upon and beneath ideas that fear, dread, or confusion, can the earth's surface, and in the water of the offer to them. To bring back gently their ocean. The most abundant of these comwandering thoughts by leading them into pounds is common culinary salt, which is the path, and by going before them in the a chloride of sodium; this, indeed, is the train they should pursue, without any re- source from which all the compounds of buke, or so much as taking notice (where chlorine used in the arts are ultimately it can be avoided) of their roving, Ì sup-derived. When this substance is heated pose would sooner reconcile and inure them to attention than all those rougher methods which more distract their thoughts, and, hindering the application they would promote, introduce a contrary habit." On the other hand, it is possible for the tutor to speak and to give his explanations too quickly, so that those pupils who are slow in their intellectual operations, are unable to follow him, and can obtain only an imperfect and confused notion of the subject; and, as confused thoughts always fatigue, it is not to be expected that, under such circumstances, the attention can be long preserved. In order to fix, the attention, it is well, as far as practicable, to bring under the with oil of vitriol, a gas is evolved composed of chlorine and hydrogen. We shall presently describe this compound more particularly; but for the present it is sufficient to say, that it possesses acid properties, and is hence called hydrochloric acidthat it is easily soluble in water, and forms a solution possessing the acid properties of the gas itself. Now, when strong hydrochloric acid is mixed with finely pounded black oxide of manganese, and a gentle heat applied, both substances are decomposed-the hydrogen of the acid taking up the whole of the oxygen contained in the oxide of manganese, to form water; while the manganese combines with half the chlorine, and the other half is set free in the form of gas. A mixture of sulphuric acid and salt may be substituted in this process for the hydrochloric acid; it is more economical, and is, therefore, always used in preparing chlorine on the large scale. The materials may be put into a Florence flask, fitted with a cork and a glass tube having two bends, as shown in Fig. 18. The tube passes to the bottom of a wide-mouthed bottle, so that the chlorine, Hi Fig. 18. which is a heavy gas, may displace the air and gradually fill the bottle. The mouth of the bottle should be loosely covered with a piece of card, to prevent the too rapid escape of the gas. Chlorine may also be collected over warm water (cold water absorbs it too quickly); but there is this inconvenience attending the method, that the gas, when it enters the bottles, has the temperature of the water through which it has passed; and as it afterwards contracts on cooling, a partial vacuum is left in the bottle, and, consequently, the pressure of the external air fixes the stopper firmly in the neck. In such a case, you may often loosen the stopper by immersing the bottle in hot water, so as to expand the gas; but it is best to avoid all risk, by collecting the gas by simple displacement, as shown in Fig. 18. Chlorine, under ordinary circumstances, is a gas of a greenish-yellow colour; its name is derived from the Greek word XAwpos, signifying that colour. By great cold and pressure it may be reduced to the liquid state.* The specific gravity of the gas is 2-4587; in other words, it is nearly The method of liquefying and solidifying gases will be explained hereafter. 23 times as heavy as common air. It has an extremely suffocating odour, and exerts a very painful and injurious action on the respiratory organs; even small quantities. considerably diluted with air, excite coughing; larger quantities produce painful inflammation of the air-passages, and often spitting of blood, which may last for several days; and the inhalation of a large quantity of the pure gas would, in all probability, be attended with fatal consequences, You must, therefore, be extremely careful in all your manipulations with it. The inconvenience attending its preparation may be considerably lessened by sprinkling ammonia, which absorbs it rapidly, about the table. Chlorine has very strong combining tendencies; it unites with most other elements, even more energetically than oxygen. Many substances introduced into it at ordinary temperatures, take fire spontaneously this is the case with phosphorus, with antimony in the state of fine powder, and with thin copper-leaf, or Dutch metal. The compound formed in each case is a chloride of the substance burned: the chlorides of phosphorus and antimony thus produced are oily liquids. But the most remarkable and important property of chlorine is its strong tendency to combine with hydrogen. These gases unite in equal volumes, forming the acid