Page:Proceedings of the Royal Society of London Vol 60.djvu/178

Rh molecules, a process which, in the case of the silver-lead, might almost amount to the separation of the alloy into conjugate liquids near 50 atomic percentages of lead.

The lead-copper affords an excellent example of a phenomenon which has been predicted, we believe, by Ostwald, but, so far as we know, has not hitherto been examined experimentally. It is that of the solidification of a system consisting of two conjugate liquids, a saturated solution of lead in copper, and a saturated solution of copper in lead. For dilute solutions of lead in copper, as far as 7 atoms of lead, the curve is in harmony with equation (2) ; but as more lead is added its effect rapidly decreases, and from 17 to 65 atoms of lead the freezing point remains constant at 954° C. With more lead the freezing point again falls, until it reaches the eutectic point. An examination of the solid alloys shows that the flat part of the curve corresponds to alloys which have separated into two layers, while still liquid.

The copper-tin curve embraces all the remarkable bronzes, gun metal, bell metal, speculum metal, and it is not surprising to find that it presents singularities. The rapid increase in the steepness of the curve as tin is added suggests that the tin is combining with the copper to form complex molecules, perhaps of SnCu3 or SnCu4, which exist in solution. An abrupt change, not only in the direction of the curve, but also in the character of the freezing point, and the nature of the precipitate at 15'2 atoms of tin is in accordance with the great changes in the physical and microscopical character of the alloy noted by Behrens as occurring here. The remarkably straight line of freezing points from here up to 20 atoms of tin is best explained on the assumption that an isomorphous mixture of SnCu4 and another body are separating. The very flat part of the curve between 20 and 25 atoms of tin, along which each freezing point is an extremely constant temperature may be due to another case of isomorphism, or may be due to the separation of conjugate liquids. The existence of a body SnCu3 is not clearly indicated by our curve, although not inconsistent with it. Double freezing points occur on the horizontal lines stretching to the left from 15 2 and 20 atoms of tin.

The silver-antimony curve shows an angle at Ag3Sb, but the eutectic point, though near Ag3Sb2, is not at this formula.

It is worthy of note that in three cases in our curves an angular depression, and not a summit, occui’s at a formula point. We have made a few experiments on alloys of gold, nickel, and iron, in copper. The two latter cause a rise, but gold produces a fall in the freezing point.

From what we have hitherto done, silver bismuth promises to resemble silver-antimony, copper-bismuth to resemble copper-lead. Ihe silver-gold curve, as is already known, rises above the freezing