in others these images widely diverge from them. Thus, we seldom find in them those purple and violet tones which seem to characterize the subchloride of silver before fixing. On the other hand, we observe two classes of developed images :-the one is of a dull metallic appearance, of a slaty grey character by transmitted light, and in but a feeble degree opake; the other varies in colour, exhibiting brown or red hues, and sometimes even presenting perfect opacity to transmitted light, closely similar to the picture formed by direct processes. But, on testing these two varieties of image by the method of conversion into sulphide of silver before described, it is found that the dull translucent metallic image teems with silver, and becomes very opake in the form of sulphide, while the more richly coloured and dense-seeming image loses opacity under the sulphurizing action, and exhibits at last a subdued tone of colour that brings it more on a par with the sulphuretted metallic image. Clearly then, here, density, and the qualities which give photographic value to an image, do not depend on the amount of metal that goes to form it, so much as on the chemical, and even perhaps mechanical state, in which that silver is present in it. The several causes which determine the deposit of the images in these several states appear to be these:— 1. The materials forming the sensitive film.-Pyroxyline, in chemical purity, has little tendency to form the darker image. Albumen and the heterogeneous substances (including decomposed collodions), which we have had to yoke in the same class with it, have this tendency. In general (speaking of the ordinary moist process) the tendency to produce the darker image is found to be in something like an inverse ratio, cæteris paribus, with the sensitiveness. The use of the bromide of silver with the iodide imparts to a collodion film a tendency to deposit the grey metallic image, at the same time that a more powerful reducing agent is needed to develope it. It is a remarkable fact, bearing upon this singular property of bromide, that no compounds analogous to that formed by A. Kremer with the iodide have yet been formed with it. In the case of albumen, this influence of bromide is not felt; for with albumen, bromide of silver is held to increase the opacity of the image. 2. The nature of the developing agent. The substances used to develope the latent image, besides the free nitrate of silver invariably necessary, embrace also without exception one ingredient, the character and the purpose of which is to reduce the salts of silver. In some cases organic bodies are employed for this purpose, in others the reducing agent is inorganic. Now, whether the grey or metallic form of image is completely reduced silver, and the more opake forms are an argentous compound (mixed or not with metallic silver), or whether all the forms of image are silver in different mechanical states of deposition, is a very important inquiry, and one on which the facts of the development and the nature of the developing agent may throw some light. But no one who is intimate with the complex and perplexing details of this step in the photographic process will expect the chemist to come in and remove the difficulty by the use of a few formulæ. All we can hope to do is to point to a few sure results of experience, and indicate any explanation which may be suggested by facts from the laboratory analogous to these. It is known, then, that to produce a "positive" picture in the camera, the developing agent should be sulphate of iron, acidified in some cases even by nitric acid. The result is the crystalline white deposit of metallic silver. Protonitrate of iron is used with a similar result. So likewise in the laboratory it is known that a neutral mixture of the ferrous sulphate and nitrate of silver forms the grey deposit, but that the addition of a little acid produces the white and brilliant form of the metal. If now we would take a result opposite to this from the experience of the photographist, we may select an ordinary collodion plate prepared by the usual negative process, and we shall find that protacetate of iron developes the image of a black colour. Now Rose, in the remarkable experiments on the production of argentous compounds with the higher oxides of iron, &c., to which we have called attention, shows that whereas the argentic salts containing strong mineral acids are precipitated as grey metal by ferrous salts containing similar acids, the deposit formed by uniting the ferrous oxide and the argentic oxide, or the compounds of these with organic weak acids, contain the suboxide of silver and are black. When to this is added the circumstance that the white and grey photographic images are with facility amalgamated