VIOLETS.

Last in order and least in size comes the violet. For "the flower of sweetest smell is shy and lowly," and has taken a modest place in the paper.

Violets are planted out in October or April. October is preferred, as it is the rainy season; nor are the young plants then exposed to the heat of the sun or to the drought, as they would be if starting life in April.

The best place for them is in olive or orange groves, where they are protected from the too powerful rays of the sun in summer and from the extreme cold in winter. Specks of violets appear during November. By December the green is quite overshadowed, and the whole plantation appears of one glorious hue. For the leaves, having developed sufficiently for the maintenance of the plant, rest on their oars, and seem to take a silent pleasure in seeing the young buds they have protected shoot past them and blossom in the open.

The flowers are picked twice a week; they lose both color and flavor if they are allowed to remain too long upon the plant. They are gathered in the morning, and delivered at the factories by the commissionnaires or agents in the afternoon, when they are taken in hand at once.

The products yielded by this flower are prized before all others in the realms of perfumery, and cannot be improved; for, as one great authority on all matters has said: "To throw a perfume on the violet ... were wasteful and ridiculous excess."

HOW TO MAKE PHOTO. PRINTING PLATES.

The drawing intended for reproduction is pinned on a board and placed squarely before a copying camera in a good, even light. The lens used for this purpose must be capable of giving a perfectly sharp picture right up to the edges, and must be of the class called rectilinear, i.e., giving straight lines. The picture is then accurately focused and brought to the required size. A plate is prepared in the dark room by the collodion process, which is then exposed in the camera for the proper time and developed in the ordinary way. After development, the plate is fixed and strongly intensified, in order to render the white portions of the drawings as opaque as possible. On looking through a properly treated negative of this kind, it will be seen that the parts representing the lines and black portions of the drawing are clear glass, and the whites representing the paper a dense black.

The negative, after drying, is ready for the next operation, i.e., printing upon zinc. This is done in several ways. One method will, however, be sufficient for the purpose here. I obtain a piece of the bichromatized gelatine paper previously mentioned, and place it on the face of the negative in a printing frame. This is exposed to sunlight (if there is any) or daylight for a period varying from five to thirty minutes, according to the strength of the light. This exposed piece of paper is then covered all over with a thin coating of printing ink, and wetted in a bath of cold water. In a few minutes the ink leaves the white or protected parts of the paper, remaining only on the lines where the light has passed through the negative and affected the gelatine. We now have a transcript of the drawing in printing ink, on a paper which, as soon as dry, is ready for laying down on a piece of perfectly clean zinc, and passing through a press. The effect and purpose of passing this cleaned sheet of zinc through the press in contact with the picture on the gelatine paper is this: Owing to the stronger attraction of the greasy ink for the clean metal than for the gelatine, it leaves its original support, and attaches itself strongly to the zinc, giving a beautifully sharp and clean impression of our original drawing in greasy ink on the surface of the zinc. The zinc plate is next damped and carefully rolled up with a roller charged with more printing ink, and the image is thus made strong enough to resist the first etching. This etching is done in a shallow bath, which is so arranged that it can be rocked to and fro. For the first etching, very weak solution of nitric acid and water is used. The plate is placed with this acid solution in the bath, and steadily rocked for five or ten minutes. The plate is then taken out, washed, and again inked; then it is dusted over with powdered resin, which sticks to the ink on the plate. After this the plate is heated until the ink and resin on the lines melt together and form a strong acid-resisting varnish over all the work. The plate is again put into the acid etching bath and further etched. These operations are repeated five or six times, until the zinc of the unprotected or white part of the picture is etched deep enough to allow the lines to be printed clean in a press, like ordinary type or an engraved wood block. I ought perhaps to explain that between each etching the plate is thoroughly inked, and that this ink is melted down the sides of the line, so as to protect the sides as well as the top from the action of the acid; were this neglected, the acid would soon eat out the lines from below. The greatest skill and care is, therefore, necessary in this work, especially so in the case of some of the exquisitely fine blocks which are etched for some art publications.

There are many details which are necessary to successful etching, but those now given will be sufficient to convey to you generally the method of making the zinc plate for the typographic block. After etching there only remains the trimming of the zinc, a little touching up, and mounting it on a block of mahogany or cherry of exact thickness to render it type high, and it is now ready for insertion with type in the printer's form. From a properly etched plate hundreds of thousands of prints may be obtained, or it may be electrotyped or stereotyped and multiplied indefinitely.—G.S. Waterlow, Brit. Jour. Photo.

