The apparatus devised by Fresenius and Will, and shown in Fig. 3, may also be em ployed in making alkalimetrical estimations. It is used in precisely the same way as in acidi metry. The alkali to be tested is carefully weighed, and dissolved in water in the flask A ; con centrated sulphuric acid is plac,ed in the flask B, and the apparatue is accurately weighed. After closing the end of the tube c, suction is applied to the tube a, so as to draw over a small quantity of air from A into B through the tube b ; on withdrawing the lips, the pressure of air forces a little of the acid over into A, by which means the alkali is decomposed. This is continued until the evolution of carbonic acid ceases, when heat is applied gently for a few moments. Air is then drawn through, and the apparatus is cooled and weighed. The loss in weight gives the amount of earbonio acid evolved, as in the previous case.
Works for reference ;—Fresenius's Quantitative Analysis '; Sutton's Volumetric Analysis.' are compounds of two or more metals ; thus brass is an alloy of copper and zinc, type-metal an alloy of lead and antimony, bell-metal and gun-metal alloye of copper and tin. Although there are fifty-one metals known to the chemist, only a comparatively small number have been largely used for industrial purposes. Every alloy may, however, be regarded as a new metal, since it genemlly possesses properties entirely different frum those of the metals of which it is composed. Hence, as the properties resulting from the combination of two metals rarely represent the mean of those metals, it is impossible to foretell the nature of a new alloy, and it is therefore probable that such combinations may be made as will adapt themselves to every imaginable want created by our advancing civilization. Only ft few hundred alloys have been prepared up to the present time, and only about sixty have really been carefully studied, although there is scarcely a limit to the number which might be made ; so that there is yet much to be learnt on this interesting and important subject.
All true alloys consist of compounds of metals in their definite chemical proportions ; it is, how ever, a matter of some difficulty to obtain them in a separate state, owing to the readiness with which they dissolve in the excess of that metal which may happen to preponderate. There are some alloys also in which the constituent metals seem to be merely mechanically mixed, but these are exceptions rather than the rule. Alloys possess the properties which are characteristic of metals, such as metallic lustre, conductivity of heat and electricity, and, in a greater or less degree, malleability, ductility, and tenacity. The specific gravity of an alloy appears to depend
upon the amount of cohesion or attraction exerted by the constituent metals for one another, and to bear no reference whatever to the high or low specific gravity of those constituents in their free state. The specific heat of alloys was found by Regnault to be very nearly the mean of the specific heats of the constituents. The following rule for obtaining the specific heat of alloys gives a very close approximation to the figures obtained by actual experiment :—Multiply the specific heat of each constituent into the percentage amount of it contained in the alloy, and divide the sum of the products by 100. Alloys are not as a rule such good conductors of heat and electricity as the metals of which they are made. Their fusibility does not at all depend upon that of their consti tuents, but is generally greater ; thus the melting point of tin is 230° C. and that of lead 334° C., whereas a compound of 5 parts of tin and 1 part of lead melts at 194° C., and a compound of equal parts of both metals melts at 241° C. The ductility of alloys is usually slightly less than that of their most ductile constituent, and their hardness is greater than the mean hardness of both or all the metals. The tenacity of an alloy is often much greater than that of either of the metals alone.
To make an alloy, the metals must be melted together. This operation may be carried on in an earthenware crucible when very small quantities are being operated upon ; but when large masses of metal have to be dealt with, as in the case of statues, &c., a reverberatory furnace must be employed to effect the melting. As a rule, the least fusible metal is placed in the crucible first, unless it be in very small quantity and will dissolve readily in the other metal, in which case it goes in last ; and if, as in the case of zinc, the volatilization of the metal be extremely rapid, it is introduced only the moment before the fused mass is ready to be poured into the mould or other receptacle. The order in which the metals are melted has a material effect upon the nature of the resulting alloy, for it has been proved by experiment that the latter often possesses different properties when the mixing has taken place in a different order. The fused metals should be kept thoroughly well stirred up until the mixture is complete ; otherwise the heaviest metal will sink to the bottom of the mass, and the alloy will not be of uniform composition. This contingency is sometimes avoided by melting the mass a second time. When three metals have to be united together, they should first be melted in pairs, and afterwards together.