area to be examined. When large pieces of quartz, the tempered glass, or selenite figures, are examined under these circumstances, the superiority of the illuminating arrangement is very decided, and allows of the picture being displayed on a much larger scale than would be otherwise practicable.

"2. Since Newton's prime experiment on the prismatic dispersion of white light, many contrivances have been used to produce the synthetical counterpart to this grand result of experimental analysis. The various methods of uniting the prismatic colors again, so as to restore the white light, are enumerated by Moigno, in his Repertoire d'Optique, Vol. IV. p. 1370, as given by Dove.

"It is desirable to obtain the original pure prismatic tints, and then unite the rays, if possible, by some process independent of refraction, and involving no obscure process. Von Münchow does this by giving a reciprocating motion to the prism, and Steinheil, by turning a prism, one side of which is ground or blackened so as to intercept the light, rapidly round an axis parallel to the refracting edge. In either case, as soon as the motion acquires a certain velocity, the colored spectrum vanishes, and a streak of white light appears in its place. This subjective mixture of the colors may be effected in a cheaper manner, by any one in possession of a water-prism, as follows. The light is introduced through one of the inclined glass sides of the prism, at such an angle as to emerge from the upper surface of the water. If the prism stands firmly upon a table, a spectrum will be projected upon the ceiling of the room. But a moderate tap with the finger upon the table will communicate a rapid vibration to the hollow prism, and thence to the water contained in it, and the little change in the refracting angle which ensues will make the spectrum dance backward and forward in the direction of its length. As soon as the dance begins, the spectrum, which hitherto had been of the usual colors, is converted into a long streak of white light.

"3. The only remaining experiment to which I shall allude in this communication carries us out of the province of Optics, and into that of Acoustics. It is well known that, when a tube filled with air is skilfully blown by the mouth at the embouchure, or a vibration in the inclosed column of air is otherwise excited, it is capable of rendering, without any fingering and without holes to finger, a certain series of sounds, which are called harmonics. In the humming-top, we may presume that the reaction is the same between the air inside

and outside of the top, as it is in the musical pipe: the only difference being, that in one case the air is driven by the mouth against the lip of the embouchure, and in the other the lip of the top is made to strike against the air. I have not been able to find any allusion to the question whether the note which the top yields varies with the force of the blast which the top itself gives by its own motion. But I have succeeded myself in producing two of the harmonics, that is, one besides the fundamental sound. Success in this experiment requires that an extraordinary velocity should be given to the top, greater than is possible except when the top is small. But if a small top is started with great energy, it gives out at first a high sound. As the velocity slackens, this sound dies out, and the top is silent for some time. Afterwards, as the motion diminishes, another and lower sound starts up, which is the one commonly heard in humming-tops, and the only one possible to excite by the usual way of starting the top, if it is large. This curious experiment may be worth recording, as carrying one step further the analogy between the theories of the sounds of the humming-top and of other wind instruments."

Mr. Sherwin alluded to a phenomenon accompanying the recent burning of a large building in Cambridge. The appearance to him, at the distance of several miles, was that of a tall column of light elevated about 20° above the horizon, appearing at first like some extraordinary meteoric phenomenon. It might be accounted for, he thought, by a series of reflections from clouds at different elevations, or perhaps more probably by mirage. In 1811 he had seen a similar appearance 24° high, which proved to have been caused by the burning of a paper-mill forty miles off.

Professor Horsford exhibited specimens of Cochituate water, together with the various products of his recent examination and analysis of it.

Four hundred and eighth meeting.

December 26, 1854. MONTHLY MEETING.

The Academy met by invitation at the house of the President. The PRESIDENT in the chair.

Professor Felton, at the invitation of the President, gave an account of the present aspect of Greece, from personal observation during a recent visit to that country. He represented its condition as being extremely interesting, from the promise which it gives of literary and scientific development. He spoke of the schools and universities, the public press, the condition of the language, and the prevailing spirit of the people, as giving ground for sanguine expectations of the regeneration of that classic land.

Dr. A. A. Hayes read the following paper on the present condition of the Cochituate water, entitled, "On a Remarkable Change which has taken place in the Composition and Characters of the Water supplied to the City of Boston from Lake Cochituate, by A. A. Hayes, M.D., Assayer to the State of Massachusetts."

"In the study of the chemical composition of waters used for domestic purposes, a wide field is open for inquiries of high scientific interest; as the accurate comparisons of different waters lead us through both departments of modern chemistry, the organic and inorganic. This interest is, however, secondary to the importance of careful inquiries in an economical view, as we have actions of waters on substances with which they come in contact at one point, modifying their composition, so as to render them purer, or less salubrious; and when a water passes some distance, its characters may thus be made to differ at different points. Not only is the water changed by different bodies with which it is brought in contact, but conduits of masonry or iron are in special cases rapidly destroyed.

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Although my observations on the water supplied to this city were among the earliest made before its introduction, they have been continued since, and within two years partial analyses have been made almost weekly for the purpose of learning the cause of any changes occurring. The results thus obtained will be given in a future paper, and the conclusions arrived at in a general form, while at present it is my intention to call attention to the condition of the water as it has existed for about ten weeks.

"Cochituate water, derived mostly from surface drainage, as it is found in the pond, or lake, belongs to the class of peaty waters so common in New England. It has not characters in common with the green or colorless waters of limestone formations, nor the medium or mixed qualities of our river waters.

"In its normal state, it may be considered as a pure water, holding in solution four or five grains of mineral salts in one United States standard gallon; and these consist of compounds of chlorine with sodium, potassium, calcium, and magnesium. Carbonates and silicates of these bases exist, in varying proportions, at different seasons.

"Its organic constituents, including the gases dissolved, are those of the most importance, as these give it particular characters, modifying its chemical relations, and affecting the taste, color, and purity of the fluid. In the spring and autumnal seasons there are found ulmic, humic, crenic, and apocrenic acids, and sparingly soluble compounds of these acids and bases, including alumina and oxide of iron. With these is a neutral body, which resembles mucilage from gum, and is usually in a changing state, especially while the water is warm in the summer season. The gases dissolved are oxygen, nitrogen, and carbonic acid; the nitrogen never has the volume relation to the oxygen which exists in air, being, except in rare instances, in smaller proportion, and instances have occurred when the nitrogen was no more than twenty volumes to eighty of the oxygen. The volume of carbonic acid also varies; while about one volume of all the gases exists in thirty-six of normal water. There are present also numerous animalcules and infusoria, fresh-water sponges, and abundance of ochrey matter, resulting from the chemical action of the water on the iron pipes. The animalcules indicate a state, which really exists, of a disturbed balance between the fish, the crustacea, animalcules, and subaqueous vegetation of the lake. Although throughout the year the water, at times increased by rains and melting snow, cannot be

classed with putrid waters, there are periods every warm season during which it closely approaches to these in characters.

"In the latter part of October last, I was watching for the increased amount of organic acids due to the decomposition of vegetable matter after a season of drought, succeeded by copious rains, when I was greatly surprised to find the humates and apocrenates giving place to crenic acid and crenates, accompanied by a perceptible odor of decomposing vegeto-animal matter, such as is emitted by freshly disturbed soil. This odor, which characterizes the humus from animal matter, continued several days; the water became colorless, while the organic matter, including carbonic acid, increased so as to exceed nineteen times the minimum amount previously found.

“The condensed vapor from the water had a strong odor of earth, or precisely that of guano from humid climates, and possessed an acid reaction. No more than mere traces of ammonia could be thus detected. When the water was mixed with lime and distilled, the condensed vapor was ammoniacal; proving that no carbonate of ammonia from the soil was present, but a salt of ammonia, due to decomposition. The earthy odor, or so-called taste, was succeeded by one closely resembling that of fresh-water fish, which, with slight variations of intensity, has continued nearly ten weeks.

"Before the water throughout the city became thus contaminated, the suggestion arose that the cause was local; the secondary main pipe supplying my dwelling having perhaps retained some decaying parts of eels or fish. A careful examination of the water was made, and by analysis a portion of oil was separated from water, which had been filtered through muslin to remove suspended impurities.

"By distillation the odor could be isolated from the water, which thus lost what was pronounced by good judges to be the flavor of fishoil; while the water retained the oil, almost destitute of odor.

"The general supply of water to a populous city had thus become very offensive, without any adequate cause appearing, and the evil led to the expression of many hypotheses and suppositions, chiefly without reliable support. As the subject was one which was within the reach of experiment, the course adopted was the following.

"A displacement apparatus of glass was charged with recently calcined animal charcoal, of medium fineness; over this was placed a conical filter of clean cotton, so that any water falling on the charcoal would first pass through the cotton filter. The water from a contracted supply pipe was allowed to flow slowly on the cotton filter, and passed