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softer part than on the harder, and by the time you have polished the harder parts you wear away the softer; but I think when you consider the porphyries of Egypt, of which Sienite is a common variety, orthose of Sweden and Russia, which have come into use for ornamental pur poses, you cannot regard porphyry as, in any case, a rotten material. I do not think porphyries, in general, can be considered as rotten in the sense of having clay cracks, and it is certain that, if these were common in any sample, it would not be selected for ornamental purposes. The Cornish granites are sometimes called porphyries. Some of them are crystals embedded in a base, and very often the base is a fel spar that decomposes rapidly, or is a quartz, which rapidly disin tegrates. Large crystals of felspar, and crystals of mica, are the usual constituents of porphyry. Sometimes the crystals are hard and bear exposure for a very long time indeed. This is particularly the case with the crystals which stand out on the pavement of London Bridge. Sometimes the felspar decomposes, and the quartz stands out in an irregular manner. In all cases there is a liability in rocks of this kind to unequal weathering, but, in spite of that, the porphyritic rocks of all kinds are usually selected as the hardest and best materials where great durability is required. There is, however, a very important cause of change in rocks of this kind, which, perhaps, is not sufficiently regarded by architects and engineers. A granite may look well and appear to be of first-rate quality. It may be exceedingly hard, and may seem as if it would last for centuries, and even for thousands of years; but the composition of that granite may show a source of weakness. All fel spar contains a certain quantity of alkali, either soda or potash, Now, there is little difference in appearance between granites composed of these two varieties, but a very important difference as far as the dura bility of the material is concerned. Thus a knowledge of the chemical composition of a building material, and some reference to the science of chemistry applied to matters of this sort may, in some cases be ex ceedingly useful. Unfortunately, this has not always been attended to. A case of this kind came before me not long ago, in the course of a pro fessional enquiry. A large and important work was in progress, and the contractor for the work desired that the materials used should be of the best possible quality. I was asked to examine the material sub mitted by the contractor, and to give an opinion as to its quality. If I considered it good, this material was to be used, as in other respects the contract was a satisfactory one. I doubted the quality of the material, and had it examined chemically. The opinion of the chemist who ana lysed it, and ultimately that of the engineer himself, was not altogether favourable. It was a stone which would wear rapidly in comparison with other granites. Still, as the contract was a good one, and as there was liberty of reference to government engineers, who had used it for public works, our opinion was overruled, and the work is now going on with the material in question. In this case the whole weight of the responsibility rests upon a report of a Colonel of the Royal Engineers, that the material was a good one, and that he had himself used it, although without special examination, for government purposes. It seems to me, however, that in matters of this kind it is not quite safe to refer entirely to military authorities, who, after all, are not omnis cient by intuition. Such is the force of authority and interest that those engaged in public works will sometimes act against their judg ment for want of faith in science. As to the theory of the primitive rocks and the composition of granite, and matters of that kind, I should be happy to enter into those subjects, as suggested by my friend Professor Donaldson, but I should occupy too much of your time were I to do so now, and explain the whole of the reasons why geologists have come to the conclusions to which I have already made allusion. I may say, however, that most geologists of the modern school, and those who have thought for themselves on the matter, and are indifferent to the voice of established authority, appear to have come to the conclusion that all rocks that have yet been examined are recomposed from other rocks, and that at the present time on the face of the earth we do not really see anything that belonged to its original nature. What we now see are the results of changes that have been going on upon and beneath the surface of the earth from a time so distant that we cannot judge of its amount, or of what took place before those changes commenced. We have no evidence of the real interior of the earth, for all we have to deal with is its crust. We do know something about that—not very much perhaps—but it is enough for our purpose as practical men, and enough to lead us to theo retical conclusions which, as scientific men, we feel an interest in. As far as architecture is concerned, it is enough to know what is the pro bable origin of these rocks, without considering how far that origin is connected with the interior condition of the earth. There was another subject referred to by several gentlemen who have spoken—That is the subject of slates. With regard to this matter there is no doubt whatever that slate, in the ordinary sense of the word, is simply a compressed clay, but, like all other substances that have been exposed for a long time to the action of forces that are slowly producing their effects in the interior of the earth, the clay, like the limestone and sand, has undergone a certain amount of change. The effect