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: