indicated in geological language as the Mesozoic period--the middle of the secondary rocks.

But Australia presents us with a yet more interesting case of "survival." Certain fish-teeth had from time to time been found in deposits of oolitic and triassic date, and the unknown creature to which they once belonged had received the name of Ceratodus. Only five years ago this animal, supposed to have been extinct for untold ages, was found still living in Queensland, where it goes by the name of "flat head." It is a fish of somewhat amphibious habits, as at night it leaves the brackish streams it inhabits, and wanders amongst the reeds and rushes of the adjacent flats. The anatomy of this animal has been carefully described for us by Dr. Günther.

We have, then, in Australia what may be termed a triassic land, still showing us in life to-day the more or less modified representations of forms which elsewhere have long since passed away from amongst us, leaving but rare and scattered fragments -relics "sealed within the iron hills."

No member of the Australian families of the kangaroo's order has left its relics in European strata more recent than the secondary rocks. But the American family, Didelphida, is represented in the earliest tertiary period by the remains of an American form (a true opossum) having been found by Cuvier in the quarries of Montmartre. He first discovered a lower jaw, and, from its inflected angle, concluded that it belonged to a marsupial animal, and that therefore marsupial bones were hidden in the matrix. Accordingly he predicted that such bones would be found; and, proceeding to remove the enveloping deposit with the greatest care, he laid bare before the admiring eyes of the bystanders the proof of the correctness of his prediction. It is noteworthy, however, that had this fossil been that of an animal like the Tasmanian wolf, he would have been disappointed, as, though marsupial, it has, as has been already said, not marsupial bones, but cartilages.

But relics of creatures more closely allied to the kangaroo existed in times ancient historically, though, geologically speaking, very recent. Just as in the recent deposits of South America we find the bones of huge beasts, first cousins to the sloths and armadilloes which live there now, so in Australia there lived beasts having the more essential structural characters of the kangaroo, yet of the bulk of the rhinoceros. Their bones and teeth have been found in the tertiary deposits of Australia. They have been described by Professor Owen, and are now to be seen preserved in the British Museum and that of the Royal College of Surgeons. It may be that other fossil forms of the middle mesozioc or even of triassic times may, as some believe, have belonged to creatures of the kangaroo's family;

but at least there is no doubt that such existed in times of posttertiary date.

As to our third point-the geological relations of the kangaroo-we may say, then, that "the kangaroo is one of an order of animals which ranged over the northern hemisphere in triassic and oolitic times, one exceptional family lingering in Europe to the Eocene period, and in America to the present day. That the kangaroo itself is a form certainly become fossil in its own region, where, in times geologically recent, creatures allied to it, but of vastly greater bulk, frequented the Australian plains."

We may now, then, proceed to answer finally the question, "What is a kangaroo?" We may do so because the meaning of the technical terms in which the answer must necessarily be expressed (if not of undue length) have been now explained, as far as space has allowed.

We may say, then, that "the kangaroo is a didelphous (or marsupial) mammal, of the family MACROPODIDE; an inhabitant of the Australian region, and connected, as respects its order, with triassic times, and possibly even as regards its family also, though certainly (as regards the latter) with the time of the post-tertiary geological deposits."

We have seen what are didelphous and what are menadelphous mammals; what are the respective values of the terms "order," "family," and "genus," and also in what respect the kangaroo differs from the other families of the marsupial order. We have also become acquainted with the distribution of organic life now and with the inter-relations of different geological strata, as far as those phenomena of space and of time concern our immediate subject.

By becoming acquainted with these matters, and by no other way, is it possible to give an intelligent answer to the question, "What is a kangaroo?"

DESCRIPTION OF PLATE CXXVIII.

FIG. 1. Macropus Parryi. (From the Transactions of the Zoological

3.

Society, vol. i. p. 300.)

Sole of Right Foot of Kangaroo.

Cuscus Orientalis. (From the Proceedings of the Zoological Society for 1858, Pl. LXI.)

4. Thylacinus Cynocephalus. (From the Proceedings of the Zoological Society for 1850, Pl. XVIII.)

395

RECENT DISCOVERIES IN PHOTOGRAPHY.

BY J. TRAILL TAYLOR.

A

MONG recent discoveries in photography there are two which appear to exercise an important influence upon the future practice of the art-science. One of these refers to the production of negatives, the other to the printing of positives, or proofs upon paper.

As hitherto practised, the production of a negative involves several manipulations of a somewhat messy nature. The argentic halogens, iodide and bromide of silver, which constitute the sensitive body in the collodion film, have been formed by double decomposition, a collodion containing a haloid salt being immersed in a bath of nitrate of silver, by which iodide of silver is precipitated in the film, together with the nitrate of the base of the salt, which, being soluble, remains in the bath to contaminate the silver. If the plate is to be dried and stored away for future use, it is now subjected to a series of washings to eliminate the soluble salts, after which it is "preserved" by the application of an organic solution such as gelatine, tannin, gum, and other bodies.

