THE

MONTHLY MICROSCOPICAL JOURNAL.

OCTOBER 1, 1870.

I.-The Patterns of Artificial Diatoms.

By HENRY J. SLACK, F.G.S., Sec. R.M.S.

(Read before the ROYAL MICROSCOPICAL SOCIETY.)

I HOPED by this time to have laid before the Society the results of a series of examinations of a number of species of Pinnulariæ, showing that these diatoms have been much misunderstood by microscopists, who have accepted their separation from the Navicula on the ground of their so-called costa not being resolvable into dots, or beads. I was led some time since, by carefully viewing the row of Pinnulariæ in Möller's "type slide" with Beck's th, to doubt the correctness of the usually-received views, and on repeating the investigation with Powell and Lealand's new immersion 1th it became evident that the costa of most, if not all, were less simple in structure than had been imagined, and could be resolved into more or less complex beaded forms. An application to Möller, through Mr. Baker, for a set of the Pinnulariæ mounted dry has not yet proved successful, probably owing to the disturbance of rational pursuits consequent upon the war, and though Mr. Norman supplied some excellent slides, he had not in his collection all the species required.

A general survey of the diatoms in Möller's "type slide" enables a gradation to be traced between beads of large size, widely separated under moderate powers, and similar beads closer and closer together, until real or closely-approximate contact is obtained. Another gradation may likewise be traced in size, from beads which are conspicuous like little marbles with a magnification of a few hundred diameters, to others that appear extremely minute when high powers with deep eye-pieces are used for their examination. The question naturally arises, Where does the beading stop? Does it stop at all?

A living object immersed in, or supplied with, water having silica, or a silicate in solution, and causing, during its life processes, a deposition of silica to strengthen its own tissue, must evidently obtain it by the method of chemical precipitation, and probably, in the case of the diatom, by decomposing a soluble silicate of soda or potash. If all diatoms were constructed upon the same principle, and by the same process of chemical precipitation, a uniform plan

VOL. IV.

would probably be found in their silicious structure, and this expectation, rather than any anxiety to see dots that others had not discerned, interested me in the Pinnularian inquiry.

One portion of such an investigation obviously relates to the patterns which can be obtained by purely chemical processes, and without the intervention of any kind of life.

In this paper those forms only which occur in Schultze's artificial diatoms will be alluded to, and it may be as well to premise that although no diatom is likely to obtain its silica exactly in that way that is by means of a silicic fluoride-Schultze's formations may still throw out much light upon the whole subject.

To obtain the artificial diatoms, powdered glass and fluor spar are acted upon by sulphuric acid, or hydric sulphate as the new school of chemistry prefer to call it. If no heat is used, the silicic fluoride gas rises slowly, and if allowed to impinge against threads of cotton moistened with water, a decomposition takes place, "one third of the silicon unites with the oxygen of the water, and is thrown down.”* If the gas is passed through water the precipitation takes place in white flakes quickly choking up a small tube. When the moist cotton threads are used, and the process goes on slowly, a great quantity of irregular sausage-like tubes are formed, and when these are well washed, to get rid of the hydric fluo-silicate, and crushed under a covering slide to obtain surfaces flat enough for convenient examination, the diatom patterns will be seen with various powers from a 3rds to the highest that are made.

The Society will excuse in these last remarks my repeating some matters that are now old, and which has been done to make the subject more intelligible to that, I fear, very numerous class of microscopists who have not paid to Schultze's artificial diatoms the attention they deserve.

The mode of their production just described will suffice to indicate that the circumstances determining the patterns in which the silica is precipitated will depend upon conditions presenting only slight differences. If the whole process is carried on in a room of ordinary temperature, say about 60° or 70°, and completed in a few hours, changes resulting from more or less heat will have little effect. The rate at which the silicic gas is evolved, the quantity of moisture it meets on the cotton threads, and such facts as the closer or looser arrangement of those threads, would seem to be the chief causes of any modification that occurs. Thus we may

safely ascribe all the patterns we obtain to very slight chemical and physical conditions of decomposition and deposition, and yet what varieties of arrangement and size of dots or beads we obtain. The following is a description of some of the varieties in a single slide. The sizes are apparent with a magnification of about 700.

* Barff's 'Chemistry.'

No. 1. Pattern regular, quincuncial, beads appearing about 1-4" diameter, interspaces double that of the beads' diameters.

