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Rambler's Top100
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Project Gutenberg's Encyclopedia, vol. 1 ( A - Andropha

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microscopical examination gives valuable information concerning 
the suitability' of a sample of steel for special purposes. 

Mixture by fusion is the general method of producing an 
alloy, but it is not the only method possible.  It would 
seem, indeed, that any process by which the particles of 
two metals are intimately mingled and brought into close 
contact, so that diffusion of one metal into the other can 
take place, is likely to result in the formation, of an 
alloy.  For example, if vapours of the volatile metals 
cadmium, zinc and magnesium are allowed to act on platinum or 
palladium, alloys are produced.  The methods of manufacture 
of steel by cementation, case-hardening and the Harvey 
process are important operations which appear to depend on 
the diffusion of the carburetting material into the solid 
metal.  When a solution of silver nitrate is poured on to 
metallic mercury, the mercury replaces the silver in the 
solution, forming nitrate of mercury, and the silver is 
precipitated; it does not, however, appear as pure metallic 
silver, but in the form of crystalline needles of an alloy 
of silver and mercury.  F. B. Mylius and O. Fromm have shown 
that alloys may be precipitated from dilute solutions by zinc 
cadmium, tin, lead and copper.  Thus a strip of zinc plunged 
into a solution of silver sulphate, containing not more than 
0.03 gramme of silver in the litre, becomes covered with a 
flocculent precipitate which is a true alloy of silver and 
zinc, and in the same way, when copper is precipitated 
from its sulphate by zinc, the alloy formed is brass.  They 
have also formed in this way certain alloys or definite 
composition, such as AuCd3, Cu2Cd, and. more interesting 
still, Cu2Sn.  A very similar fact, that brass may be formed 
by electrodeposition from a solution containing zinc and 
copper, has long been known.  W. V. Spring has shown that by 
compressing a finely divided mixture of 15 parts of bismuth, 
8 parts of lead, 4 parts of tin and 3 parts of cadmium, an 
alloy is produced which melts at 100 deg.  C., that is. much 
below the melting-point of any of the four metals.  But these 
methods or forming alloys, although they suggest questions 
of great interest, cannot receive further discussion bore. 

Our knowledge of the nature of solid alloys has been much 
enlarged by a careful study of the process of solidification.  
Let us suppose that a molten mixture of two substances A and 
B, which at a sufficiently high temperature form a uniform 
liquid, and which do not combine to form definite compounds, 
is slowly cooled until it becomes wholly solid.  The phenomena 
which succeed each other are then very similar, whether A 
and B are two metals, such as lead and tin or silver and 
copper, or are a pair of fused salts, or are water and common 
salt.  All these mixtures when solidified may fairly be termed 
alloys.1 If a mixture of A and B be melted and then allowed 
to cool, a thermometer immersed in the mixture will indicate 
a gradually falling temperature.  But when solidification 
commences. the thermometer will cease to fall, it may even 
rise slightly, and the temperature will remain almost constant 
for a short time.  This halt in the cooling, due to the heat 
evolved in the solidification of the first crystals that form 
in the liquid is called the freezing-point of the mixture; 
the freezing-point can generally be observed with considerable 
accuracy.  In the case of a pure substance, and of a certain 
small class of mixtures, there is no further fall in temperature 
until the substance has become completely solid, but, in 
the case of most mixtures, after the freezing-point has been 
reached the temperature soon begins to fall again, and as the 
amount of solid increases the temperature becomes lower and 
lower.  There may be other halts in the cooling, both before 
and after complete solidification, due to evolution of 
heat in the mixture.  These halts in temperature that occur 
during the cooling of a mixture should be carefully noted, 
as they give valuable information concerning the physical and 
chemical changes that are taking place.  If we determine the 
freezing-points of a number of mixtures varying in composition 
from pure A to pure B, we can plot the freezing-point 
curve.  In such a curve the percentage composition can be 
plotted horizontally and the temperature of the freezing-point 
vertically, as in fig. 5. In such a diagram, a point P defines 
a particular mixture, both as to percentage, composition and 
temperature; a vertical line through P corresponds to the 
mixture at all possible temperatures, the point Q being its 
freezing-point.  In the case of two substances which neither 
form compounds nor dissolve each other in the solid state, 
the complete freezing-point curve takes the form shown in 
fig. 5. It consists of two branches AC and BC, which meet 
in a lowest point C. It will be seen that as we increase 
the percentage of B from nothing up to that of the mixture 
C, the freezing-point becomes lower and lower, but that if 
we further increase the percentage of B in the mixture, the 
freezing-point rises.  This agrees with the well-known fact 
that the presence of an impurity in a substance depresses its 
melting-point.  The mixture C has a lower freezing or melting 
point than that of any other mixture; it is called the eutectic 
mixture.  All the mixtures whose composition lies between 
that of A and C deposit crystals of pure A when they begin to 
solidify, while mixtures between C and B in composition deposit 
crystals of pure B. Let us consider a little more closely the 
solidification of the mixture represented by the vertical line 
PCRS.  As it cools from P to Q the mixture remains wholly 
liquid, but when the temperature Q is reached there is a 
halt in the cooling, due to the formation of crystals of A. 
The cooling soon recommences and these crystals continue to 
form, but at lower and lower temperatures because the still 
liquid part is becoming richer in B. This process goes on 
until the state of the remaining liquid is represented by 
the point C. Now crystals of B begin to form, simultaneously 
with the A crystals, and the composition of the remaining 
liquid does not alter as the solidification progresses.  
Consequently the temoerature does not change and there is 
another well-marked halt in the cooling, and this halt lasts 
until the mixture has become wholly solid.  The corresponding 
changes in the case of the mixture TUVW are easily understood 
--the first halt at U, due to the crystallization of pure 
B, will probably occur at a different temperature, but the 
second halt, due to the simultaneous crystallization of A and 
B, will always occur at the same temperature whatever the 
composition of the mixture.  It is evident that every mixture 
except the eutectic mixture C will have two halts in its 
cooling, and that its solidification will take place in two 
stages.  Moreover, the three solids S, D and W will differ 
in minute structure and therefore, probably, in mechanical 
properties.  All mixtures whose temperature lies above the 
line ACB are wholly liquid, hence this line is often called 
the ``liquidus''; all mixtures at temperatures below that 
of the horizontal line through C are wholly solid, hence 
this line is sometimes called the ``solidus,'' but in more 
complex cases the solidus is often curved.  At temperatures 
between the solidus and the liquidus a mixture is partly solid 
and partly liquid.  This general case has been discussed at 
length because a careful study of it will much facilitate the 
comprehension or the similar but more complicated cases that 
occur in the examination of alloys.  A great many mixtures 
of metals have been examined in the above-mentioned way. 

