THE CHEMICAL NEWS. VOL. V. No. 112.-January 25, 1862. SCIENTIFIC AND ANALYTICAL CHEMISTRY. On the Adulteration of Butter with Animal Fats, by EDWARD BALLARD, M.D. Lond., M.R.C.P., Medical Officer of Health for Islington, and Parochial Analyst under the Adulteration of Food Act. [THIRD COMMUNICATION.] I FIND from my friend Mr. Horsley's last letter that I must recur to the question of the solubility of butter (pure and adulterated) in ether. He very courteously gives me credit for "meaning well," but I claim something more than this. I claim to be right. Nor can I endorse his ultimatum. I think that there is yet more to be said upon the subject, and this I propose partly to say in the present paper. It seems that it must partake also somewhat of the form of a reply. And my reply is this: 1. In Mr. Horsley's first communication he writes: "If a piece of this prepared butter be introduced into a wide-mouthed stoppered bottle and surrounded with ether at the temperature of 65° Faht., it ought to entirely dissolve, forming a clear, lemoncoloured liquid." He now says that he dissolves the butter in a phial "held in the warm hand for a minute or so." This is not dissolving it at 65°. That, when thus dissolved at a higher temperature, it should still be held in solution at 65° is altogether a different statement. That all his experiments were performed in a similar manner is a fact which, to my mind, vitiates them. 2. I must decline to bow to Mr. Horsley's criticism, on the ground that in his first paper he mentions no proportions of butter and ether, and, therefore, ought not to blame me for using those which are incorrect in my trial of his test. Doubtless, if an excess of ether be used, or the temperature be sufficiently raised, the solution might, as he describes, be perfect. But I also decline to submit to his criticism, because in the experiments he relates he uses the same "immoderate" quantities that he accuses me of experimenting on. If he reads my last paper, he will find experiments where the proportion of 10 grs. to 3j. of ether were those employed; still with a residue. 3. I assert that these last described experiments of his are not fairly conducted, even allowing the question of tempera ture to stand aside. He did not use adulterated butter where the ingredients had been fairly mixed by melting together, but the butter and the adulterant separate from each other. This is no trifling error, especially when lard was used; inasmuch as the physical conditions of the latter causes it, when unmixed with butter, to resist the action of the ether. Again, the appearance of the deposit is no criterion of its absolute quantity; a loose deposit, though bulky, may weigh wonderfully little, and vice versa. I know by observation that this is so in the instance in hand. And now, before passing to observations of a different character, one word in favour of Mr. Horsley. I think it probable that there may be butters which do dissolve in ether at 65°, in the proportion even of 20 grs. to 3j., although I hold that he has not proved it, nor have I yet met with such a sample. And the reason of my opinion is, that I have found butters which I have every reason to believe pure (barring the salt and water which all shop butters contain) leaving different amounts of residue when acted upon in my cylinder. I append the result of experiment in three of these, 20 grs being used with 3j. of ether and left in the water-bath at 65° for one hour: I. "Dorset " butter, sold at 1 s. 3d. per lb., melted with boiling water in a beaker formed a layer nearly uniform, the cellulations being so fine as to require a lens to distinguish them, and breaking down when cold and dried into a very fine mealiness, tasting only of butter, residue 33 grs. of the form and appearance of that from pure butter. 2. "Fresh" butter, sold at 1s. 5d. per lb., professedly "unadulterated," by the agent of a farmer in the country, melted with boiling water, presented in the melted form, and when cold, the same physical characters as above; i.e., those of pure butter, residue 1.8 grs, of same form and appearance. Here there are two samples which leave residues, the one greater the other less than in the pure sample I described in my last paper. The third sample stands alone. 3. Butter imperfectly churned, very soft and creamy, obtained from a friend, who prepares it for his own table. When melted and cold, physical characters those of pure butter as described above, residue 57 grs. When dry, bulky, loose, and honey-combed; deeply fissured in several places, and not aggregated like other samples of pure butter into the typical form. The cow from whom this butter was prepared was stall-fed, and the butter was white. Here there are all three grades of residue. It is possible to imagine butter with even less residue, till it amounts to nil. But I say this has not yet been fairly shown. And now for another branch of the subject. I think it is Dr. Hassall who has somewhere suggested that the consistence of butter at various temperatures may possibly furnish a test of their adulteration with foreign fats. It appeared to me that this suggestion was worth carrying out. But the consistence of a substance which may present every degree between complete fluidity and absolute hardness, passing through every stage of soft solidity, seemed at first so difficult to represent that for a long time I despaired of a method of testing it. And the difficulty was increased by the irregularity with which caloric diffused through a hard niass of butter or fat exposed to increased temperature. It also occurred to me that possibly the appearance of the butter, its clearness and limpidity at different temperatures, might tell us something; but here I found that the rapidity or slowness with which the heating was effected modified 44 On the Adulteration of Butter with Animal Fats. the result, as did also every irregularity in the rate of warming by introducing a disturbance either from convection or gravitation. The only satisfactory mode of carrying on such an inquiry thus turned out to be by at once raising the substance to a high temperature and allowing it to cool slowly, gradually, and uniformly. But then, how to measure and represent the consistency? I experimented thus: 3 10 My apparatus consisted of a test tube, length 44 in.; diameter, in.; an ordinary chemical thermometer, diameter of containing tube, in.; diameter of bulb, in.; length of bulb, 3 in.; and a half-pint beaker, to serve as a water bath. I used throughout the same tube, thermometer, and beaker, so that the comparison of the experiments might be fair and incontestible. The weight of the tube was 175 grains, and the quantity of butter, &c., acted on was in each instance the same, viz., 155 grains, making a total weight of 330 grains. Having weighed the butter, &c., in the tube, I pressed the thermometer to the very bottom of its centre and supported the whole upright in the beaker, which I filled to the I noted the point to which brim with boiling water. the thermometer rose, and then allowed the whole to cool spontaneously, noting also the time it took to attain various temperatures; and, lastly, I noted the time and temperatures at which various changes in appearance of the liquid took place, and at which it had acquired sufficient solidity to enable me to raise the loaded tube by the thermometer, which now became fixed in the fat or butter, first an inch or so above the water in the beaker, and next completely out of it, so as to allow of its being oscillated without permitting the thermometer to slip. I experimented upon pure butter,-the sample I mentioned in my last paper,-upon beef and mutton dripping, upon lard (home made), and finally on the several adulterated butters. I will endeavour to compress as much as possible the results of my experiments. First Experiment.-PURE BUTTER-Carefully dried, raised to 169°. Liquid, cloudy from minute points. Thermometer visible, but not the markings on it. After a few minutes the points became more distinct, and after 20 minutes and at 122° they had aggregated into an universally diffused fine flocculence, and the mercurial column had become visible. After 35 minutes, and at 108°, the first evidence of gravitation of the flocculi became apparent by the formation of a line of clear fluid at the top, which now gradually increased, until after 78 minutes, and at 82°, the clear layer, through which the numbers on the thermometer were quite legible, had attained the depth of half an inch (the entire of the butter occupying about an inch and three-quarters); the bulb had become now obscure by gravitation of the flocculi, but both this and the lower part of the stem were still visible through them. In 110 minutes and at 75° the clear layer had attained about one inch in depth, and the bulb, which at 76° was still obscurely visible, was now quite lost in the deposit. This total obscurity of the bulb was synchronous with a change in the clear layer (a change which occasioned the total obscurity by filling in the interstices of the deposited flocculi), viz. the appearance of minute points in the clear top layer. In 118 minutes and at 74° no further gravitation of flocculi had occurred, but the points in "clear" layer had become more distinct, and the deposit was to the eye denser and more opaque, obscuring totally all that part of the thermometer it surrounded. In Jan. 25, 1862. 