duct, and specifies briefly the manner in which he conducted the process. As however his method is not noticed in other chemical works, and as our mode of proceeding and some of the results we have obtained, are, we think, not without novelty and interest, we deem them worthy of a brief notice in your Journal.

When alcohol is added to a strong aqueous solution of chromic acid in a retort, a very brisk reaction ensues, and upon applying a gentle heat there passes over a clear liquid, containing a considerable amount of aldehyde, with a faint trace of acetic and probably formic acids. The presence of the aldehyde is readily shown by adding a few drops of the liquid to a solution of nitrate of silver previously curdled by ammonia, and then gently heating the mixture. The oxide is speedily reduced, forming a brilliant metallic coating on the sides of the glass.

Substituting for the chromic acid of this experiment, a mixture of bichromate of potassa and sulphuric acid, and blending with this a quantity of common alcohol, the reaction is extremely violent, a large volume of carbonic acid is evolved, and the liquid which distils over, contains, with other products, much aldehyde and acetic acid. To obtain either of these substances but little mingled with the rest, special attention must be paid to the proportions in which the bichromate, sulphuric acid, and alcohol are mixed, and to the order in which they are brought together. Thus, in all our experiments, we found, than when alcohol is added in small quantities at a time to a mixture of the bichromate and sulphuric acid the distilled product is almost pure acetic acid, but when sulphuric acid is slowly dropped into a mixture of the salt and alcohol, the liquid which passes over contains little else than aldehyde.

This remarkable difference in the products is, we think, readily explained by the different intensity of the oxidating power in the two cases. In the former, the alcohol, as it falls into the mixture. of bichromate and sulphuric acid, being surrounded on all sides by free chromic acid, is carried rapidly through the lower stages of oxidation, corresponding to aldehyde and aldehydic acid, until by the addition in all of 4 equivalents of oxygen, and the elemination of 2 equivalents of water, it is converted into acetic acid.

In the latter case, the sulphuric acid, dropping slowly into

the mixture of alcohol and bichromate, liberates but a small quantity of chromic acid at any one time, and thus limits the oxidation of the alcohol in great part to the first stage, or that of the formation of aldehyde.

From the observation of these facts, we were led, after a number of comparative trials, to the following process for the preparation of aldehyde.

Equal weights of powdered bichromate of potassa and alcohol, sp. gr. 0.842 being placed in a capacious retort, connected with a receiver and the usual means of refrigeration, we adapt to the tubulure a pipette, charged with sulphuric acid, and whose stem reaches nearly to the surface of the liquid, the top of the pipette being furnished with a strong gum-elastic bag for injecting the acid into the mixture beneath. We now slowly add the acid, taking care to avoid excessive reaction, by sometimes allowing it merely to drop spontaneously from the pipette, and again when the action subsides accelerating its flow by pressure. At this period of the operation, the heat evolved in the retort is sufficient to carry over into the receiver a considerable volume of the aldehydic liquid; and, as much carbonic acid is at the same time disengaged, the tubulure of the receiver should only be loosely closed. Having thus added gradually a weight of sulphuric acid equal to about 1 times that of the bichromate, we apply a gentle lamp heat and continue the distillation as long as the aldehydic liquid passes over. When the reaction is most energetic, white fumes are evolved, which, falling from the beak of the retort into the receiver, are so dense that they may readily be poured from the latter through a funnel into a narrow necked bottle. These, when condensed, form a clear liquid consisting chiefly of aldehyde.

By this process 1500 grains of bichromate of potassa and the same amount of alcohol have on repeated occasions yielded us about 8 cubic inches of a clear liquid, containing but slight traces of acetic acid or other extraneous matters, and possessing all the characters of a nearly pure mixture of aldehyde and water.

The product thus obtained is sufficiently rich in aldehyde to exhibit instantly and strikingly all the characteristic reactions of that substance. It may, therefore, without rectification, be employed in class-room experiments and in testing for silver.

A few drops of this liquid, added in a test tube to a solution of nitrate of silver previously curdled by ammonia, quickly converts the oxide into metallic silver, which attaches itself as a brilliant coating to the glass. In describing this characteristic property of aldehyde, Liebig and others appear to regard the application of heat to the mixture as necessary to the effect, (Chem. Org., p. 377.) We have found however that the aldehydic liquid of the above process, as well as the more concentrated aldehyde procured from it by distillation over chloride of calcium, causes a complete and brilliant reduction of the oxide of silver in a few seconds at ordinary temperature, and that the same effect results even when the tube is immersed in snow, but in this case the change requires two or three minutes for its completion.

Heated with hydrate of potassa the liquid becomes yellow, then of a deep reddish brown color, and in a little while yields floating flakes of the characteristic resin of aldehyde.

On adding to the liquid an excess of caustic baryta in the cold, little or no action is manifested; but as soon as heat is applied, the mixture assumes an intense opaque yellow color like that of chromate of lead, which by continuing the heat passes into a deep rich brown, as in the preceding experiment.