No. 2. Pattern regular, quincuncial, beads about half the size, and interspaces about half those of No. 1.

No. 3. Beads appearing about 1-7" in diameter and touching. This pattern readily gives the hexagonal aspect obtainable with many diatoms.

No. 4. Regular rectangular pattern, beads appearing about 1-20" in diameter.

No. 5. Similar beads to 4, grouped very irregularly with wide irregular interspaces, in which no beads could be discerned.

No. 6. Similar beads in rows wider apart in one direction than in another, the interspaces being unequal in rectangular directions and many times the diameter of the beads.

No. 7. Rather larger beads irregularly arranged, some lines straight, and others curved.

No. 8. Large beads appearing from 1-8" to 1-7′′ in diameter surrounded by beads a quarter their size.

No. 9. Large composite bead, as if composed of three adhering and compressed beads, surrounded with smaller beads irregularly arranged, and the interspaces filled with innumerable minute beads. No. 10. Large bead, surrounded by concentric circles of smaller beads.

No. 11. Small beads in concentric rows, like a fragment of a delicate circular diatom.

No. 12. Beading so fine and close as not to admit of distinct separation with mag. 1000.

No. 13. Beading only indicated, and distinguished from clearlooking spaces somewhat as a surface of dead-gold is distinguished from a burnished one.

No. 14. A form approximating to some of the polycistina, but with large beads as well as holes.

The above by no means represent all the varieties noticeable in a single slide. In one case large beads appeared built up of smaller ones, but the optical appearances were too puzzling to say exactly what the true form was. It seems probable that the clear spaces only differ from the beaded ones in the fact that the silex, though precipitated in spherules, has formed a pattern too minute to be discerned.

An examination of the artificial diatoms shows that purely chemical and physical considerations will account for the varieties of pattern we notice in natural diatoms, and their living structure appears only to provide the conditions under which the silicious precipitation takes place, according to the ordinary laws of chemical action and molecular coalescence.

Journal,

II.-On Ancient Water-fleas of the Ostracodous and Phyllopodous Tribes (Bivalved Entomostraca).

By Professor T. RUPERT JONES, F.G.S.

PLATE LXI.

Part I.-The LEPERDITIADÆ.

THE "water-fleas" are a mixed group of minute Crustaceans, some in fresh and others in salt water, which received their common English name from their jerky movements, and their continental name of "Entomostraca," or "shell-insects," from their somewhat insect-like appearance and their shell-like cases. The "water-fleas" proper, however, are the Daphniada and their near allies, mostly of fresh-water habits. These have for the most part a very delicate, thin, horny envelope, or carapace, bent down on either side over, and enclosing, the body, but not divided and jointed along the back. Of these there are no well-known fossil representatives; but certain remains of such soft-shelled water-fleas are recorded by M. Mahony, in the Geological Magazine,' No. 63, p. 392, as occurring in a Post-tertiary clay in Renfrewshire; and some possibly Daphnioid Entomostracans are noticed in the same Magazine, No. 71, p. 219, as coating a layer of coal-shale from South Wales. Some old fossils have been referred to Daphnia on a mistaken notion of similarity of form.

Other water-fleas are termed "water-shell-fleas (Cypris, &c.) and "sea-shell-fleas" (Cythere, Cypridina, &c.), having more distinct shells, with two valves, jointed on the back-line. These are abundant enough fossil in many strata of different ages.

The English names, however, are too general for special use in indicating the different kinds, the family and generic distinctions being very numerous and decided in this important sub-class of the great Crustacean group or class. Indeed it comprises these and other many-featured, minute, aquatic, articulate creatures, for which the term Entomostraca has been accepted as the systematic name.

Of these the really bivalved forms (namely, the Ostracoda and some of the Phyllopoda) have left their carapace-valves in the muds of lakes and seas of all ages as far as the history of the globe can be traced back geologically. The geologist recognizes three main stages of the world's progress, which he terms the Paleozoic (old life) or Primary, the Mesozoic (middle life) or Secondary, and the Cenozoic (new life) or Tertiary and Post-tertiary. These periods had their own characters of life, as shown by the fossils preserved in the respective groups of successive strata; and as regards the Bivalved Entomostraca there are many noticeable facts as to the per

* 'Catalogue of Crustacea,' in the British Museum, 12mo, 1850.