gas already mentioned by the name of hydrochlocation of flame, or when an electric spark ric acid. The mixture explodes on the appliis passed through it, with much greater violence than a mixture of hydrogen and oxygen-so violently, indeed, that only the strongest vessels will bear it. The action of light also determines the combination. A mixture of chlorine and hydrogen may be left in the dark for any length of time; but when it is exposed to ordinary daylight, combination takes place gradually; and if a direct ray of the sun falls upon the mixand with explosion. If & Florence flask, ture, the gases combine instantaneously filled with a mixture of equal volumes of chlorine and hydroge., and having its cork secured by a string, be thrown up in the air from the shade into the sunshine, the mixstrike upon it, and the flask will be shivered ture will explode as soon as the sun's rays in pieces. The affinity of chlorine for hydrogen causes it to decompose and destroy a great number of organic substances, which are compounds of the latter element with carbon, either alone, or with the addition of oxygen and nitrogen. Oil of turpentine is a compound of carbon and hydrogen. If a piece of paper soaked in that liquid be introduced into chlorine gas, the turpentine instantly takes fire, burning with a red flash and depositing a large quantity of black, finely divided charcoal. A lighted taper burns in chlorine gas with a dull red flame, and produces a mixture of black and white smoke; the white smoke is hydrochloric acid, the black is carbon separated in the form of minute solid particles. Chlorine destroys all vegetable colours, and is consequently a powerful bleaching agent. Leaves, flowers, paper, or linen, stained with litmus or indigo, &c., are instantly bleached when introduced into this gas. It is necessary, however, that water be present; for dry chlorine has no action whatever on dry vegetable colours; hence it is supposed that chlorine bleaches by decomposing water, taking up the hydrogen and setting the oxygen free, and that the oxygen is the real bleaching agent. That Oxygen possesses strong bleaching powers is known by the action of peroxide of hydrogen. Chlorine is extensively used as a bleaching agent on a large scale: the most usual mode of applying it is to steep the articles to be bleached, in a solution of chloride of lime, a substance to be presently described. The same affinity of chlorine for hydro- | gen causes it to act with great energy in destroying those organic compounds which infect the atmosphere and render it prejudicial to health; the poisonous exhalations from decomposing animal and vegetable substances, and the infectious matter of fevers and other disorders, are speedily removed by introducing a small quantity of chlorine into the infected atmosphere. Cold water absorbs twice its volume of chlorine, the solution possessing the odour and bleaching power of the gas itself. Chlorine and water likewise form a definite compound, a solid hydrate, containing 35.5 parts of chlorine and 90 of water. This compound is easily formed by introducing a small quantity of water into a vessel filled with chlorine gas, and exposing the whole to a temperature of 32° Fahrenheit; the hydrate is then produced in the form of small needles. Chlorine and Hydrogen-HYDROCHLORIC ACID. It has already been stated that this compound is formed of chlorine and hydrogen united in equal volumes. Now, chlorine is 35 times as heavy as hydrogen (compare their specific gravities); hence it follows that the composition by weight must be 35.5 chlorine to one part of hydrogen. Moreover, the gases unite without condensation-that is to say, one volume of chlorine and one volume of hydrogen united make exactly two volumes of hydrochloric acid gas.* To find the specific gravity of this gas, then, we have only to add together the specific gravities of chlorine and hydrogen, and divide by 2; thus Sp. gr. of hy- = drochl. acid You have already seen that this compound may be formed by the direct union of its elements; this, however, is by no means an advantageous method. The practical mode of obtaining it consists in decomposing a metallic chloride with oil of vitriol: the most advantageous, because the cheapest that we can use, is the chloride of sodium, or common salt. The hydrogen of the oil of vitriol then combines with the chlorine, forming hydrochloric acid, which escapes as gas; and sulphate of sodium (a compound of sulphur, oxygen, and sodium) remains behind in the vessel. The action may be thus represented: Before decomposition. COMMON SALT. OIL OF VITRIOL. 1.0 hydrogen 16.0 sulphur 320 oxygen f 107.5 After decomposition. 36.5 hydrochloric acid. 710 sulphate of sodium. 107.5 The gas may be collected over mercury; accompanied by condensation, as we shall see by numerous instances. *The combination of gases is, in most cases, |