with mercury, but that the coloured and black images are not, it may be treated as a matter of high probability that the black and coloured images are formed by compounds of the suboxide of silver. A directive energy is exercised upon the nature of the deposit by the various kinds of organic matter employed in the development. These all seem to restrict the limits of variation to the dark bluish-black (given by citric acid when present), on the one hand, and various reds and browns upon the other; while, again, the presence of the albuminous and other substances, so often before referred to, is, as was above remarked, a sure means of forming these darker and coloured images. Indeed, albumen will determine such images notwithstanding that even free nitric acid be present with it. If it be a suboxide that causes the dark precipitate, that suboxide must go down in combination, and so resist the action of the fixing solvents. But, 3. The character of the light has also a remarkable influence in inducing a grey or a dark character on the developed image. If the picture has been produced by an intense light, as by a lens of large aperture, or as in the case of an exterior as contrasted with an interior view of a building, or as on a dull, misty day in contrast with a bright and sunny one, it will be found that, cæteris paribus, the tendency of the weaker action of the light is to allow the reduction of the silver in the metallic form. On the other hand, the more intense light has given to the molecules of the sensitive film a controlling energy which they exercise on the deposit, and which appears analogous to that of the light in the direct process, in its modifying the reduction and giving it the form of a production of an argentous compound; as though the iodic compound became in a certain sense phosphorescent to the chemical rays of the light, and operated on the mixed silver-salt and reducing agent as they float over it in the manner that the direct light might be supposed to do. Of course, the materials must be nicely balanced, as regards their tendencies to produce the black or the grey images, for the peculiar action of an intense or a weak light to be made fully evident. Albumen or powerful organic agents will usually destroy this balance. One fact remains to be observed. Whatever may have been the character of the first particles deposited on the plate, that character will be maintained thenceforward, and fresh deposits may be, so to say, piled upon the first by the singular agglutinative tendency of crystalline deposits, so long as the necessary conditions of fresh silver solution and of fresh stores of the reducing agent be supplied to keep up the action. Our task has been, by an investigation of the chemistry of the image in its different varieties, to afford some data, at least, by which the further step may be hereafter taken of determining the precise character of the photochemical agency, to whose marvellous influences art owes so many beautiful results, and science is indebted for more than one intricate problem. Report of the Belfast Dredging Committee for 1859. By GEORGE C. HYNDMAN, President of the Belfast Natural History and Philosophical Society. THE Committee appointed at the Meeting of the British Association at Leeds, to proceed with the investigation of the Marine Zoology of the north and north-east of Ireland, consisted of Mr. Patterson, Dr. Dickie, Dr. Wyville Thomson, Mr. Waller, and Mr. Hyndman, who took measures to commence their operations early in June, from which time till the end of August various explorations were made along the coast and in the sea adjacent, extending from the south side of Belfast Bay (county Down) to the deep water north of the Maidens on a line with Glenarm (county Antrim). Those acquainted with dredging operations will understand the difficulties and delays to which such work is liable, calms and storms equally interfering with progress. At the first meeting on the 7th of June, the weather was too fine to enable the party to reach the desired ground in due time; the few specimens of living Brachiopoda then obtained were forwarded to Mr. Hancock, who has been engaged in the investigation of that tribe, but owing to his absence from home the opportunity of seeing them alive was lost. On a second occasion, 22nd of June, the party engaged a steamer and succeeded in reaching the chosen ground for dredging in the deep water off the Maiden Islands, when a sudden storm arose, more violent than usual at that season, which obliged them to cease work and make for the shelter of land with all expedition, glad to save their ropes and dredges. A boat belonging to a ship of war then in Belfast Bay was not so fortunate, being upset in the squall, by which lamentable occurrence