The following passage is, in our opinion, inimitable :"Mulder's analysis is considered the best that has been made of Tea. It appears not to be a very good one; for Muriatic Acid has a strong sour taste, and if Tea had, onefifth of its total constituents, Muriatic Acid Extract, it should have a strong sour taste, which it has not; probably the Theine in the Tea which he analysed had turned into Muriatic Acid. As a proportional analysis, Mulder's can carry with it but little authority, for the totals of the parts of each of the Teas disagree. Still it has its use, for where Mulder states that there was no trace of a constituent, probably there was none."

iron prolongations, c, forming two cylindrical cavities in which the two armatures, m, n, are rotated in the same direction by two bands passing over a common drum. The armature, n, returns its currents into the electro-magnet, while the currents from m are utilised, as in producing the electric light between the carbon terminals, D.

In another form of the apparatus constructed by Mr. Ladd, the two armatures of the former are combined in one, and the coils are wound at right angles to each other as shown at A and B in Fig. 2.

Of these coils the shorter, B, feeds the electro-magnet, and the longer, A, furnishes the utilised current.

The results to be derived from these dynamo-magneto machines are always in proportion to the power expended,

WE noticed the first edition of this useful little volume on its appearance in 1866. It gives us pleasure to have the opportunity of here recording the issue of the third edition, containing several additions and improvements. Amongst these we notice a description of Wheatstone's and Siemens' automatic or selfcharging electro-magnet, and the ingenious method of utilising the current energy evolved by these machines in the modification constructed by Mr. Ladd. Dr. Noad's description of the somewhat complicated principle involved in these machines is so clear that we are induced to quote it for the benefit of our readers.

On page 32 he writes: "In this curious contrivance the terminals of a revolving armature are (with the intervention of a commutator, to change the direction of the alternate induced currents) placed in connection with the terminals of the electro-magnet coil. Now if the iron body of the electro-magnet were absolutely free from magnetism, no action would issue from the rotation of the armature; but practically that is never the case, for the mass of iron is never absolutely pure and soft; it consequently always retains a small portion of the magnetic polarity which was last induced in it, by the transmission of an electric current through its coil, consequently on the first semirotation of the armature, a minute induced current is evolved; but this, in consequence of the connection described, is returned into the electro-magnet coil, and slightly exciting the electro-maguet, a slightly stronger induced current is delivered by the armature, during its second semi-revolution; and so on successively, until the currents induced in the rotating armature acquire a very high degree of energy. If the primary coil of an inductorium be now interposed in the circuit as in Professor Wheatstone's apparatus, sparks of considerable length may be obtained between the terminals of the secondary coil; or a certain length of platinum wire will become incandescent. It may

here be remarked that as the energy of the induced currents is augmented, the resistance of the armature to rotation, and consequently the amount of dynamic energy necessary to rotate it is similarly augmented. This fact presents a strong confirmation of the theory that an electric current is merely an undetermined form or mode of energy, and that, by the mediation of the machine here described, dynamic is converted into electric energy."

Mr. Ladd's machine is similar to the above: he employs a second armature, placed between the contrary ends of the plates which constitute the electro-magnet, in which an equal current is induced as in the armature already described; this latter current may be utilised in any required manner.

Mr. Ladd has allowed us to use some of the very beautiful wood-cuts which adorn this book, to illustrate his dynamomagneto machine which gained the silver medal at the Paris Exhibition, 1867. In this machine (Fig. 1), the ends A of the electro-magnets, B, are firmly screwed to two pairs of

armatures can be rotated.

Mr. Beanes's ozone generator, of which so much has been heard and so much more is expected, is here described. The FIG. 1.

berless useful and agreeable products. Their numbers are constantly increased by new members, who enter upon science with the desire of seeing what it is, and are drawn on by an admiration of its beauty and harmony, to become themselves workers in its domain.

"Popular writers who hover upon its outskirts, describe to the world what they have seen, and the world judges science to be mainly destined as a handmaid to the industrial arts, and claims that as her highest use.

