[46] The theory on which these object-glasses are constructed is contained in a paper of Euler’s, published at St. Petersburgh, in 1774. Messrs. Chevalier have caused it to be inserted entire in the “Bulletin de la Société d’Encouragement de Paris,” No. CCLIV., for Aug. 1825.
[47] This is the theoretical view of the case, the practical one is different, as frequently happens. Opaque objects are not affected at all by the eccentricity of the axis of the body: nor can I recollect that I ever felt any particular inconvenience from the motion of the optical part, even with transparent subjects, unless it was thrown very much indeed out of the axis of the illuminating mirror. On the other hand, it is notorious that living aquatic insects and animalcules are the most popular and entertaining objects which microscopes can show. These are, for the most part, abundantly restless; and if the stage on which they are placed has any motion, their natural unquietness is so much exasperated, that it becomes almost impossible to get a good observation of them at all. I once set to work at making some drawings of a variety of new and original objects of this class (which, I trust, will one day be published) with a microscope having all the requisite motions applied to its stage, and am confident that I had thrice the labour fairly appropriate to the execution of my task from this oversight. The very tremor produced by the transition of a carriage in the street, is frequently sufficient to unsettle live objects when disposed to be still and quiet, and put them in a fidget for a quarter of an hour. It is very unfortunate that the mountings of optical instruments are made in general by mere mechanics, who seldom or never observe with them, and consequently know not the exigencies which occur in practice.
It is still more unfortunate, that in the science of fitting up microscopes, an ounce of a man’s own wit is worth about a ton of his neighbour’s. Was it not that I dislike to verify this adage myself, I should recommend the following motion to be applied to the body:—let the socket of the arm which carries it have a smooth rotatory motion on the head of the bar in the usual way, conjoined with another horizontal one produced by rackwork attached to the said socket. Let the pinion which belongs to the latter movement be made very strong, so that a lever about three inches long may project from its centre: this is to be held in the hand, the thumb and index finger operating on the rackwork, while the two little fingers give a rotatory motion by working the lever end. This rapid double motion is here completely under the command of one hand, while the other is at liberty to adjust the focus. I know of nothing better for general purposes, or in particular for following the motions of live insects, &c.; but when only inanimate ones are to be the subject of microscopical study, I prefer the motion of the stage, for the reasons stated by Mons. C.
On the Existence of Chlorine in the Native Black Oxide of Manganese. By John M’Mullen, Esq. [◊]
IN the paper relating to this subject, which the editors of the Quarterly Journal of Science obligingly inserted in the forty-fourth number of that work, I described some experiments which I had made, to show that chlorine is uniformly evolved from the Native Black Oxide of Manganese by the action of sulphuric acid, under certain circumstances, which I endeavoured to detail with strict accuracy, so as to prevent any mistake or failure in the event of the experiment being repeated.
Upon this paper Mr. Richard Phillips has made some observations in the Philosophical Magazine and Annals of Philosophy for April last, to which I am desirous of briefly adverting. He says, “Mr. M’Mullen having observed, when sulphuric acid is added to peroxide of manganese, that chlorine is evolved, he conceived it might be derived from an admixture of muriate of manganese, iron, or copper; but having washed some of the peroxide of manganese with water, he did not find that any chloride of silver was precipitable from it; he, therefore, concluded that the peroxide in question contained no muriatic salt.” If Mr. Phillips will take the trouble to refer to my paper, he will find that this is by no means the statement which it contains. I observed that, in order to separate any soluble [p259] muriates which the oxide used might, in the first instance, have contained, “I washed it every day, for three weeks successively, using sometimes hot and sometimes cold water: at the end of that time, I tested the water, which was then decanted from the washed manganese, by the nitrate of silver, but without finding the slightest appearance of precipitated chloride. I then poured upon the manganese four times its weight of dilute sulphuric acid; allowed the mixture to stand for about four weeks, occasionally agitating it, and at the end of that time, found, when the dilute acid, now of a deep crimson colour, was removed from the subsident manganese, and the latter agitated, that the most decisive evidence of the presence of chlorine was exhibited in the vapour evolved from it.” I stated further, that I had “carefully preserved this particular mixture, and that after a lapse of more than twelve months, the residuary manganese, when the supernatant acid was removed, continued to evolve chlorine.”
In this experiment my object was effectually to purify the manganese used from any soluble muriate which might by possibility have been mixed with it: I did not, however, test the water first used in washing it, but merely that which was last removed from it.
Mr. Phillips proceeds to observe that he had prepared some observations, and at considerable length, to prove that the author of the above paper has been completely misled by “forced analogies” and “erroneous experiments;” but that it afterwards occurred to him, that it would be better to show, in a few words, the real source of the chlorine in question, the evolution of which from peroxide of manganese he had noticed some time previous to the publication of my papers. That with this view he had procured various specimens of the peroxide of manganese, (one of them in the crystallized state,) which were reduced to powder, and on the addition of sulphuric acid, chlorine was evolved from each. That he then washed separate portions of them with distilled water, and on the addition of nitrate of silver to the washings, chloride of silver was immediately precipitated: sulphuric acid being poured upon the washed peroxide, no chlorine whatever was evolved. That he further added sulphuric acid to an unwashed portion, and to one which had [p260] been washed, and referred both to a bystander, who immediately detected the odour of chlorine in the former, but not in the latter. He then proceeds to show that the specimens of manganese which he had made the subject of this experiment contained a portion of lime, and he infers that the black oxide of manganese consequently contains muriate of lime.
Mr. Phillips asserts that I have been misled by erroneous experiments. My reply is, that the experiment to which he refers, and which I have recapitulated, was carefully made, and is truly and faithfully detailed. In what, then, is it erroneous? It is not incompatible with that which he has produced as a refutation of it, inasmuch as he did not wait the result for which the perusal of my statement should have prepared him, and which he clearly should not have anticipated. He states that chlorine was not evolved from washed manganese at the instant when sulphuric acid was affused upon it. This is not a contradiction of my statement: I affirmed that, after washed manganese had been exposed to the action of sulphuric acid for a very considerable period, I distinctly observed the evolution of chlorine, and that for twelve months afterwards, under the circumstances detailed, this continued to be the case:—all this I deliberately re-assert. I have frequently met with specimens of manganese which, upon the first affusion of sulphuric acid, gave off chlorine; but in general, as far as my experience goes, this is not the case: were it of uniform occurrence, and that the mixture of sulphuric acid and oxide of manganese rendered chlorine evident to the smell, the fact could not have remained unnoticed till now.