127 minutes and at 73° the readings through "clear" layer slightly obscured; and in A 145 minutes and at 71° the numbers on thermometer in "clear" layer illegible, and scarcely even visible. slight secondary gravitation apparent from the greater obscurity of the markings through the upper than through the lower part of this layer. The stem of thermometer now gradually became lost to view, the opacity proceeding from below upwards, so that after 165 minutes and at 69° none of the stem of the thermometer was visible, the upper part of the "clear" layer, however, remaining more translucent than the lower. The punctiform appearance of the opacity still remained. 178 minutes and at 68°. The loaded tube could be raised one inch out of the water, and then the thermometer began to slip. In 222 minutes and at 66° it could be raised completely out of the water, but, after a few seconds of supporting, it began to slip. On repeating this experiment the same results were obtained, but the cooling was still further continued, viz. for 505 minutes and to a temperature of 60°, at which the thermometer still slid out. It was now left for the night, and in the morning, at a temperature of 55° the loaded tube could be supported out of the water and oscillated. Second Experiment.-BEEF DRIPPING—At 170° was liquid and almost clear, presenting a very trifling milkiness, the readings being clearly legible through it. By the lapse of 25 minutes and at 120° the milkiness had assumed the form of very fine points. After 60 minutes and at 90° these points had become more distinct, but not flocculent, and at the upper part the readings were slightly clearer than at the lower, showing a trifling gravitation. After 80 minutes and at 80° the opaque points were very distinct, and either larger or more numerous, for they were so diffused throughout the liquid as to render the After readings every where more obscure, but still legible. 85 minutes and at 79° the numbers on the thermometer were illegible; the bulb and lower part of the stem were slightly more obscure than the rest. In 88 minutes and at 78° the numbers and mercurial column were invisible. In 96 minutes and at 76° the thermometer was wholly invisible. In 107 minutes and at 74° the loaded tube could be raised a quarter of an inch out of the water. In III minutes and at 73° it could be raised nearly out of the water; and after 115 minutes and at 71° it could be raised and supported out of the water, and oscillated without slipping. Third Experiment.-MUTTON DRIPPING-At 177° was liquid and colourless, presenting a very slight milkiness, apparently due to very fine points; numbers on the thermometer were all very legible and distinct. After 64 minutes and at 91° points had gradually become larger and more distinct; the readings were equally legible, but rather less so at the lower than the upper part. In 71 minutes and at 88° points much larger, but no flocculence; numbers less legible. In 80 minutes and at 870 numbers illegible, but still visible. In 84 minutes and at 86° bulb and lower part of stem CHEMICAL NEWS, Jan. 25, 1862 On the Estimation of Sulphur in Iron and Copper Pyrites. 45 lost to view, and within one minute the whole of the stance which corresponded with that forming the floccustem was invisible. At 85° the thermometer was not lent deposit in butter, but the solid material began to be yet fixed. In deposited in points at a much higher temperature, viz. 95 minutes and at 84° the loaded tube could be sup-in mutton and beef dripping considerably above 90° and ported out of the water and oscillated without slipping. in lard about 88°. The difference is probably due less Fourth Experiment.-LARD (home made)--At to the difference in the melting point of stearine and 168 liquid and as clear as water, and thus without margarine than to the amount of the liquid elements in any remarkable change it continued for about an hour. these several fats. 8. The time occupied in acquiring After the same degree of solidity was for these four fats least in mutton fat and longest by far in butter, lard occupying a shorter time than beef fat. 9. Lard presents in the act of solidifying a remarkable rise in temperatureabout 83°. I found a similar phenomenon occur with a sample of lard which was purchased. 59 minutes and at 88° a diminution of clearness became evident. After 66 minutes and at 85° obscurity had increased, numbers everywhere still legible, but less distinctly at lower than at upper part, and bulb not quite so distinctly visible as the stem. After 68 minutes and at 844° numbers everywhere illegible. After 71 minutes and at 84° numbers and mercurial column only obscurely visible. Opacity distinctly punctiform. In 75 minutes and at 83° stem of thermometer only obscurely visible. And now occurred a remarkable phenomenon which did not occur with butter, or beef, or mutton dripping, and which was apparently due to rapid solidification. The temperature fell about a quarter of a degree, and then began to rise until it reached 83°. Thus, after Islington Parochial Laboratory, January 1, 1862. On the Estimation