EQUIVALENTS, CHEMICAL 301 



Another machine for the same object was invented by Mr. A. Remond, of Birming- 

 ham, and is that employed by Messrs. Dickinson & Co. The distinguishing feature 

 of this arrangement is the employment of atmospheric pressure to feed in the paper 

 which is to form the envelope, and to deflect the flaps of the envelope into inclined 

 positions, to facilitate the action of a plunger, which descends to complete the folding. 

 The pieces of paper, cut to the proper form, are laid on a platform, which is furnished 

 with a pin at each corner, to enter the notches in the pieces of paper, and retain them 

 in their proper position, and such platform is caused alternately to rise and bring the 

 upper piece of paper in contact with the instrument that feeds the folding part of the 

 machine, and then to descend until a fresh piece is to be removed. The feeding in- 

 strument consists of a hollow horizontal arm, with two holes in the under side, and 

 having a reciprocating movement. When it moves over the upper piece of paper 

 on the platform, a partial vacuum is produced within it, by a suitable exhausting 

 apparatus, and the paper is thereby caused to adhere to it at the holes in its under 

 surface by the pressure of the atmosphere. The instrument carries the paper over a 

 rectangular recess or box ; and then, the vacuum within it being destroyed, it deposits 

 the paper between four pins, fixed at the angles of the box, and returns for another piece 

 of paper. As the paper lies on the top of the box, the flap which will be undermost 

 in the finished envelope is pressed by a small bar or presser on to the upper edge of 

 two angular feeders, communicating with a reservoir of cement or adhesive matter, and 

 thereby becomes coated with cement ; and at the same time, the outermost or seal flap 

 may be stamped with any required device, by dies, on the other side of the machine. 

 A rectangular frame or plunger now descends and carries the paper down into the box ; 

 the plunger rises, leaving the flaps of the envelope upright ; streams of air, issuing 

 from a slot in each side of the box, then cause the flaps to incline inwards : and the 

 folding is completed by the plunger again descending ; the interior and under surface 

 of such plunger being formed with projecting parts, suitable for causing the several 

 flaps to hold in proper superposition. The bottom of the box (which is hinged) opens, 

 and discharges the envelope down a shoot to a table below ; the feeding instrument 

 then brings forward another piece of paper ; and a repetition of the above movements 

 takes place. 



EPIIESITE, A micaceous mineral accompanying the emery of Ohunmacli-Dagh, 

 near Ephesus. It has been described by Dr. J. L. Smith. 



EPIDERMIS. The outer covering or scarf-skin. The fibrous horny coating of 

 some shells which sometimes are used in the arts. 



EPXDOTE. A complex mineral silicate, usually containing alumina, peroxide of 

 iron, lime, magnesia, and protoxide of iron. It crystallises in the oblique or monoclinic 

 system, often in long well-shaped glassy crystals. The colour is generally some shade 

 of green, often of a characteristic pistachio green, but sometimes passing into black. 



Epidote is not uncommon in granite, and other crystalline rocks ; whilst it forms 

 the main constituent of a rock known as Epidosite or Epidote-rocJc. The first crystals 

 occur in limestone, in trappean rocks, and in deposits of iron ore. The name (eTriSocns, 

 cpidosis, increase) was originally proposed by Haiiy on crystallographic considera- 

 tions, but has since been much extended, and the species now includes a large number 

 of varieties, the more important of which will bo noticed under their respective names. 



EPXZOA. Those animals which live parasitic upon other animals. See ENTOZOA. 



EPSOZVXITE. Native Epsom salts, or sulphate of magnesia. 



EPSOM SALTS. A sulphate of magnesia, consisting of magnesia 16-26, sul- 

 phuric acid 32-52, water 51-22. It derives its name from a mineral spring containing 

 the salt at Epsom. It is largely manufactured by the evaporation of sea-water, after 

 crystallisation of the common salt ; or by precipitating the magnesia from sea-water 

 by means of lime, and then dissolving this precipitate in sulphuric acid. It may also 

 be prepared from carbonate of magnesia, or from magnesian limestone (dolomite). See 

 MAGNESIA, SULPHATE OF. 



EQUISETACE.&:. (Equus, a horse, seta, a hair.) A group of plants commonly 

 called ' horse-tails.' The species of equisetum grow very abundantly in ditches, lakes, 

 and rivers, in this and other parts of the world. The stems are striated, hollow, arti- 

 culated, usually branched, having no true leaves, but in their place green-jointed 

 branches arranged in whorls at the articulations of the stem. In South America they 

 grow to a large size. The stems usually contain a great deal of silica in their compo- 

 sition, and the Equisetum hyemale, or Dutch rush, finds its use in the arts on this 

 account. See DUTCH RUSH. 



EQUISETUM. A genus of Equisetacese. 



EQUIVALENTS, CHEMICAL. By this term is understood the proportions 

 in which substances combine with each other to form definite compounds. These 

 proportions are referred to the common standard, hydrogen, which is taken as unity. 

 The limits of this work preclude the possibility of entering into the history of 