"Take as an illustration the report of the great Macaulay; he says that Bacon's philosophy aimed at things altogether different from those which his predecessors had proposed to themselves; that its object was to increase human comforts, to relieve more effectually the inconveniences of human life, to endow human life with new inventions and forces;' that it began in observations and ended in arts.'

"Again: he says that a follower of Bacon, if asked what the new philosophy has effected for mankind, would name the prominent material results which have followed from it. It has lengthened life, it has mitigated pain, it has extinguished diseases, it has increased the fertility of the soil, it has given new securities to the mariner,' and so forth; but not one word of teaching a better use of human reason, or of showing the true helps to the human understanding.

"So truly does this represent the commonly prevailing opinion of this country, that experimental science is almost exclusively studied by those who wish to apply it to some technical purpose: and most persons would be surprised to hear that the most important of all the applications of science is to the business of general education.

"A brewer wishes his son to learn chemistry, because he knows that the quality of beer depends upon chemical processes which can be regulated by one who understands them. A dyer values the science upon similar practical grounds. But a schoolmaster, who has to watch the direct growth of young minds, and to supply each with the food best calculated for its development, is satisfied that the knowledge of a couple of dead languages and of mathematics, fully qualifies him for the work; and while admitting the utility for technical purposes of scientific studies, he usually ignores their educational value."

"A century has not elapsed since a discovery was made which must ever rank amongst the most remarkable achievements of philosophy. From the first dawn of human reason, the process of combustion had been an object of wonder not unmixed with awe. Men gradually learnt to obtain fire and to use it for various purposes, while the nature of the process by which it was maintained continued an impenetrable mystery to them. Various substances were found, such as wood and resin, capable of feeding a flame and of keeping it alive, but the fuel gradually disappeared whilst supporting the flame, and was supposed to be destroyed by the process. Powerful intellects strove to work out an explanation of the process of combustion, and gave words instead of facts. The process remained a complete mystery till experiment was brought to aid the reasoning powers.

"In the year 1774, Priestley obtained a gas by heating calx of mercury, as it was called. He found that combustible bodies burn in this gas with far greater intensity and brilliancy than they burn in common air; and that charcoal forms carbonic acid by burning either in air or in this gas. He declared it to be the active principle to which air owes its power of supporting combustion. We now know that combustion could not be explained without a knowledge of this gas, and that in reality Priestley's discovery supplied the key to the whole mystery.

"His mind was, however, so constituted as to discover facts by experiment, and not to discover their place in the order of nature. Priestley's energies were not devoted to explaining the process of combustion; he was a conservative in theory, and he called his wondrous gas 'Dephlogisticated air '—a name which was retained until the new theory of combustion suggested a better one.

NEWS

July, 1869

"It was, however, not long before Lavoisier turned the discovery to account in his brilliant and masterly theory of combustion. He proved that this gas, which he named oxygen, unites with the materials of combustible bodies when they are burned in the air, and that the compounds thus formed contain the oxygen and the combustible materials. "Combustion is a combination which gives off heat and light, but which does not destroy the elements taking part in it. If we burn wood or tallow or resin, and collect the products formed by their combustion, we can recover all the materials of the wood, or tallow, or resin, and the oxygen which had united with them.

"Those who know the wondrous light which this theory has thrown upon natural processes, and the vast amount of knowledge of the properties of transformations of matter which it has rendered accessible to us, cannot hesitate to class it among the most important results which man has yet attained.

"It enables us to understand, to classify, and to describe an infinite number of processes of change discovered by chemists; processes of which a few only have been explained in any other way at all, and those few in a most inconvenient and clumsy way.

"If we compare man's present insight into nature (imperfect as it is) with that which he would have if this idea and its fruits were taken from him, it is like comparing daylight to the faint glimmer of starlight. We now see changes in the properties of matter which takes place when different kinds come together under particular conditions, and we learn how to regulate those changes by our knowledge of their nature and conditions, where formerly our uncertain glimpses gave us confused and very untrue impressions of destruction and production of different kinds of matter. We are delighted with a perception of natural forces working silently and irresistibly: and the more we observe and compare the results produced by these forces under various conditions, the more order and harmony do we find pervading the infinite variety of their manifestations.