of Sulphur in Iron and Copper SULPHURIC ACID has of late years been almost exclu- At the present time sulphur is being replaced by iron. pyrites or by ferruginous pyrites, more or less rich in 82 minutes and at 83 the temperature became Sulphide of copper. This latter kind of pyrites is chiefly stationary for a minute or two, and then began again to obtained from the shores of Spain, whence it is exported fall. During this time the stem of the thermometer was to England. It serves at the same time for the manuquite invisible from opacity and solidity of mass. It facture of sulphuric acid and for the extraction of copper. attained after 104 minutes and at 79° it could be supported out of the water, and oscillated without slipping. 2. France possesses several deposits of pyrites. The factories of Paris, Lille, Chauny, Rouen, &c., are principally supplied from Chessy and Sain-Bel, near Lyons. Those of the south obtain their pyrites from the neighbourhood of Alais, and other factories derive it from Belgium and Rhenish Prussia. It will readily be imagined that several sources are necessary for a substance the annual demand for which approaches 100,000 tons. The composition of these pyrites being extremely variable, the mercantile transactions connected with them are necessarily based on the amount of sulphur they contain, and it is requisite to determine it frequently and with care. On the other hand, it is not requisite for the manufacturer to know the quantity of sulphur left in the residue after roasting the pyrites. He should endeavour to exhaust these residues as much as possible, for hitherto the roasted pyrites has not been usefully employed. It has been recently sought to be utilised for the manufacture of cast iron of an inferior quality, but appears to have been given up. This is easily explained when it is remembered that the unburnt sulphur which remains mixed with the oxide of iron amounts to 3, 4, and 6 per cent., and sometimes to a more considerable amount. The differences in the phenomena of cooling of these four substances may be summed up as follows:-1. The formation of flocculi in the cooling liquid is a peculiarity distinctive of butter. In the sample described the first opacity observed was in points. In another sample of pure butter-that which gave 18 grains residue with ether-mentioned in an earlier part of this paper, the flocculi were visible from the first, viz. at 171°. Butter at all temperatures above 55° is less solid and consistent than beef or mutton dripping or lard. It is still quite liquid when mutton fat and lard have solidified, and is only on the eve of solidifying when beef fat is solid. 3. The order of solidification of the four fats on cooling is mutton, lard, beef, butter. 4. Butter is quite solid only at a temperature of about 55° (in an experiment with the other sample of butter mentioned above it was solid at 59°). Perhaps it may be said that its solidifying point is at least from 12 to 16 degrees below that of beef dripping. 5. This difference is observable throughout the cooling. Thus complete opacity was observed in the mutton dripping at 84, in lard at 83° or thereabouts, in beef dripping at 76°, and in butter at 69°. 6. There exist in butter two substances at least which are insoluble in the olein and other conjoined liquid elements of butter. One of these is insoluble at temperatures between 74° and the highest temperature I have applied, viz. 212°. It is this which forms the gravitating flocculi. The other is I know that manufacturers of sulphuric acid are insoluble only below 74° or 75°. It is this which forms anxiously seeking for a simpler and more rapid process. the points during solidification in the "clear" layer. That which I am about to propose cannot fail to come 7. In mutton fat, beef fat, and lard there was no sub-into use, for it is in principle nothing else but an alkali In the present state of things, analyses of metallic sulphides are in general performed with accuracy, but unfortunately with extreme slowness. They are treated with aqua regia, the solution diluted with water filtered, and the sulphuric acid precipitated by a baryta salt. From the weight of the sulphate of baryta the proportion of sulphur may be calculated. This method requires, like all the methods of analysis by the wet way, a certain skill in chemical manipulations. metrical assay-of all industrial processes the one with- crucible itself. I throw it on a filter and wash it again out exception best known and practised. with boiling water. That will be understood when it is remembered that the manufacture of salts of soda is so bound up with the manufacture of sulphuric acid that a soda furnace is never seen in a factory without at the same time the leaden chambers being met with. My new process is based upon the property which chlorate of potash possesses in the presence of an alkaline carbonate of transforming into sulphuric acid the sulphur contained in metallic sulphides, especially those of iron and copper, the only ones employed in the manufacture of sulphuric acid. This re-action, if well managed, is complete; that is to say, the whole of the sulphur passes into the state of sulphuric acid. This unites with soda or potash, or with both these bases at once, which is immaterial when regarded from a purely analytical point of view. It is necessary to employ more carbonate of soda than is pointed out by theory, so as to avoid losing sulphuric acid. This excess of carbonate of soda is easily estimated by the ordinary alkalimetrical means. The neutralisation of the carbonate of soda is therefole performed twice, first by the sulphuric acid formed at the expense of the sulphur during the calcination of the above-named mixture, and secondly by dilute sulphuric acid of any known standard. Normal sulphuric acid being met with in laboratories, I employ this in preference to any other acid solution. This is of such a strength that 10 grammes of pure and dry carbonate of soda are exactly neutralised by 924 cubic centimètres of normal acid. These numbers correspond to equal equivalents of carbonate of soda and of monohydrated sulphuric acid. A litre of normal acid contains 100 grammes of monohydrated acid, or 32.653 of sulphur. Suppose, now, that in an analysis of pyrites I have employed five grammes of carbonate of soda. I know that it would have required 462 cubic centimètres of normal acid to neutralise it directly; but if after the combustion of one gramme of pyrites, for example, I only require 30'2 cubic centimètres of acid, that shows that there has been formed by the oxidation of the sulphur en amount of sulphuric acid precisely equal to that contained in 16 cubic centimètres of normal acid for 16 cubic centimètres and 30'2 cubic centimètres make together 462 cubic centimètres. There only remains, therefore, to calculate how much sulphur there is in 16 cubic centimètres of normal acid. I obtain this by the following proportion : 1000 c. c. 32 653 :: 16 c. c. : x=0'522 of sulphur. Thus I gramme of such a pyrites contains o'522 sulphur, or 52.2 per cent. of I now pass on to the description of my process. I will suppose that the analysis of an iron pyrites is to be performed. I accurately mix, in a porcelain mortar, 1 gramme of porphyrised pyrites, 5 grammes of pure and dry carbonate of soda, 7 grammes of chlorate of potash, and 7 grammes of fused or decrepitated marine salt. introduce this mixture into a platinum crucible, and gradually expose it for eight or ten minutes to a dull red heat; the marine salt is added to prevent too violent an action. When the mixture is nearly cold I add warm distilled water to it, and remove the solution by means of a pipette, and filter it. I repeat the washing five or six times, and finally boil the residue with water in the A little experience soon enables one to effect completely, and without any loss, the thorough washing of the substance operated upon. The solution and the washing waters are lastly neutralised with the normal sulphuric acid, without any modification of the method and precautions recommended by Gay Lussac. Supposing that it has been found necessary to employ 34 cubic centimètres of normal acid; subtract this from 46 2 cubic centimètres, and there will remain, therefore, 12.2 cubic centimètres, which represents the sulphuric acid formed by the pyrites. This number multiplied by 32.653, and divided by 100, gives the weight of the sulphur sought,-0398, or 39.8 per cent. A quartzose, barytic, or calcarious gangue does not in the least interfere with this process. The residue, after washing, should dissolve in hydrochloric acid without deposition of sulphur. It is easy to ascertain this, for, in a badly-managed assay, the sulphur separates from the gangue in the form of light flocks, recognisable by the blue flame and by the odour of sulphurous acid which they give when burning. When this happens, which is very rare, and indicates generally a badly-incorporated mixture, the analysis should be recommenced. I am satisfied (and this is an essential point) that there is no disengagement of sulphurous acid during the combustion of the pyrites by receiving the gas either in a warm solution of aqua regia, containing a small quantity of chloride of barium, or, which is still better, in a solution of permanganate of potash. I have detected neither the precipitate nor the decolouration, which are characteristic of sulphurous acid. I have made some other experiments to ascertain the accuracy of my process; they are the following: sharpness, for which I am indebted to the kindness of I. Specimens of pyrites in cubes of the most perfect M. Combes, yielded me in six analyses quantities