several men were drowned. During the season the Committee were assisted by the co-operation of several gentlemen who took an interest in their work. In August, J. Gwyn Jeffreys, Esq., visited Belfast, and made one of a party for dredging off Larne, where a fortnight was spent in examining the coast and deep water adjacent, extending as far north as opposite to Glenarm. Mr. Jeffreys' experience and acuteness in discriminating species were of great service in adding to the lists and correcting some previous errors. During this period a steamer was again engaged from Belfast, which enabled a number of gentlemen to join in the labour and rendered good service. It was originally contemplated to extend the investigation as far as Rathlin Island, but want of time and other circumstances prevented this from being accomplished. Very comprehensive lists having been already published in the Reports of the British Association for 1857 and 1858, it is thought needless on the present occasion to do more than record such additions as have been made, with any further information that may be considered interesting regarding some particular species. List of Species referred to in the Report of the Belfast Dredging Committee for 1859. Philine quadrata, dead. In 80 fathoms off the Maidens. Cylichna Lajonkaireana (Baster). From the Turbot-bank, dead; determined by Mr. Jeffreys in Mr. Hyndman's cabinet. Mangelia attenuata, dead. Turbot-bank sand, Mr. Waller. reticulata, dead. A single specimen of this rare and beautiful shell was found by Mr. Jeffreys in dredging from the deep water north of the Maidens. New to the costata, var. coarctata, dead. Near the Turbot-bank. Fusus Islandicus, var. gracilis (Alder), living. In 60 fathoms, about six miles from the Gobbins. Buccinum undatum, var. striatum, Pennant; living. With the last. Cerithiopsis pulchella, dead. In Turbot-bank sand, Mr. Waller; erroneously recorded in the list of 1857 as Cerithium metula. Trichotropis borealis, living. Turbot-bank. Lamellaria perspicua, living. In 80 fathoms north of the Maidens. This is usually a sublittoral species. Natica helicoides, dead. A single young specimen by Mr. Jeffreys. Cerithium metula, of the list for 1857, was found by Mr. J. to be Cerithiopsis pulchella. In dredged sand, Turbot-bank. Euomphalus (Omologyra) nitidissimus (Skenea nitidissima), living on Zostera marina. Shores of Larne Lough. Skenea divisa, living. Off Larne, 1858, Mr. Hyndman. planorbis, living. A small variety occurs in Larne Lough, has a more convex spire, and it appears to bear the same relation to the typical form that the Helix rupestris of Continental authors does to our H. umbilicata, Mr. Jeffreys. Jeffreysia Gulsonæ, dead. Turbot-bank sand. In Mr. Hyndman's cabinet, determined by Mr. Jeffreys. Lacuna crassior, living. Coast of Antrim. Mr. Jeffreys observed that the shell has a distinct canal or groove in the columella, evidently showing its generic position. The animal, which he examined, settles the question. It is of a yellowish white colour, having two subulate and slender tentacles, with the eyes placed on short peduncles at their external base; proboscis long and narrow; two rather long caudal filaments, one on each side of the operculigerous lobe. The creature is active in its habits, and seems fond of crawling out of water. labiosa, Lovèn, dead. In Turbot-bank sand, Mr. Jeffreys. Littorina fabalis, living. Found by Mr. Jeffreys on the shore of Larne Lough, and considered by him to be only a variety of L. littoralis. tenebrosa, living. In the same locality as the last, and considered only a variety of L. rudis. Scissurella crispata, dead. A fresh specimen taken in 80 fathoms, 5 or 6 miles north of the Maidens. Margarita costulata (Skenea), dead. In Turbot-bank sand, Mr. Waller. Trochus Montagui, living. An exquisite scalariform variety found by Mr. Jeffreys and Mr. Waller off the coast of Antrim; the animal does not differ from that of the usual form. -striatus, dead. In Turbot-bank sand, Mr. Jeffreys. Emarginula reticulata, living. In 80 fathoms north of the Maidens. Mr. Jeffreys found the fry, which closely resembles a Scissurella, and has a regular Trochoidal spire, with the edges of the slit inflected. Propilidium ancyloide, living. On stones and shells in 70 to 80 fathoms. They were of different sizes, the largest not exceeding one-eighth of an inch, and evidently adult. The Patella cæca of Müller, of which the authors of 'British Mollusca' supposed this might be the young, appears to be a very different species, if indeed it belongs to the same genus. (J. G. J.) Patella athletica, living. Coast of Down, in Mr. Hyndman's cabinet. Chiton cancellatus, living. Not uncommon in deep water. Hanleyi. A fine living specimen on a shell, and one on a stone in 80 fathoms. Argiope