"But while our power over matter has increased, we have become aware of its limits; for we now know that we can neither destroy nor create matter, only arrange, distribute, and alter its properties.

"Greatly as we must rejoice at these results, viewed as mere instruments of thought, they sink into comparative insignificance beside the method which led to their discovery, and which leads onwards to all future extensions of our knowledge. What made Priestley examine the properties of air itself and compare them with the properties of air in which a piece of charcoal had been burnt? What made him examine the gas given off by heating mercuric oxide or by heating nitre ? He was cognisant of the facts and ideas established by previous investigators respecting combustion in air; and he put such questions to Nature as were suggested to him by a consideration of the results before him, receiving in reply facts of momentous importance. What made Lavoisier establish the theory of combustion by arranging Priestley's facts in natural order among other facts relating to combustion, and describing that order? Both men had the same materials of the thought before them, and both were interpreters of Nature; but two minds more different in their constitution could hardly have been, and two pieces of work more different than theirs can hardly be found. Priestley found the materials, but could not arrange or explain them. He could not even see the explanation when given by another. Lavoisier's chief merit was in arranging the facts given to him. He measured the amount of the changes which had been discovered, and his theory of combustion is the simplest statement of the result of a quantitative comparison of those changes. He had a genius for discovering order-Priestley had a genius for discovering isolated facts. The work of each harmonises with each other, and their very differences made them the more necessary to one another.

"Some persons might attribute this great result to chance (a common name for ignorance of causes); but those who ex

palate. The taste communicated to water by even very minute quantities of iron, is one which most people would at once recognise. This source of error being known, nothing is easier, for a person possessed of only a smattering of chemical knowledge, than to determine whether the decolouration of the permanganate arises from iron. But the grand object of a popular water-test, so far as organic matter is concerned, is to afford the public a ready and simple means of detecting and rejecting water that may prove prejudicial to health. This the permanganate test satisfactorily accomplishes. If its use may sometimes lead to the temporary rejection, as unwholesome, of water charged with nothing worse than iron, the mistake will be on the safe side, and can never give rise to injurious results.

The popularisation of the permanganate test would assuredly tend to put an end to the occurrence of those outbreaks of fatal disease which are caused by the use of polluted water; and if this result were obtained at the expense of the occasional rejection of an innoxious water, it would be very cheaply purchased. But, what is more, the popular use of this substance would not only prove an effectual safeguard against those accidents to the public health which arise from the use of befouled water, but would afford the means of extemporarily purifying such water, and rendering it fit for consumption. Water that has been treated by permanganate will be found to be deprived of putrescible matter. Even repulsively foul water, after purification by this means, is not liable to become offensive. Permanganate solution, therefore, is not only of great value as a popular test for polluted water, but a safe and reliable means of freeing potable water from deleterious putrescible matter.

To Professor Attfield, I would reply that I am quite ready to admit that, instead of having proposed a new method of detecting organic pollution of water by the olfactory sense, as I, it seems, erroneously put it, he has merely suggested an improved means of rendering the nose a more efficient detective. As stated in my former communication, I consider Professor Attfield's suggestions on this subject well worth knowing, and I give that able chemist every credit for freely offering them to the general public. When he found fault with my letter in the CHEMICAL NEWS of the 3rd of April (Am. Repr., June, '68, page 288), for being similar to one which had appeared previously in the Medical Times and Gazette, he was not, perhaps, aware that the communication which called it forth had already appeared almost verbatim in another periodical.—I am, &c.,

Science Teaching in Schools.

J. MUTER.

To the Editor of the CHEMICAL NEWS. SIR,-In the last number of this journal, your correspondent “D." (Am. Repr., June, 68, page 292) criticises some lectures of mine recently given at Eton College, and I feel constrained to offer a word in reply, although it is to be wished that the writer had given his name in full.