of sulphur comprised in each case between 53 and 54 per cent. The formula FeS, contains 53.3 per cent. II. Specimens of natural and roasted pyrites from the manufactories of Channy have been analysed both in the laboratory of the factory and in my own, by the aqua regia and baryta method, and also by my new process. case These substances have furnished, by this double treatment, quantities of sulphur which in no differed from each other more than 1 per cent., and which generally corresponded. III. The product of the calcination of the mixture above named, well extracted with water and saturated with hydrochloric acid, gave with baryta the same weight of sulphate of baryta as by the ordinary aqua regia process. I have also found similar results with several specimens of copper pyrites. I have not hitherto mentioned any but iron and copper pyrites. I will now explain, in a few words, the appliIcation of my process to roasted pyrites in which the makers of sulphuric acid have so much interest in knowing the amount of sulphur, and of which they are constantly obliged to analyse a great number of specimens. In this case I dispense with, as useless, the employment of marine salt. I mix accurately five grammes of roasted pyrites, five grammes of pure and dry carbonate of soda, and five grammes of chlorate of potash. I expose the mixture to a dull red heat in a platinum crucible. The oxidation of the sulphur takes place The rest of the slowly and without any deflagration. experiment does not differ from that pointed out for iron and copper pyrites. If it has required 40 cubic centimètres of acid to neutralise it, it shows that the 5A grammes of roasted pyrites contained o 202 grammes of sulphur, or o'0404 for 1 gramme, or 4'04 per cent. When the roasting has been badly performed, it is not uncommon to find pieces of pyrites still containing from 12 to 15 per cent. of sulphur; in these cases the abovenamed proportions of carbonate of soda and chlorate of potash must be increased. In conclusion, I must urge the necessity of washing with boiling water, which is a matter of no difficulty; washing in the cold is long, and generally insufficient. The reason is, undoubtedly, that with pyrites having a silicious gangue, there is formed a small quantity of alkaline silicate which only dissolves easily in warm water. I will add, that any loss of carbonate of soda occasions an apparent increase of sulphur; this is evident, since the amount of sulphur is judged from the volume of normal acid employed to complete the saturation. The carbonate of soda lost will wrongly be supposed to have passed into the state of sulphate, and the calculation of the proportion of sulphur will be established on a false basis. It is, however, easy, with a little care, to avoid errors of this sort, and also of others which I will now mention. I need scarcely say that the carbonate of soda should be perfectly pure and dry, and that it must be weighed with as much accuracy as the pyrites itself. This care is not necessary in the case of the chlorate of potash and chloride of sodium. The proportion of the latter salt may be varied according to the combustibility of the pyrites, and must be increased until the oxidation of the mixture takes place without deflagration. Finally, the most necessary precaution of all consists in very finely powdering the pyrites, and mixing the whole very intimately together. To sum up, the new method of analysing the metallic sulphides consists in the combustion of the sulphur by means of chlorate of potash in the presence of carbonate of soda. The sulphur passes entirely to the state of sulphuric acid, which neutralises a portion of the alkaline carbonate. The excess of this salt is ascertained by the volume of normal sulphuric acid employed to complete the saturation; this volume is subtracted from that which would have been required, by five grammes of pure carbonate of soda to directly neutralise it, and the difference shows the amount of sulphuric acid produced by the pyrites. From the amount of sulphuric acid that of the sulphur may be obtained by calculation. The process does not occupy more than thirty or forty minutes; the errors involved in it do not exceed 1 to 1 per cent. of the weight of the sulphur to be determined. Ann. de Chim. et de Phys., Third Series, Vol. lxiii. Royal Institution.-On Tuesday, January 21, in the afternoon, John Marshall, Esq., delivered a lecture or "The Physiology of the Senses." On Thursday, January 23, in the afternoon, Professor Tyndall delivered a lecture on "Heat." Last evening Professor Rolleston delivered a lecture on "The Affinities and Differences between the Brain of Man and the Brains of certain Animals." On Saturday (this day) January 25, the Rev. A. J. D'Orsey will deliver a lecture on "The English Language," at three o'clock. PROCEEDINGS OF SOCIETIES. 