ANALYSIS OF A HAND FIRE GRENADE.

By CHAS. CATLETT and R.C. PRICE.

The analyses of several of these "fire extinguishers" have been published, showing that they are composed essentially of an aqueous solution of one or more of the following bodies; sodium, potassium, ammonium, and calcium chlorides and sulphates, and in small amount borax and sodium acetate; while their power of extinguishing fire is but three or fourfold that of water.

One of these grenades of a popular brand of which I have not found an analysis was examined by Mr. Catlett with the following results: The blue corked flask was so open as to show that it contained no gas under pressure, and upon warming its contents, but 4 or 5 cubic inches of a gas were given off. The grenade contained about 600 c.c. of a neutral solution, which gave on analysis:

		In 1000 c.c.	In the Flask.
		Grammes.	Grains.
Calcium	chloride¹	92.50	850.8
Magnesium	"	18.71	173.2
Sodium	"	22.20	206.9
Potassium	"	1.14	10.6
		———	———
		134.55	1241.5

¹Trace of bromide.

As this mixture of substances naturally suggested the composition of the "mother liquors" from salt brines, Mr. Price made an analysis of such a sample of "bittern" from the Snow Hill furnace, Kanawha Co., W.Va., obtaining the following composition:

		In 1000 c.c.	In the Flask.
		Grammes.	Grains.
Calcium	chloride¹	299.70	925.8
Magnesium	"	56.93	175.7
Strontium	"	1.47	4.5
Sodium	"	20.16	62.2
Potassium	"	5.13	15.8
		———	———
		383.39	1184.0

¹Trace of bromide.

There is of course some variation in the bittern obtained from different brines, but it appears of interest to call attention to this correspondence in composition, as indicating that the liquid for filling such grenades is obtained by adding two volumes of water to one of the "bittern." The latter statement is fairly proved by the presence of the bromine, and certainly from an economical standpoint such should be its method of manufacture.—Amer. Chem. Jour.

MOLECULAR WEIGHTS.

A new and most valuable method of determining the molecular weights of non-volatile as well as volatile substances has just been brought into prominence by Prof. Victor Meyer (Berichte, 1888, No. 3). The method itself was discovered by M. Raoult, and finally perfected by him in 1886, but up to the present has been but little utilized by chemists. It will be remembered that Prof. Meyer has recently discovered two isomeric series of derivatives of benzil, differing only in the position of the various groups in space. If each couple of isomers possess the same molecular weight, a certain modification of the new Van't Hoff-Wislicenus theory as to the position of atoms in space is rendered necessary; but if the two are polymers, one having a molecular weight n times that of the other, then the theory in its present form will still hold. Hence it was imperative to determine without doubt the molecular weight of some two typical isomers. But the compounds in question are not volatile, so that vapor density determinations were out of the question. In this difficulty Prof. Meyer has tested the discovery of M. Raoult upon a number of compounds of known molecular weights, and found it perfectly reliable and easy of application. The method depends upon the lowering of the solidifying point of a solvent, such as water, benzine, or glacial acetic acid, by the introduction of a given weight of the substance whose molecular weight is to be determined. The amount by which the solidifying point is lowered is connected with the molecular weight, M, by the following extremely simple formula: M = T x (P / C); where C represents the amount by which the point of congelation is lowered, P the weight of anhydrous substance dissolved in 100 grammes of the solvent, and T a constant for the same solvent readily determined from volatile substances whose molecular weights are well known. On applying this law to the case of two isomeric benzil derivatives, the molecular weights were found, as expected, to be identical, and not multiples; hence Prof. Meyer is perfectly justified in introducing the necessary modification in the "position in space" theory. Now that this generalization of Raoult is placed upon a secure basis, it takes its well merited rank along with that of Dulong and Petit as a most valuable means of checking molecular weights, especially in determining which of two or more possible values expresses the truth.—Nature.

[Continued from Supplement, No. 642, page 10258.]

THE DIRECT OPTICAL PROJECTION OF ELECTRO-DYNAMIC LINES OF FORCE AND OTHER ELECTRO-DYNAMIC PHENOMENA.[¹]

By Prof. J.W. MOORE.

II. LOOPS.

If the wire, with its lines of force, be bent into the form of a vertical circle 1⅛ in. in diameter, and fixed in a glass plate, some of the lines of force will be seen parallel to the axis of the circle. If the loop is horizontal, the lines become points.

Fig. 14.

Fig. 14a.