apparent coloured bands. The light may be held a few inches before the eye, and so that the incident and reflected rays may make but a small angle. This experiment is due to Mr. Whewell of Cambridge; but M. Quetelet, on repeating it, found that it was not constantly produced, and that the necessary condition was the presence of a slight film of vapour on the glass*. To make the experiment it is sufficient to breathe upon a cold mirror at the place where the image of the candle is to be reflected.

M. Quetelet has found that the experiment succeeds as well when the mirror is not silvered; even a piece of crown glass will do; but, from its irregularities, the bands are not so distinct. Daylight does not interfere with the observation. A drop of oil behind the glass makes the colours disappear. A line from the image of the eye to the image of the light is always perpendicular to the direction of these coloured lines. The bands affect the form of curved lines, which, in certain cases, degenerate into straight linest. They do not extend far beyond the image of the light. The colours proceeding from the light are bluish-green, yellow, red; bluishgreen, yellow, red, &c. Other circumstances being the same, the bands are larger as the observer is farther from the mirror, as the light is nearer to the eye, and in fact as the angle between the incident and the reflected rays is smaller.

This phenomenon does not appear to be related to that which Newton observed with concave mirrors. It appears, as to the

colours, to have more connexion with the effect observed when the sun or a light is seen through a transparent plate upon which has been spread a very fine powder.

The breath forms but a transient haze; but M. Quetelet has found an easy mode of rendering the preparatory state of the glass permanent. It consists in extending a very thin regular film of fatty matter, as oil or tallow, over the glass; a soft cloth is to be pressed lightly or dabbed over the whole surface of the film to destroy the parallel lines otherwise existing, and then the effect is obtained as well as with the breath‡.

9. SIZE FOR ILLUMINATORS, ARTISTS, &c.

Four ounces of Flanders glue and four ounces of white soap are to be dissolved on the fire in a pint of water, two ounces of powdered alum added, the whole stirred and left to cool. It is to be spread cold with a sponge or pencil on the paper to be prepared, and is much used by those who have to colour unsized paper, as artists, topographers, &c.§

* Many mirrors produce the effect without the film, in consequence of a slight granulation left upon the surface of the glass by the manufacturer.-Ed. We have never seen the bands in a flat piece of glass otherwise but straight. -Ed. § Ibid. E. xiv. 344.

Bull. Univ., A. xiii., 190, 192.

§ II-CHEMICAL SCIENCE.

1. GALVANIC CURRENTS DURING THE DECOMPOSITION OF

WATER.

The following description is from the personal observation of Professor Silliman. In the decomposition of water by the galvanic power, two tubes being filled with water, and inverted in a vessel filled with that fluid, their orifices being about one inch apart and the connexion established through the fluid by slips of platina, I had recently the satisfaction of observing distinctly the currents of gas as they took their departure to their respective poles. It has been a problem, whether the water is decomposed under one tube, or the other tube, or at some intermediate point; but, in the experiment referred to, ocular demonstration was exhibited, that the decomposition took place simultaneously, under both tubes, and not at any intermediate point. This appeared from the fact, that under each tube a current of gas rose vertically from the platina slip, and collected in the top of the tube, while another current shot off laterally and took up its march towards the opposite pole beneath the contiguous tube: as this process was going on at the same time under both tubes, it follows that there were opposite currents of gas, but they occasioned less mutual disturbance than might have been supposed; because the levity of the hydrogen and the gravity of the oxygen determined them to pass each other at different levels, and although many bubbles were buoyed up in the passage, and made their escape, and were lost by passing through the water intermediate between the two tubes, a large part of the gases was collected in the respective tubes. The process was continued for several hours with a large battery, and the currents were palpable to all the bystanders. With a magnifying-glass the appearance was beautiful, and nothing can exhibit more decisively the all-dominant power of the galvanic influence in causing even gaseous elements to separate at different points, and to pass horizontally, in opposition, through at least two inches of water, until they arrived at the poles by which they were respectively attracted: but, on examining the gases in the two tubes, so far from finding the oxygen gas in the one and the hydrogen in the other, there was found in both a highly explosive mixture, which gave a very sharp report when a flame was applied; and in fact the result was precisely the same as when the two tubes, standing in different vessels and furnished with metallic caps and depending platina wires, to connect them with the slips of the same metal below, are joined by a good conductor touching the caps.

