for the bell-jar to cover. The neck of the bell-jar is attached to a Liebig's condenser. Should a substance require exhaustion with the solvent, the Liebig is placed in an upright position; should an evaporation or distillation be required, the condenser is placed in the usual slanting position, and in this way all the liquid evaporated is saved. As a matter of convenience it is well to have a pair of these apparatuses in a laboratory, one with an upright, the other with a slant condenser.* § 52. The Spiral Balance.-A spiral balance, proposed and used some years ago by Professor Jolly,† has been figured and

Fig. 5.

R

Fig. 6.

The apparatus can be obtained from Messrs. Cetti, Brook Street, Holborn.

+ V. Jolly, Sitzungsber. Baey. Akad., 1864, i. 162.

described in a former edition of this work, but the author's experience of the instrument is not favourable.

53. Vacuum Processes.-There are a variety of analytical operations, especially those employed in toxicological and food chemistry, which for their proper performance require an efficient vacuum. A short time ago the Sprengel pump was generally employed for this purpose, and, indeed, is so now. Obtaining a vacuum by the Sprengel pump, however, in large retorts or large flasks is a most tedious operation, and as a technical process vacuum working with a Sprengel would become impossible. The author, therefore, uses the same instrument which is employed by the Swan Electric Light Company to exhaust the globes for the thread of incandescent carbon. This pump-the patent mercury pump of Mr. Lane Fox-the author has modified slightly, and with these modifications it is now adapted for every species of laboratory work. The pump (see fig. 6) consists of a glass tube, AA, with a large bulb, B, and a thistle-head, C, in which a ground stopper, F, is fitted, and the whole made tight by a little mercury in C. When gas has to be collected, F is replaced by the apparatus SS', which consists of thickwalled capillary tubing, having either India-rubber pressure tubing at S', with a clamp, or a glass stopcock at S'. The side tube, G, is provided with a glass stopper-float, I, ground accurately into X. It allows air or gas to go in the direction of the arrows only, any back pressure carrying up the mercury, and floating and firmly fixing the float into X. In the glass cup, Z, is ground a stopper of angle tubing, with which the apparatus it is intended to exhaust is connected. To work the pump the stopper is taken out of C, and the mercury reservoir, A, is raised until the globe is filled and mercury rises into C. At this moment the stopper is inserted and the reservoir lowered to the ground; this causes a vacuum in the apparatus, and air-bubbles pass into B in the course of the arrows, and collect in the globe. By now raising the reservoir, and at the proper moment loosening the stopper, the air is expelled; on closing the stopper and again lowering, a fresh quantity of air escapes into B, and so on until a perfect vacuum is obtained. A very large retort may be exhausted by working the reservoir up and down about a dozen times, while smaller vessels are made vacuous in three minutes. In collecting gas, as, for example, nitrogen and carbon dioxide in an organic analysis, the stopper F is replaced by the tube SS'P. By dipping the end of the tube P into the mercury trough, having the clamp or stopcock open, and lowering the reservoir, the capillary thread is readily filled with mercury, and the mercury retained by closing the stopcock. When the combustion

tube is vacuous, the beak of the tube is inserted under the eudiometer (or whatever special gas apparatus the analyst has), and the combustion tube made red-hot in the usual way, the gas being readily pumped out and delivered into the eudiometer. The purposes to which such an instrument are applicable are so very various as to render it absolutely necessary in all laboratories.*

THE MICROSCOPE, THE SPECTROSCOPE, AND THE ART OF PHOTOGRAPHY AS APPLIED TO THE CHEMISTRY OF FOOD.

§ 54.-There are so many special works describing the microscope that it will be quite unnecessary to burden the pages of this book with information so readily accessible. The chemist, as a rule, will find a binocular most suitable for his purpose, for it is only with a binocular that it is possible to have a really good view of crystals. Besides, the instrument is so readily converted into a monocular, that it possesses the advantages of the latter combined with its own. For certain branches of research, and more especially for observing reactions under the microscope, the inverted microscope of Dr L. Smith, of Merton College (or those of similar pattern), by which the object glass is placed below the substance to be examined, has this advantage, that it is possible without injury to the instrument, and without being annoyed by acid fumes, to treat substances under observation with strong acids, even at a boiling temperature.

In

The analytical student will require to familiarise himself with the use of the micrometer and the polariscope. The most suitable micrometer for the measurement of starches and similar substances, is what is called an eyepiece micrometer. A glass, ruled, either in squares, or as a simple scale, is placed between the eye and field piece, so that both the object magnified and the scale are seen clearly at one and the same time. order to find the value of the divisions of the eye micrometer, it is necessary, in the first place, to determine them by noting how many divisions correspond with one or more of a slip of ruled glass placed on the stage, and containing divisions equalling the hundredths of an inch, or any other convenient measurement. Suppose, for example, that it is found that one-hundreth of an inch on the stage when measured by the eyepiece required 18

*For a variety of purposes a good water pump is more convenient than a mercury pump, and, provided the fall tube is sufficiently long, a vacuum within inch of the barometric height may be obtained.

=

18609

or

of the eyepiece divisions, then it is obvious that each one of the divisions is of or of an inch; therefore, any object that measured, say four divisions, would be 4 × 1800 would measure the one four hundred and fiftieth of an inch. There is another method of measurement which is extremely accurate and applicable to all cases; this is, to take a microphotograph of the subject, and to photograph a glass with suitable ruled divisions, with the same arrangements and with the same powers; afterwards a measurement with ordinary compasses can, with great ease and convenience, be made.

Chemical reactions, under the microscope, are either observed in shallow cells ground in the glass slide itself, or simply on the ordinary flat slide, or, as is sometimes convenient, in almost capillary tubes with flattened sides, the microscope being in a horizontal position. Reactions, as a rule, should be observed with only a moderate magnifying power. It is quite possible to execute, on a very small amount of material, a complete qualitative analysis on the stage of the microscope, mixing with drops of the solution under observation droplets of the ordinary test solutions, such as sulphuretted hydrogen water, ammonium sulphide, ammonia, oxalic acid, sodic phosphate, etc. Dr. Beale has recommend glycerin to be used instead of water for these reactions, and he states that although the reactions are slower, yet that they are more perfect.* The method of subliming alkaloids, and its important bearing in the determination of the nature of substances in tea or coffee, is described in the article on "Tea," together with the microscopic appearance of the ash of various leaves, and the method of obtaining "skeleton ashes."

In cutting sections of seeds, leaves, &c., no difficulty is experienced when they are in the entire state, nor are any special instruments required save a sharp razor, for with a little practice sections quite as fine as those it is possible to cut by a sectioncutting machine, can be made with a razor. It is, however, quite different with such matters as tea leaves which have been dried and crumpled, or seeds in the state of powder. Here considerable difficulty may be experienced, and it is often not possible to get a section at all satisfactory of any given dark microscopic particle. The author has had tolerably fair results by sprinkling opaque powders on a piece of smooth wood, and embedding the powders in a tenacious glue. When the cement has set, there is no difficulty in getting sections.† Similarly, the known processes for

"How to Work with the Microscope." London, 1880.

+ Sealing-wax does admirably, the particles of powder are placed on a thin layer of sealing-wax, and the wax softened by heat; on cooling, the particles are held with sufficient firmness to allow of sections.

A simple

embedding soft substances answer well with tea. method is also to gum the leaf, or fragment of leaf, on to a solid substance, and then horizontal sections can be obtained. Sometimes scraping a leaf in the same manner as when a blot is being erased from paper, brings away very beautiful pieces of the epidermis and stomata. Sections of leaves are easily obtained by placing the leaf between two pieces of cork, pressing them well together, and then cutting the finest possible layers with a sharp razor. In all these cases the razor should be wet with some fluid, either water or (which is for the most part better) glycerine, a little diluted. The section floats on the water, and may be transferred to a dish of dilute glycerine. It is well to cut a great number of sections in this way, and select the most transparent from the dish for microscopic examination. The author's new method of observing and preparing leaves is described in the article on "Tea."

$55. Micro-Spectroscope.-The micro-spectroscope has become a leading instrument in food-analysis, more especially since the introduction of so many artificial colouring materials. Fig. 7 shows the various parts of the "Sorby-Browning" microspectroscope. An eyepiece fits into the microscope tube, having

Fig. 7.

FOLLINGS.