It would appear to be unwise to judge of the nature of a course of lectures from the concluding sentences of the last lecture, because a generalisation founded upon the assumption that the whole is similar to an infinitely small part cannot be very trustworthy. In the case in point, the small part differs materially from the whole, because the concluding portions of a course of lectures, to a greater or less extent, summarise the matter of the entire course, and usually give the more plausible and dominant deductions. A work on arithmetic may commence with simple addition, and end with adfected quadratic equations; but it would be obviously an erroneous practice to deduce the nature of the whole book from its last chapter. As in the book, so in the course of lectures; we ascend from simple matters to those which are more difficult:-" Neque enim in plano via sita est," says Francis Bacon, "sed ascendendo et descendendo; ascendendo primo ad axiomata, descendendo ad opera.'

"If we may judge of the lectures themselves," writes

"D.," "from these extracts, I would say that such lectures are not adapted to attract boys to science." Allowing for a moment, for the sake of argument, that the lectures may be judged of from the extracts (which I trust has been disproved above), I may say that I fully concur with “D." "that such lectures are not adapted to attract boys to science." They would not be fulfilling their object if they sought to do this; for if science, in its plain unvarnished form, does not present sufficient attractions to the student, it ill becomes the teacher to make it externally showy and factitiously attractive, while the reality must ever remain the same. It would be like binding a book, on say, Hebraic punctuation, in the same kind of binding as a popular novel. Those who require to be attracted to science by forced means can never study it with advantage, nor should we ask such to commence the study.

I do not mean to say that lectures on natural science should be sparsely illustrated by experiments: there should always be a sufficiency, and there is a limit in both directions. A lecture which is insufficiently illustrated is quite as bad as -some would say worse than-one which is over-illustrated; in the latter, there are so many experiments, that it is impossible to have them properly explained; in the former there is too much dry explanation, which could as well be acquired from a book. It is the happy mean that we all desire to attain. A lecture on natural science can rarely be sufficiently illustrated by less than fifteen experiments, and it is usually over-illustrated when the number of experiments exceeds twenty-five.

Because science happens to be popularized in certain places and in certain well known magazines, there is a general impression that it is to be viewed as an amusement, and that it does not require serious study and application to master it. Nothing can be more erroneous. Until this idea is disproved, natural science can never be taught in schools with advan tage. If science is to be taught at all, it must receive from the pupil as much attention as he gives to Latin and Greek; it must not be regarded as an amusement or light study; notes must be taken, and themes must be written after each lecture, and a two-hour examination must be held at the conclusion of the course. Popular science is as much out of place in a school as popular Greek would be. To popularise a thing, is usually to degrade it. It is indeed right for us to rend the dark veil of the temple of nature, so as to expose the shrine to the worshippers without; but I think it is unnecessary to make the shrine garish with a false lustre. -I am, &c., G. F. RODWELL.

Estimation of Potash.

To the Editor of the CHEMICAL NEWS. SIR,-With reference to the abstract of our paper on the "Estimation of Potash," which appeared in last week's impression (Amer. Repr., June, '68, p. 284), we beg to state that on account of its requiring to be posted here on the evening following that on which it was read, we were unable to revise it.

Will you therefore kindly allow space for the following corrections in this week's issue, in order to prevent the necessity of any remarks or correspondence on the part of your numerous readers ?-We are, &c.

THE AUTHORS.

2nd, bottom line, p. 284, Am. Repr.),read sulphate of soda. Col. Corrections.-Col. 2nd, line 10th from bottom (Col. 3, line 12 from bottom (Col. Ist, line 4th from bottom, p. 285, Am. Repr.), read hydric sulphate or hydric chloride. Col." 3, line 17 (Col. Ist, line 26th, p. 285, Am. Repr.), read precipitates and evaporated washings. Col. 4, line 12 (Col. 2nd, line 19th, p. 285. Am. Repr.), read to dissolve the pure potassic chloride. Col. 4, line 13 (Col. 2nd, line 19th, p. 285, Am. Repr.), introduce the following paragraph:

The method which we ultimately adopted for purifying recovered platinum, is that by reduction with alcohol and