of pressure on a substance like clay is very different from that produced in sand or limestone, and is far greater than we can perhaps certify. The result of pressure is two-fold; first, it has a physical effect of a very important kind, bringing into play the attrac tion of cohesion. When brought about in this way cohesion is some times complete enough to produce a material perfectly compact. The particles are in fact cemented together; and in this way slate, in the natural course of things, is quite as compact as quartz, and, in this sense, I believe practically indestructible. But if you employ friction, and rub over the surface of the slate, you will of course obtain a removal and severance of the particles, and it will be the same if you do so with quartz ; only, in one place you get a clay, and in the other you get sand. The sand may seem very natural, and the clay may seem strange, but the case is very simple. In that sense slate is liable to deterioration. Of course there are bad slates, and why are there bad slates ? Because all slate having been compressed, and the compression being unequal, there are differences of cohesion. The slates also being formed originally of very different materials—never of pure clay—it is only when exposed to those electric and magnetic currents.which produce an effect in the interior of the earth that it becomes comparatively pure : and one of the results of change is the production of those peculiar veins of quartz to which Professor Donald son directed attention, and which are amongst the most remarkable phenomena of quartz. In the composition of slate it seems likely that the removal of a certain amount of moisture produces contraction in the mass, and thus are derived fissures, which are of necessity filled up by the substance which remains after the silicate of alumina has been brought together; but every one who knows the appearance of a slate quarry knows that besides veins of quartz there are also numerous crystals of sulphide of iron (iron pyrites), which are derived from the sulphur and iron present as impurities in the clay of which the slate was formed. The slate becomes a pure silicate of alumina, the foreign substances having been separated by the action of those forces which are constantly arranging and re arranging matters in the interior of the earth. It is in this way that veins of quartz have been produced in the great masses of rock 1 refer to. There are many varieties of slate—some good, some indifferent. Some are capable of being split into exceedingly thin films—so thin indeed that it is not determined what the limit is. I believe the highest quality of slates are those which split most perfectly. At the same time there are excellent slates which do not split thin, which are only fissile to a certain extent, but which may be trusted thoroughly for architectural purposes. Thus, it is not absolutely the case that slates which will not split finely are not as good as those that will. Generally those slates which split most finely are manufactured into slates of comparatively small size. The explanation of the matter is simple. I believe the thick slates or slabs have been less completely pressed, but are just as pure in their composition as the others ; some may have been less squeezed by the circumstances and conditions of upheaval, others may not only have been squeezed vertically, but also laterally, as against some unyielding wall in the interior of the earth. The former may also have been less exposed to those forces which re-arranged the particles, and which perhaps produced the most delicate of the splitting planes. There is but one more subject to which I will direct attention, and that is the question of the material that may possibly render stones more able to resist the weather than they would be naturally. There is no doubt whatever that a considerable variety of materials may be used for this purpose. Paint, we know, preserves stone for a short time, for months at any rate, and under favourable circumstances, for a few years. Soap and alum can, I think, do little more than that, but may also be suc cessful for a short time. But though useful in that way, they cannot have a permanent effect on the material, or communicate the power of resisting exposure and weathering. As I have said before, I believe the only material that is likely to answer the purpose is one of those that produce a coating of silicate of lime over the surfaces of all the different particles of limestone near the surface. This is most easily done in the porous limestones which absorb readily ; and I think it would be effected if the stones were washed with the silicate of soda and chloride of calcium in the manner patented by Mr. Frederick Ransome, and kept sheltered for a certain time before being used. The probability is that, so treated, very inferior and cheap limestones would last as long as the most costly qualities of limestone. Such an operation may be performed after the stone has been dressed for use ; and in this way stone may be much improved. I have said before that I do not believe any process has been introduced, and that I doubt whether any can be introduced—which will prevent stones placed in a building and already showing signs of decay, from continuing to decay. You may temporarily check the decay; you may do it for many years ; but that you can stop the decay I do not think is likely ; for when once the moisture has thoroughly penetrated into stone, we know that the split ting power of frost will throw off any films of foreign material from the surface, and then the whole thing has to be done over again ; and this would practically be as difficult and costly as repainting or recoating with any of those materials that have been suggested. ♦