ter of the organic nature of those serpentine limestones in the Laurentian formations of Canada and elsewhere, which are products of the growth of the gigantic foraminiferal Eozoon Canadense, over immense areas of the ancient sea-bottom, is one of still greater interest both to the student of Geology and of Biology.

This immense rhizopod appears to have grown one layer over another, and to have formed reefs of linestone as do the living coral-polyps. Parts of the original skeleton, consisting of carbonate of lime, are still preserved, while certain interspaces have been filled up with serpentine and white augite.

Microscopic Palæontology.—As a general rule it is only the hard parts of animal bodies that have been preserved in a fossil state.

It will often occur that the inspection of a microscopic fragment of such a fossil will reveal with certainty the entire nature of the organism to which it belonged. Thus minute fossil corals, the spines of Echinodermata, the eyes of Trilobites, etc., will determine the position to which we should ascribe the specimen, or a section of tooth or bone will enable the microscopist to assign the fossil to its proper class, order, or family. Thus Professor Owen identified by its fossil tooth, the Labyrinthodon of Warwickshire, England, with the remains in the Wittemberg sandstones, and declared it to be a gigantic frog with some resemblances both to a fish, and a crocodile. This prediction the subsequent discovery of the skeleton confirmed.

The minute structure of teeth differs greatly in different animals. In the shark tribe of fishes the dentine is very similar to bone, excepting that the lacunae of bone are absent. In man and in the Carnivora the enamel is a superficial layer of generally uniform thickness, while in many of the Herbivora the enamel forms with the cementum a series of vertical plates which dip into the substance of the dentine. Enamel is wanting in serpents, Edentata,

and Cetacea. Such differences make it quite possible to distinguish the affinities of a fossil specimen from a small fragment of tooth.

In a similar way the microscopic characters of bone vary. The bones of reptiles and fishes have the cancellated structure throughout the shaft, while the lacunæ present very great varieties, so that an animal tribe may be determined by their measurement. In this way many contributions have already been made to palæontology.

CHAPTER VIII.

THE MICROSCOPE IN CHEMISTRY.

THE value of microchemical analysis, and the simplicity of its processes, commend this department of microscopy to general favor.

A large proportion of the actions and changes produced by reagents may be observed as satisfactorily in drops as in larger quantities. The decompositions effected by a galvanic battery far smaller than that contained in a lady's silver thimble, which deflected the mirror at the other end of the Atlantic Telegraph Cable, may be readily observed with a microscope.

Apparatus and Modes of Investigation.-A few flat and hollow glass slides, thin glass covers, test-tubes, small watch-glasses, a spirit-lamp or Bunsen's burner, constitute nearly all the furniture which is essential.

Dr. Wormley* directs that a drop of the solution to be examined should be placed in a watch-glass, and a small portion of reagent added with a pipette. The mixture

* The Microchemistry of Poisons, by Dr. Wormley.

may then be examined with the microscope. If there is no precipitate, let it stand several hours and examine again. Dr. Beale prefers a flat or concave slide, and suggests that if a glass rod be used for carrying the reagent, it must be washed each time, or a portion may be transferred from the slide to the bottle. He also advises the use of small bottles with capillary orifices for reagents. Dr. Lawrence Smith uses small pipettes with the open end covered by india-rubber.

If heat be required, the drop may be boiled on the slide over a spirit-lamp, or a strip of platinum-foil or mica may be held with forceps so as to get a red or white heat from the lamp or a Bunsen burner. This is especially needed to get rid of organic matters.

For the examination of earthy materials, as carbonate or phosphate of lime, phosphate of ammonia and magnesia, sulphates or chlorides, a small fragment may be placed on a slide and covered with thin glass. A drop of nitric acid is then put near the edge of the cover. If bubbles escape a carbonate is indicated. Neutralize the acid with ammonia; let the flocculent precipitate stand awhile; cover and examine with the microscope. After a time, amorphous granules and prisms will show phosphates of ammonia, magnesia, and lime. Sulphates are shown by adding to the nitric acid solution nitrate of barytes, and chlorides by nitrate of silver.

Dr. Beale recommends adding glycerin to the test solutions. The reactions are slower but more perfect, and the crystalline forms resulting are more complete.

If a sublimate be desired, a watch-glass can be inverted over another, and the lower one containing the material, as biniodide of mercury, etc., heated over a spirit-lamp, or the sublimation may be made in a reduction-tube.

Preparation of Crystals for the Polariscope.-Many specimens may be prepared by concentrating the solution with heat and allowing it to cool. It should not be evaporated

to dryness. Many salts may be preserved in balsam, but some are injured by it, and need glycerin or castor oil as a preserving fluid.

The method of crystallization may be modified in various ways so as to obtain special results. Thus if a solution of sulphate of iron is suffered to dry on a slide, the crystals will be arborescent and fern-like, but if the liquid is stirred with a glass rod or needle while evaporating, separate rhombic prisms will form, which give beautiful colors in the polariscope. Pyrogallic acid also crystallizes in long needles, but a little dust, etc., as a nucleus, brings about a change of arrangement resembling the "eye” of the peacock's tail.

A saturated solution dropped into alcohol, if the salt is insoluble in alcohol, will produce instantaneous crystals.

To obtain the best results, some crystals, as salicin, should be fused on a slide over the lamp, and the matter spread evenly over the surface. This may be done with a hot needle. The temperature greatly affects the character of the crystallization. If very hot, the crystals run in lines from a common centre. A medium temperature produces concentric waves.

Many new forms result from uniting different salts in different proportions. The knowledge of these different effects can only be attained by experience.

Sections of crystals, as nitrate of potash, etc., to show the rings and cross in the polariscope, are difficult to make. After cutting a plate with a knife to about onefourth of an inch thick, it may be filed with a wet file to one-sixth of an inch, smoothed on wet glass with fine emery, and polished on silk strained over a piece of glass, and rubbed with a mixture of rouge and tallow. The nitre must be rubbed till quite dry, and the vapor of the fingers prevented by the use of gloves.

For a general account of the use of polarized light, see Chapter VI.

The Use of the Microspectroscope.—We have already described this accessory in Chapter III. It promises important results in chemical analysis, but requires delicate observation and exact measurements, together with a careful and systematic study of a large number of colored substances.

In using the microspectroscope, much depends on the regulation of the slit. It should be just wide enough to give a clear spectrum without irregular shading. As a general rule, it should be just wide enough to show Frauenhofer's lines indistinctly in daylight. The slit in the side stage should be such that the two spectra are of equal brilliancy. No light should pass up the microscope but such as has passed through the object under examination. This sometimes requires a cap over the objectglass, perforated with an opening of about one-sixteenth of an inch for a one and a half inch objective.

The number, position, width, and intensity of the absorption-bands are the data on which to form an opinion as to the nature of the object observed, and Mr. Sorby has invented a set of symbols for recording such observations. (See Dr. Beale's How to Work with the Microscope.) These bands, however, do not relate so much to the elementary constitution as to the physical condition of the substance, and vary according to the nature of the solvent, etc., yet many structures give such positive effects as to enable us to decide with confidence what they are.

Colored beads obtained by ordinary blowpipe testing, sections of crystals, etc., cut wedge-shaped so as to vary their thickness, often give satisfactory results. But minute quantities of animal and vegetable substances, as blood-stains, etc., dissolved and placed in short tubes fastened end wise on glass slides, or in some other convenient apparatus, offer the most valuable objects of research.

To measure the exact position of the absorption-bands,