DIFFUSION 31 



retouched by the engraver, is turned, hardened, and collared, like the matrix, of 

 which it is now a complete impression in relief, and, as we have before said, is called a 

 punch. 



This punch becomes an inexhaustible parent of dies, without further reference to the 

 original matrix ; for now by impressing upon it plugs of soft steel, and by pursuing 

 with them an exactly similar operation to that by which the punch itself was obtained, 

 we procure impressions from it to any amount, which, of course are fac-similes of the 

 matrix, and these dies being turned, hardened, polished, and, if necessary, tempered, 

 are employed for the purposes of coinage. 



The distinction between striking medals and common coin is very essential, and the 

 work upon the dies is accordingly adjusted to each. Medals are usually in very high 

 relief, and the effect is produced by a succession of blows ; and as the metal in which 

 they are struck, be it gold, silver, or copper, acquires considerable hardness at each 

 stroke of the press, they are repeatedly annealed during the process of bringing 

 them up. In a beautiful medal, which Mr. Wyon executed for the Koyal Naval 

 College, the obverse represents the head of the King, in very bold relief; it required 

 thirty blows of a very powerful press to complete the impression, and it was necessary 

 to anneal each medal after every third blow, so that they went ten times into the fire 

 for that purpose. In striking a coin or medal, the lateral spread of the metal, which 

 otherwise would ooze out as it were from between the dies, is prevented by the appli- 

 cation of a steel collar, accurately turned to the dimensions of the dies, and which, 

 when left plain, gives to the edge of the piece a finished and polished appearance ; it is 

 sometimes grooved, or milled, or otherwise ornamented, and occasionally lettered, in 

 which case it is made in three separate and moveable pieces, confined by a ring, into 

 which they are most accurately fitted, and so adjusted that the metal may be forced 

 into the letters by its lateral spread, at the same time that the coin receives the blow 

 of the screw-press. 



Coins^are generally completed by one blow of the coining-press. These presses are 

 worked in the Eoyal Mint by machinery, so contrived that they shall strike, upon an 

 average, sixty blows in a minute ; the blank piece, previously properly prepared and 

 annealed, being placed between the dies by part of the same mechanism. 



The number of pieces which may be struck by a pair of dies of good steel, properly 

 hardened and duly tempered, not unfrequently amounts at the Mint to between one 

 and two hundred thousand ; but the average consumption of dies is of course much 

 greater, owing to the variable qualities of steel, and to the casualties to which the 

 dies are liable ; thus, the upper and lower die are sometimes struck together, owing 

 to an error ^n the layer-on, or in that part of the machinery which ought to put the 

 blank into its place, but which now and then fails so to do. This accident very com- 

 monly arises from the boy who superintends the press neglecting to feed the hopper 

 of the layer-on with blank pieces. If a die is too hard, it is apt to break or split, and 

 is especially subject to fissures, which run from letter to letter upon the edge. If too 

 soft, it swells, and the collar will not rise and fall upon it, or it sinks in the centre, 

 and the work becomes distorted and faulty. He, therefore, who supplies the dies for 

 an extensive coinage, has many accidents and difficulties to encounter. There are eight 

 presses at the Mint, frequently at work for ten hours each day, and the destruction of 

 eight pair of dies per day (one pair for each press) may be considered a fair average 

 result, though they much more frequently fall short of, than exceed this proportion. 

 It must be remembered, that each press produces 3,600 pieces per hour ; but making 

 allowance for occasional stoppages, we may reckon the daily produce of each press at 

 30,000 pieces ; the eight presses, therefore, will furnish a diurnal average of 240,000 

 pieces. 



DIES, hardening of. See STEEL, HARDENING OF. 



DIFFUSION*. As the principles of diffusion are sometimes involved in processes 

 of manufacture, a short notice of the phenomena appears to properly find its place 

 in this Dictonary. To no one do we owe so much for elucidating this subject as to 

 the late Prof. Graham, and from his ' Elements of Chemistry ' the following notices have 

 been selected. When light and heavy gases are mixed together, they do not exhibit 

 any tendency to separate again on standing at rest, differing in this respect from mixed 

 liquids, many of which speedily separate and arrange themselves according to their 

 densities, the lighted uppermost, the heaviest undermost, as in the familiar example 

 of oil and water, unless they have combined together. This peculiar property of gases 

 has repeatedly been made the subject of careful experiment. Common air, for instance, 

 is essentially a mixture of two gases, differing in weight in the proportion of 971 to 

 1,105 ; but the air in a tall close tube of glass, several feet in length, kept upright in 

 a still place, has been found sensibly the same in composition at the top and bottom 

 of the tube after a lapse of months. Hence, there is no reason to imagine that the 

 upper stratum of air differs in composition from the lower, or that a light gas, such as 