With a view to simplify these complex processes it had long been considered desirable that the haloid salts of silver should be mixed with the collodion, so as to ensure by a single operation the coating of the plate with a sensitive layer. But it was found that the large atoms of iodide of silver would not emulsify with collodion; and all attempts at producing a sensitive collodion failed, until about ten years ago, when two Liverpool amateurs, Sayce and Bolton, solved the problem of producing a sensitive collodion emulsion by discarding entirely the iodide of silver, and substituting for it bromide of silver. The best conditions under which this bromide emulsifies has for several years formed a theme for the investigations of scientific photographers, and these researches have greatly conduced to the high state in which the art exists at the present time. Such is the state of perfection to which the system of preparing sensitive

plates has been brought during the past year, that all baths, washings, preservatives, and organifiers may now be entirely dispensed with, the sensitive emulsion being so composed as to contain within itself everything that conduces to the rendering the sensitive film complete.

A practical difficulty that long existed in the preparation of a sensitive emulsion lay in hitting so exactly the combining equivalents of the salts used in sensitising the collodion as to leave neither of them in excess. The bromide of silver being formed in the collodion by the decomposition of nitrate of silver and a soluble bromide, there was a difficulty, well-nigh amounting to impossibility, in so combining them as not to allow either to predominate. To make a combination in water is easy; but not so is it in a thick, viseid liquid like collodion. If the silver be left in excess, fogging of the negative is certain to follow, unless a restrainer like mineral acid be added; if, on the other hand, the bromide preponderate, the plate is insensitive in proportion to that excess; and hence for some time it was customary to have a much larger proportion of bromide present than was necessary, and after coating a plate with an emulsion of that kind to confer sensitiveness upon it by washing with water, so as to effect the removal of the free bromide which acted so powerfully as a retarder. A further necessity for having the free bromide removed was found in the fact that when an image is impressed upon a film containing it, unless that latent image be quickly developed, it is rapidly destroyed by the soluble bromide.

It has already been said that a result of the decomposition arising from immersing a salted collodion plate in a nitrate of silver bath is not only the formation of bromide of silver, but also of the nitrate of the base of the haloid salt. The presence of this nitratę in a wet process is of minor consequence, but far different is it when it exists in a collodion film that is to be dried; for on crystallising out, as it must necessarily do if present in a moderately large quantity, it disintegrates the film; and even if quite innocuous in a chemical sense, its presence is fatal in a physical point of view.

The maleficent influence of the crystallisable salt resulting from the decomposition had previously been noted by Mr. J. King, of the Bombay Civil Service, during a brief visit to this country; and when making experiments with gelatine instead of collodion as a vehicle for the sensitive bromide, he, by a happy application of the principle of dialysis, succeeded in effectually removing every crystallisable compound, as will be presently shown. The method subsequently adopted by Mr. Bolton in effecting a similar removal on behalf of collodion was very complete, inasmuch as he not only eliminated the crystallisable

salt but also added the requisite organifier or preservative body, by which the pores of the film are kept sufficiently open to be permeated by the developing solutions afterwards to be applied.

The simplicity of the method adopted is great, its efficiency is obvious. A collodio-bromide emulsion that has been so nearly adjusted in relation to the predominance of one salt over another as to be in moderately good working order is poured into a large flat dish. After a few hours, when the thick film has become set, a small quantity of distilled water is poured upon it and the film divided into squares by means of a paper-knife or silver, fruitknife. By thus breaking up the film and subjecting it to a few changes of water all the soluble matter is entirely removed, this removal having been facilitated by the addition of a little glycerine to the emulsion before it was poured out to set. The function of the glycerine is mechanical, not chemical. When the whole of the crystallisable salts are removed the film is dried, and is either ready for being re-dissolved immediately, or for storing away for future use. There appears to be no limit to its keeping powers, provided it be kept in a place from which light is excluded. To render this dried pellicle ready for use it is only necessary that it be dissolved in a mixture of equal parts of ether and alcohol, adding to it a little of an alcoholic solution of tannin and a similar solution of soap.

To use a collodion prepared in this or any similar way all that is necessary is to pour a little of it on the glass plate on which the negative is to be taken, allow it to dry, and either expose it in the camera without further preparation or place the plate away until it is convenient to use it. In this way it will be seen that photography is now reduced to a state of great simplicity, so far, at any rate, as the preparation of the plates is concerned.

Armed with a bottle of this sensitive emulsion, a photographer or tourist may now visit any country with the certainty that, wherever he can procure glass plates cut to such sizes as he may require, there can he have sensitive plates-plates, too, absolutely identical with each other in respect of sensitiveness-uniformity being a necessary consequence of the method by which they are prepared. To one accustomed to the preparation of plates by the usual bath method, with the subsequent washings and preservatives, it is very difficult at first to realise the extreme simplicity of the "washed emulsion process." In the simple act of pouring the collodion from a bottle on to a glass plate every operation is now included. The result is a plate capable of yielding a high-class negative, and possessing quite as great a degree of sensitiveness as dried collodion plates prepared by any other method.