Fig. 6 gives the freezing-point diagram for alloys of lead and 
tin.  We see in it exactly the features described above.  
The two sloping lines cutting at the eutectic point are the 
freezing-point curves of alloys that, when they begin to 
solidify, deposit crystals of lead and tin respectively.  The 
horizontal line through the eutectic point gives the second 
halt in cooling, due to the simultaneous formation of lead 
crystals and tin crystals.  In the case of this pair of metals, 
or indeed of any metallic alloy, we cannot see the crystals 
forming, nor can we easily filter them off and examine them 
apart from the liquid, although this has been done in a few 
cases.  But if we polish the solid alloys, etch them if 
necessary, and examine them microscopically, we shall find 
that alloys on the load side of the diagram consist of 
comparatively large crystals of lead embedded in a minute 
complex, which is due to the simultaneous crystallization 
of the two metals during the solidification at the eutectic 
temperature.  If we examine alloys on the tin side we shall 
find large crystals of tin embedded in the same complex.  The 
eutectic alloy itself, fig. 2 (Plate), shows the minute complex 
of the tin-lead eutectic, photographed by J. A. Ewing and W. 
Rosenhain, and fig. 3 (Plate), photographed by F. Osmond, shows 
the structure of a silver-copper alloy containing considerably 
more silver than the eutectic.  Here, the large dark masses 
are the silver or silver-rich substance that crystallized 
above the eutectic temperature, and the more minute black 
and white complex represents the eutectic.  It is not safe 
to assume that the two ingredients we see are pure silver 
and pure copper; on the contrary, there is reason to think 
that the crystals of silver contain some copper uniformly 
diffused through them, and vice versa.  It is, however, not 
possible to detect the copper in the silver by means of the 
microscope.  This uniform distribution of a solid substance 
throughout the mass of another, so as to form a homogeneous 
material, is called ``solid solution,'' and we may say that 
solid silver can dissolve copper.  Solid solutions are probably 
very common in alloys, so that when an alloy of two metals 
shows two constituents under the microscope it is never safe 
to infer, without further evidence, that these are the two 
pure metals.  Sometimes the whole alloy is a uniform solid 
solution.  This is the case with the copper-tin alloys 
containing less than 9% by weight of tin; a microscopic 
examination reveals only one material, a copper-like substance, 
the tin having disappeared, being in solution in the copper. 

Much information as to the nature of an alloy can be obtained 
by placing several small ingots of the same alloy in a furnace 
which is above the melting-point of the alloy, and allowing 
the temperature to fall slowly and uniformly.  We then extract 
one ingot after another at successively lower temperatures 
and chill each ingot by dropping it into water or by some 
other method of very rapid cooling.  The chilling stereotypes 
the structure existing in the ingot at the moment it was 
withdrawn from the furnace, and we can afterwards study this 
structure by means of the microscope.  We thus learn that 
the bronzes referred to above, although chemically uniform 
when solid, are not so when they begin to solidify, but that 
the liquid deposits crystals richer in copper than itself, 
and therefore that the residual liquid becomes richer in 
tin.  Consequently, as the final solid is uniform, the 
crystals formed at first must change in composition at a later 
stage.  We learn also that solid solutions which exist at 
high temperatures often break up into two materials as they 
cool; for example, the bronze of fig. 1, which in that figure 
shows two materials so plainly, if chilled at a somewhat 
higher temperature but when it was already solid, is found 
to consist o
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