Did the strong mechanical conflict of the two opposite currents cause the gases to be intermingled and thus to be in part carried into the stream? or did a portion of each gas fail to be expelled from the tube by the attractions and repulsions, and thus rise by mere

levity, to mingle with the gas appropriate to each particular pole *?

We can by no means consider Professor Silliman's account as at all altering the state of our knowledge relative to where the decomposition of water occurs, between or at the voltaic poles. The Professor seems to imply that it takes place at both poles, quoting the two currents from each pole as the proof; but there is no proof that the two currents were not of the same gas, i.e., both oxygen at the positive and both hydrogen at the negative pole; and, in fact, that is the only way of accounting for the mixture of both gases in both receiving tubes. There is great reason to believe that the arrangement of the gas at each pole into two currents, one internal and the other external to the receiving tube, was a mere conse quence of the descending water carrying off the smaller bubbles with it.-Ed.

2. POWER OF METALLIC RODS, OR WIRES, TO DECOMPOSE WATER, AFTER THEIR CONNEXION WITH THE GALVANIC PILE IS BROKEN.-(Berzelius.)

In the experiments which I undertook in 1806, 7, in company with Mr. Hisinger, we had found that rods of metal which were employed to decompose water by means of the galvanic pile, continued to develope gas after their connexion with the pile had ceased, a circumstance which seemed to indicate a continuance of electrical state, though these rods shewed no action upon any other portion of liquid, even of the same kind, than that in which they had been placed during their contact with the pile. This observation, which I had almost forgotten, has been lately confirmed by Pfaff, who has also added to it several others of a similar kind. We might suppose such effects to be produced by a residual polarity, both in the liquid and the metal, shewing itself, as long as it continues, by a continuation of chemical action; but some of Pfaff's experiments seem to oppose this idea, for he found that the addition of ammonia to the liquid, by which all its internal polarity was destroyed, did not deprive the wires of their effect. The metals which acquire this property in the highest degree are iron and zinc, next to which is gold. He attempts to explain the phenomenon, by supposing that the continued passage of the electrical stream had brought the elements of the water nearer to a state of separation, so that a very slight influence was sufficient to destroy their union. It must be confessed, however, that we cannot at present advance a satisfactory explanationt.

3. ON PYROPHOSPHORIC ACID AND THE PYROPHOSPHATES. Mr. Clarke first pointed out the singular change induced upon the phosphates by calcination, and, conceiving the acid was changed in its nature, gave it in its new condition the name of Pyrophos* Silliman's Journ., xviii. 199,

† Berzelius, Arsberättelse, 1829, p. 33,

phoric acid.

Gay-Lussac then gave further light on the subject, and now M. Stromeyer has published an investigation of the subject, which adds very much to what was before known.

M. Stromeyer first compares the two salts of silver, namely, the phosphate and pyrosphosphate, as those compounds which most strikingly exhibit the new characters impressed on the acid. Both these salts are pulverulent, and, when well dried, anhydrous; the first is yellow, the second white; the first has a specific gravity of 7.321, the second of 5.306. The first fuses with great difficulty, requiring a very high temperature, and cools into a yellow mass. The second fuses beneath a red heat into a brown liquid, which, by cooling, becomes a colourless, crystalline mass. Both salts are insoluble in water, both dissolve in nitric and sulphuric acid, and are precipitated unchanged; but when the pyrophosphate is heated in solution, it becomes ordinary phosphate. Muriatic acid decomposes it, but without changing the peculiar character of the acid.