Cistellula, living. On stones as well as shells in the deeper water. Terebratula capsula, living. With the last. caput-serpentis, living. Of large size in the deep water. Some specimens kept living exhibited on the front margin a series of white filaments which appeared to protrude from the tubes of the shells, and not to be retractile when touched. Pecten opercularis. Mr. Jeffreys remarks that the young have a rhomboidal form, and the lower or flat valve is much smaller than the other (which overlaps it), and is perfectly smooth. The ribs do not at first appear on the larger valve, but are preceded by a shagreened reticulation. furtivus, alive. Taken in 1858 by Mr. Waller and Mr. Hyndman on both the Antrim and Down coasts along with P. striatus. It was again taken this year, and at once distinguished by Mr. Jeffreys. Pecten Danicus, dead. single valve in 80 fathoms. In the former list, 1857, with a mark as being doubtful. This proves Dr. Dickie to have been correct. Modiola modiolus, living. A small variety, three inches in length, occurs in deep water. The same at the Copelands. It is stated that specimens have been found on the West coast of Scotland, seven or eight inches long. phaseolina, living. With the last in deep water. Astarte compressa, dead. A few valves of the smooth variety, found by Mr. Jeffreys in the Turbot-bank sand. Tellina pygmæa, dead. Valves united; from the Turbot-bank sand, in Mr. Hyndman's cabinet. Solecurtus candidus, dead. In the Turbot-bank sand. Sphænia Binghami, dead. Not uncommon in pieces of rolled chalk, and among the roots of Laminaria digitata by Mr. Grainger. Mr. Jeffreys doubts its having the power of burrowing or excavating. See Mr. Jeffreys' " Gleanings" in the 'Annals of Natural History' for Sept. 1859. Mya truncata. A young living specimen was brought up by the dredge from 80 fathoms on stony ground; its usual habitat being low-water mark in mud. Saxicava arctica, living. Not uncommon, moored in cavities or crevices of stones and shells. Mr. Jeffreys considers it to be merely a variety of S. rugosa, differing in habitat. The latter, when enclosed in stone, loses the sharp keel and teeth of S. arctica, and is more rugged in appearance. Pholadidea papyracea, living. At a depth of 80 fathoms North of the Maidens, in small pieces of soft sandstone. The smaller specimens want the cup-shaped appendage, whether the effect of insufficient space or immature growth. An examination of these smaller specimens affords means of correcting an error in the first list of 1857. The so-called Pholas striata, being identical with these, is therefore to be expunged. Cynthia limacina, living. On stones and shells from deep water. Balanus tulipa alba (Hameri of Darwin) is not uncommon, living in the deep water. Balanus? Of another species, not yet determined, a single dead specimen was found in 80 fathoms. Sphænotrochus Wrightii. A few dead specimens were found in the Turbot-bank sand by Mr. Hyndman in 1852, and subsequently by Mr. Waller. Dr. Perceval Wright, having seen these specimens in Mr. Hyndman's collections, received permission to hand them over to Mr. Gosse, who has described and figured them in the 'Dublin Natural History Review' for April 1859. Sagartia coccinea. A sea anemone appearing to be this species is not unfrequent on stones and shells from deep water. Appendicularia flagellum. On the 7th of June, 1859, a bright calm day, this curious and interesting animal was seen in great abundance floating through the water at the northern entrance of Belfast Bay. It has not hitherto been recorded as Irish; but has been fully described by Professor Huxley in the Microscopic Journal,' vol. iv. Sagitta bipunctata. Several specimens were taken in the towing net along with the former. Dr. Wyville Thomson had discovered it a short time previously in Strangford Lough. Not hitherto recorded as Irish?. It has been described by Dr. Busk in the 'Microscopic Journal,' vol. iv. Hippolyte spinus. In the deep water off the Maidens: determined by Dr. Kinahan. A Northern species, inhabiting the seas of Iceland and Greenland. New to the Irish list. Acanthonotus testudo. Taken with the last. A pleistocene bed of stratified gravel was observed on the side of the road between Larne and Glenarm, and was examined by Mr. Jeffreys and Mr. Hyndman. It was found to contain several species of shells, corresponding with those from a bed at the Belfast Water Works, recorded in Portlock's Report on the Geology of Londonderry. The following is a List of the species obtained, which will no doubt be augmented on further investigation : Pholas crispata, fragments. Tellina solidula. calcarea (Möller). Mactra subtruncata. Astarte compressa, var. globosa. Astarte elliptica. Natica clausa (nana Möller). Pingelii (Möller). |