47 ROYAL INSTITUTION OF GREAT BRITAIN. Course of Six Lectures on Light (adapted to a Juvenile LECTURE III. (Dec. 31, 1861.) NOTES TO THE LECTURE: When a ray of light passes from a rarer to a denser medium it is bent towards the perpendicular-When it passes from a denser to a rarer medium it is bent from the perpendicular-But the density or rarity here meant is not that which is expressed by the weight of a body. A body may be optically denser than another, though it be the lighter of the two-Spirit of turpentine floats on water, and is therefore lighter, or, in ordinary language, less dense than water; but a ray of light, in passing from turpentine to water, is bent from the perpendicular, and in passing from water to turpentine it is bent towards the perpendicular-In optics the densest body is that which refracts most. Conceive a ray of light passing from a denser medium to a rarer, striking the common surface of both so obliquely, that on quitting the denser medium it is refracted so as just to graze the surface-The angle between that ray and the perpendicular is called the limiting angle, and for this reason-Because no ray that strikes the surface at a larger angle than the limiting angle can get out of the denser medium. All such rays as striking the surface are totally reflected, according to the law mentioned in the notes of Lecture I.-The limiting angle then marks the limits of possible transmission from a denser to a rarer me lium. This is the only case in which the reflection of light is total-A liquid vein may be filled wi h light which caunot escape from the vein in consequence of total reflection- Mirage is produce 1 by the total reflection of rays passing obliquely int, the rare air close to the hot surface of the earth. Trees and houses may be seen thus reflected from the air as if from water. The human eye is composed of three principal optical parts; the aqueous humour, the crystalline lens, and the vitreous humour Behind the vitreous humour is the retina, which forms a screen to receive the images of external objects produced by the eye; these images are always inverted-For distinct vision it is necessary that the rays from every point of an object should come to a focus upon the retina-Some eyes reftact too much and bring the rays to a focus too soon: to remedy this defect a divergent lens is placed before the eye; this is short sight-Some eyes do not refract enough, and to help them we place a convergent lens before the eye; this is long sightDivergence is promoted by bringing the object close to the eye; ccnvergence is assisted by holding the object far from the eye; hence the terms "short sight" and "long sight." The action of light upon the eye does not subside the moment the light ceases; the impression endures in some cases nearly a quarter of a second after the light has ceased to shine-A succession of sparks, therefore, which follow each other at intervals of a quarter of a second, would appear as a continuous light; in fact, each impression would arrive before the preceding one had disappeared-To the eye a luminous object appears larger than it really is, and the more intense its light, the larger does the object appear. This effect is called irradiation-The full moon appear larger when looke at with the naked eye than when looked at through a dark glass. The bright new moon appears to belong to a larger sphere than the dusky globe which it partially encircles. The white light of the sun is made up of an infinite number of rays of different retrangibilities-Each particular refrangibility corresponds to a particular colour; hence the number of colours involved in solar light is inanite-But for convenience sake we divide these colours into seven, which are called primary colours. These are red, orange, yellow, green, blue, indigo, violet-Of these colours the red is the least refrangible, and the violet the most refrangible; the other colours being intermediate between these two-The solar beam is resolved into these colours by passing it through a prism; the coloured image thus formed is called the solar spectrum-The colours of the spectrum, when suitably blended, produce white light. I have now to say two or three words in completion of the memoranda of the last lecture. I have said there that "If two bedies refract a ray of light equally, one of them Now let me say ne cannot be seen within the other." or two words upon the refraction and reflection of light in I have completion of what I have already mentioned. here a large piece of glass with parallel polished surfaces; if a beam of light falls upon that glass you know that a portion of it is reflected, making the angle of incidence equal to the angle of reflection, a portior, however, goes through, and may be for the other side (ter having passed through the p After it bad the first sur |