All the metallic pyrophosphates, boiled with phosphate of soda, become phosphates, and form pyrophosphates of soda-the reverse does not take place. Hence pyrophosphoric acid should be placed after phosphoric acid in chemical affinity; and this alone establishes an important distinction between the two. Most of the pyrophosphates recently precipitated dissolve freely in the solution of pyrophosphate of soda. The same effect does not happen with the phosphates and phosphate of soda.

Hence, that a great difference exists between the phosphoric and the pyrophosphoric acid is evident, although the latter is obtained by calcining the former, or by burning phosphorus in oxygen; still there are plenty of reasons why the difference should not be due to either an excess or deficiency of oxygenation in this respect. M. Stromeyer shews that both are alike; neither does it depend upon more or less water combined, for the two salts of silver are both anhydrous, and yet their properties are distinct.

M. Stromeyer determined the composition of these two salts, and, by various modes of experimenting, proved that they contained different proportions of acid and base. The result of all his experiments was, that the proportions per cent. were as follows:

for equal quantities of acid, therefore, the quantity of oxide of silver in the two salts is as 35. This great difference in saturating power is the cause why, when a neutral phosphate of soda is calcined, it becomes strongly alkaline, for the phosphoric acid present, by becoming pyrophosphoric acid, loses two-fifths of its neutralising power, and yet this extraordinary effect happens without any loss of acid, or any change in the quantity of its constituents. The whole difference depends upon the manner in which the elements combine, and it is one more added to the very few decisive cases previously known, in which the mere mode of combination, and that too in a

binary compound, produces such differences of properties as to constitute the products real and distinct substances*.

4. PRODUCTION OF HYDROCYANIC (PRUSSIC) ACID UNDER UNCOMMON CIRCUMSTANCES.—(A. A. Hayes.)

Wishing to decompose some nitric acid containing about one-third its weight of dry acid, it was subjected to distillation with one-third of its weight of raw sugar; the distillation was attended by the production of vapours of nitrous and hyponitrous acids, as is usual in the decomposition of nitric acid. The fluid in the receiver was slightly acid, it was therefore returned to the retort still containing the residue of the first operation, and gentle heat applied; the strong and peculiar odour of hydrocyanic acid was developed, in such a quantity as to render the atmosphere of a small room irrespirable. After cooling the apparatus and decanting the distilled fluid, a few drops of ammonia were added, and the alkaline fluid, mixed with a solution of proto-sulphate of iron, and a few drops of acid, deposited a bulky precipitate, which, on exposure, became of a fine blue colour.-Rosebury Laboratory, March, 16th, 1830 †.

5. ACTION OF CHLORINE ON CARBURETTED HYDROGen.— (Morin.)

A memoir upon the action of chlorine on carburetted hydrogen, consisting of single proportionals of carbon and hydrogen, has been read by M. Morin to the Société de Physique, &c., of Geneva, of which the following is a brief abstract. The investigation was rendered necessary in consequence of the conflicting statements put forth by different philosophers of the nature, composition, and production of the resulting substance.

When chlorine and olefiant gas are brought together over water, a compound sometimes called chloric ether, or hydrocarburet of chlorine, is formed, which was analysed several years since by MM. Robiquet and Colin: they concluded, from all their experiments, that it consisted of equal volumes of chlorine and olefiant gas combined together, and in fact, it was well ascertained, that in these proportions the substance was abundantly produced, and the gases disappeared.

M. Morin analysed it by passing its vapour through a tube heated to dull redness: carbon was left in the tube and a gaseous mixture obtained, containing two volumes of muriatic acid gas, and one volume of a peculiar carburetted hydrogen, containing twice its volume of hydrogen, in combination with 0.6 of a volume (as the hypothesists say) of the vapour of carbon; 3.7 parts of the hydrocarburet of chlorine were used, and, according to the received opinion of composition, a fourth more of muriatic acid, and a third less of the carburetted hydrogen gases ought to have been obtained. Hence it appeared, that a very considerable part of the chlorine Silliman's Journal, xviii. 201.

* Ann. de Chim., xliii. 364.