The proportion of aldehyde in the liquid as it comes from the receiver in the above process, is such, that to prepare aldehydite of ammonia, we may dispense with the two successive distillations from the chloride of lime directed by Leibig, and use the fresh product at once for this purpose. We therefore add to the liquid about half its bulk of sulphuric ether, and pass a stream of ammonia into the mixture. As the aldehyde becomes saturated, the compound in question falls in an abundant deposit of brilliant transparent rhombohedral crystals. From this, as is well known, perfectly pure aldehyde is prepared by the reaction of dilute sulphuric acid.

In some experiments made to determine the delicacy of aldehyde as a test for oxide of silver, we obtained the following results:

1. A solution of 1 part of nitrate of silver in 1000 of distilled water, when heated gently in a test tube with a drop or two of aldehydite of ammonia, formed a brilliant metallic pellicle on the inner surface of the glass.

2. A solution containing 1 part in 2000 produced the pellicle in distinct spots and not continuous as in the former case. At the same time the liquid became of a deep greenish purple color, and although only one quarter of an inch thick was nearly opaque.

3. A solution of 1 part in 10,000 gave no adherent pellicle, but on continuing the heat for two or three minutes became strongly colored, presenting a deep greenish purple by transmitted, and a dull olive by reflected light.

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4. In a solution of 1 part in 20,000, the peculiar greenish purple tint was still quite decided, and even when the solution was diluted so as to contain only th of nitrate of silver this color was very distinctly manifested after heating it sometime with aldehydite. Compared with the faint opalescence caused by the addition of chloride of sodium to the same solution, this effect of the aldehyde appeared to be the more striking of the

two.

The peculiar purplish tint of the liquid, remarked in these experiments, is evidently due to the finely divided metallic silver held in suspension, and affords therefore a striking confirmation of the statement of Dupasquier, that not only gold, but silver and other opaque bodies, present this hue when greatly subdivided and viewed through a clear suspending medium, (Comp. Rend., No. 1, July, 1845.) It may be added that the same hue is developed when a very dilute solution of nitrate of silver is subjected to the reducing action of a formiate.

Allusion has already been made to the production of acetic acid in large amount by a modification in the above process. For this purpose the powdered bichromate and the sulphuric acid in the proportion of about 2 to 3, are to be first mixed in the retort so as to develop a large amount of free chromic acid. The alcohol is then introduced from the pipette as in the former case and with like precautions. During the violent action that ensues, much acetic acid passes over without the aid of external heat. When the alcohol thus added amounts to about twice the weight of the bichromate employed, the action having subsided, we apply a gentle lamp heat and obtain a large additional quantity of the acid. This we have found to be free from sulphurous acid and to contain little more than a trace of formic acid and aldehyde.

ART. III. On the Evidence of Fossil Footprints of a Quadruped allied to the Cheirotherium, in the Coal Strata of Pennsylvania; by CHARLES LYELL, Esq., F. R. S., F. G. S., &c.

(Communicated by the Author.)

On my way from Pittsburg to Philadelphia, I visited the rocks in which Dr. King, of Greensburg, first discovered, in 1844, the footmarks supposed to have been made by a large reptile, and which stand out in relief from the lower surface of slabs of sandstone, resting on a thin layer of fine clay. Having visited the quarry and enquired into all the circumstances of the discovery, having seen almost every specimen hitherto obtained, and considered the relations of the sandstone with the rocks lying above and below it, I have no hesitation in declaring my conviction, first, that the footprints are genuine, and were made by a quadruped nearly allied to the Cheirotherium of Europe, if not the same; secondly, that they occur in the middle of the carboniferous formation, having both above and below them seams of coal and strata containing Lepidodendron, Sigillaria, Stigmaria, Calamites, &c.

Great praise is due to Dr. King for the exertions which he has made in bringing these fossils to light, and duly appreciating from the first their extraordinary importance and value. A young man, engaged in an extensive medical practice in a remote town, acting under every discouragement which want of sympathy.in those immediately around him, and of access to scientific books or museums could produce, he has persevered and succeeded in forming a fine collection of the fossil tracks, and become acquainted with the relations and organic remains of the contiguous strata.*

I shall begin by describing the first locality to which the attention of Dr. King was called in 1844, a stone quarry about five miles S. E. of Greensburg in Westmoreland county, Pennsylvania. The slabs of argillaceous sandstone are extracted here for paving, but the excavation begun in the bank of a small stream was soon desisted from in consequence of the increasing thickness of the

Three papers have appeared in this Journal from Dr. King. The first describes and figures the footmarks from five miles S. E. of Greensburg, vol. xlviii, p. 343; the second, those of Derry, vol. xlix, p. 216; and lastly, the supposed hoofed-prints at Connelsville, second series, vol. i, p. 268. SECOND SERIES, Vol. II, No. 4.-July, 1846.

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